This op-amp buffer is oscillating and I can't figure out why












11














Currently, this is the only assembled part on the circuit board. This is a simple inverting buffer circuit that should be at the input. The op-amp (LTC6241HV) is powered +/-5V from a linear bench power supply. The power pins are bypassed with 0.1uF caps.



I'm inputting a 1KHz sine and on the output I get a ~405KHz sine superimposed on the 1KHz signal. I have tried to build a second PCB but the results are exactly the same.



If anyone knows what could be the cause for this I'll be happy to hear.



LTC6241HV Datasheet
enter image description here










share|improve this question




















  • 5




    Wow, 1MEGohm: that's dangerous. Try reducing R1, R3.
    – glen_geek
    Dec 22 at 14:41






  • 3




    Most problematic: The capacitor C6 which gives the loop gain a lowpass characteristic. As a result, additional phase shift which reduces the phase margin - in particular because of unity gain configuration
    – LvW
    Dec 22 at 14:44






  • 3




    If you need high-Z, then add a tiny capacitor (even a few pf) across R1 in parallel. That should help kill oscillation. But be aware that the high-frequency response is affected. An optimum value should allow flat response to about 1 MHz.
    – glen_geek
    Dec 22 at 14:47






  • 2




    If you cannot reduce R3 (at least to 100k, better if even lower) you can shunt R1 with a capacitor, setting say 100kHz or lower bandwidth. Otherwise, you can shunt non inverting input to ground with, say 100kohm or so, reducing loop gain.
    – carloc
    Dec 22 at 14:52






  • 3




    Has anyone asked about the load capacitance for this problem? With any cable you will xx pF/m and the datasheet specifies the series R vs load pF for stability reasons. Why did you choose this device for -1 gain? What is the load pF?
    – Tony EE rocketscientist
    Dec 23 at 3:00
















11














Currently, this is the only assembled part on the circuit board. This is a simple inverting buffer circuit that should be at the input. The op-amp (LTC6241HV) is powered +/-5V from a linear bench power supply. The power pins are bypassed with 0.1uF caps.



I'm inputting a 1KHz sine and on the output I get a ~405KHz sine superimposed on the 1KHz signal. I have tried to build a second PCB but the results are exactly the same.



If anyone knows what could be the cause for this I'll be happy to hear.



LTC6241HV Datasheet
enter image description here










share|improve this question




















  • 5




    Wow, 1MEGohm: that's dangerous. Try reducing R1, R3.
    – glen_geek
    Dec 22 at 14:41






  • 3




    Most problematic: The capacitor C6 which gives the loop gain a lowpass characteristic. As a result, additional phase shift which reduces the phase margin - in particular because of unity gain configuration
    – LvW
    Dec 22 at 14:44






  • 3




    If you need high-Z, then add a tiny capacitor (even a few pf) across R1 in parallel. That should help kill oscillation. But be aware that the high-frequency response is affected. An optimum value should allow flat response to about 1 MHz.
    – glen_geek
    Dec 22 at 14:47






  • 2




    If you cannot reduce R3 (at least to 100k, better if even lower) you can shunt R1 with a capacitor, setting say 100kHz or lower bandwidth. Otherwise, you can shunt non inverting input to ground with, say 100kohm or so, reducing loop gain.
    – carloc
    Dec 22 at 14:52






  • 3




    Has anyone asked about the load capacitance for this problem? With any cable you will xx pF/m and the datasheet specifies the series R vs load pF for stability reasons. Why did you choose this device for -1 gain? What is the load pF?
    – Tony EE rocketscientist
    Dec 23 at 3:00














11












11








11







Currently, this is the only assembled part on the circuit board. This is a simple inverting buffer circuit that should be at the input. The op-amp (LTC6241HV) is powered +/-5V from a linear bench power supply. The power pins are bypassed with 0.1uF caps.



I'm inputting a 1KHz sine and on the output I get a ~405KHz sine superimposed on the 1KHz signal. I have tried to build a second PCB but the results are exactly the same.



If anyone knows what could be the cause for this I'll be happy to hear.



LTC6241HV Datasheet
enter image description here










share|improve this question















Currently, this is the only assembled part on the circuit board. This is a simple inverting buffer circuit that should be at the input. The op-amp (LTC6241HV) is powered +/-5V from a linear bench power supply. The power pins are bypassed with 0.1uF caps.



I'm inputting a 1KHz sine and on the output I get a ~405KHz sine superimposed on the 1KHz signal. I have tried to build a second PCB but the results are exactly the same.



If anyone knows what could be the cause for this I'll be happy to hear.



LTC6241HV Datasheet
enter image description here







op-amp buffer oscillation






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited Dec 22 at 14:37

























asked Dec 22 at 14:28









user733606

698




698








  • 5




    Wow, 1MEGohm: that's dangerous. Try reducing R1, R3.
    – glen_geek
    Dec 22 at 14:41






  • 3




    Most problematic: The capacitor C6 which gives the loop gain a lowpass characteristic. As a result, additional phase shift which reduces the phase margin - in particular because of unity gain configuration
    – LvW
    Dec 22 at 14:44






  • 3




    If you need high-Z, then add a tiny capacitor (even a few pf) across R1 in parallel. That should help kill oscillation. But be aware that the high-frequency response is affected. An optimum value should allow flat response to about 1 MHz.
    – glen_geek
    Dec 22 at 14:47






  • 2




    If you cannot reduce R3 (at least to 100k, better if even lower) you can shunt R1 with a capacitor, setting say 100kHz or lower bandwidth. Otherwise, you can shunt non inverting input to ground with, say 100kohm or so, reducing loop gain.
    – carloc
    Dec 22 at 14:52






  • 3




    Has anyone asked about the load capacitance for this problem? With any cable you will xx pF/m and the datasheet specifies the series R vs load pF for stability reasons. Why did you choose this device for -1 gain? What is the load pF?
    – Tony EE rocketscientist
    Dec 23 at 3:00














  • 5




    Wow, 1MEGohm: that's dangerous. Try reducing R1, R3.
    – glen_geek
    Dec 22 at 14:41






  • 3




    Most problematic: The capacitor C6 which gives the loop gain a lowpass characteristic. As a result, additional phase shift which reduces the phase margin - in particular because of unity gain configuration
    – LvW
    Dec 22 at 14:44






  • 3




    If you need high-Z, then add a tiny capacitor (even a few pf) across R1 in parallel. That should help kill oscillation. But be aware that the high-frequency response is affected. An optimum value should allow flat response to about 1 MHz.
    – glen_geek
    Dec 22 at 14:47






  • 2




    If you cannot reduce R3 (at least to 100k, better if even lower) you can shunt R1 with a capacitor, setting say 100kHz or lower bandwidth. Otherwise, you can shunt non inverting input to ground with, say 100kohm or so, reducing loop gain.
    – carloc
    Dec 22 at 14:52






  • 3




    Has anyone asked about the load capacitance for this problem? With any cable you will xx pF/m and the datasheet specifies the series R vs load pF for stability reasons. Why did you choose this device for -1 gain? What is the load pF?
    – Tony EE rocketscientist
    Dec 23 at 3:00








5




5




Wow, 1MEGohm: that's dangerous. Try reducing R1, R3.
– glen_geek
Dec 22 at 14:41




Wow, 1MEGohm: that's dangerous. Try reducing R1, R3.
– glen_geek
Dec 22 at 14:41




3




3




Most problematic: The capacitor C6 which gives the loop gain a lowpass characteristic. As a result, additional phase shift which reduces the phase margin - in particular because of unity gain configuration
– LvW
Dec 22 at 14:44




Most problematic: The capacitor C6 which gives the loop gain a lowpass characteristic. As a result, additional phase shift which reduces the phase margin - in particular because of unity gain configuration
– LvW
Dec 22 at 14:44




3




3




If you need high-Z, then add a tiny capacitor (even a few pf) across R1 in parallel. That should help kill oscillation. But be aware that the high-frequency response is affected. An optimum value should allow flat response to about 1 MHz.
– glen_geek
Dec 22 at 14:47




If you need high-Z, then add a tiny capacitor (even a few pf) across R1 in parallel. That should help kill oscillation. But be aware that the high-frequency response is affected. An optimum value should allow flat response to about 1 MHz.
– glen_geek
Dec 22 at 14:47




2




2




If you cannot reduce R3 (at least to 100k, better if even lower) you can shunt R1 with a capacitor, setting say 100kHz or lower bandwidth. Otherwise, you can shunt non inverting input to ground with, say 100kohm or so, reducing loop gain.
– carloc
Dec 22 at 14:52




If you cannot reduce R3 (at least to 100k, better if even lower) you can shunt R1 with a capacitor, setting say 100kHz or lower bandwidth. Otherwise, you can shunt non inverting input to ground with, say 100kohm or so, reducing loop gain.
– carloc
Dec 22 at 14:52




3




3




Has anyone asked about the load capacitance for this problem? With any cable you will xx pF/m and the datasheet specifies the series R vs load pF for stability reasons. Why did you choose this device for -1 gain? What is the load pF?
– Tony EE rocketscientist
Dec 23 at 3:00




Has anyone asked about the load capacitance for this problem? With any cable you will xx pF/m and the datasheet specifies the series R vs load pF for stability reasons. Why did you choose this device for -1 gain? What is the load pF?
– Tony EE rocketscientist
Dec 23 at 3:00










2 Answers
2






active

oldest

votes


















18














Chip suppliers are keen that their users avoid common design errors, shown by application examples in their data sheets. This one is addressed by Linear Technology in their data sheet for LTC6241. It also applies to many other opamps:




The good noise performance of these op amps can be attributed to large
input devices in the differential pair. Above several hundred
kilohertz, the input capacitance rises and can cause amplifier
stability problems if left unchecked. When the feedback around the op
amp is resistive (RF), a pole will be created with RF, the source
resistance, source capacitance (RS, CS), and the amplifier input
capacitance. In low gain configurations and with RF and RS in even the
kilohm range (Figure 4), this pole can create excess phase shift and
possibly oscillation. A small capacitor CF in parallel with RF
eliminates this problem.






schematic





simulate this circuit – Schematic created using CircuitLab






share|improve this answer





















  • As was suggested by glen_geek, I've added a 15pF cap across R1. At the freq. of oscillation (~400KHz) this has an effective impedance of just over 25KOhm. Paralleled with 1MOhm R1 this figure remains almost unchanged. At that freq. the gain is about -0.025 so high freq. get filtered out. The output is now an inverted sine wave, as was expected. Thank you all for your contributions!
    – user733606
    Dec 22 at 15:15












  • At that freq. the gain is about 0.025 so high freq. get filtered out. Can you explain what you mean by that? I thought the gain of this op-amp is (-1). How did it get to 0.025 and why is it affected by the frequency?
    – Eran
    Dec 22 at 15:36










  • @Eran at 400Khz the 15pF cap has an impedance of about 26.5Kohm and R1 almost does not change that figure so the gain that the op-amp has at that freq. is -26.5K / 1M = -0.0265 which is attenuation at that higher freq.. This is compared with the gain at a lower freq. of say 5KHz where the cap has much higher impedance so the gain of the opamp is closer to -1. This is a typical behavior of a low pass filter.
    – user733606
    Dec 22 at 15:49










  • Right! Even though you wrote that, I didn't think about the impedance of the capacitor and resistor in parallel changing the overall gain of the op-amp - I thought the gain was still (-1) because there are two 1M resistors. Thanks!
    – Eran
    Dec 22 at 16:05






  • 1




    +1 One of the CMOS-input parts I've used a lot has a front end that consists of scores of MOSFETs in parallel, arranged in an X-Y array with half the transistors for each input. That way the variations across the wafer are minimized and Vos is minimized. Neither that nor the consequences (high input capacitance) are disclosed in the datasheet despite being aimed at low power applications where high value feedback resistors are common. So maybe TI isn't as keen as LTC was.
    – Spehro Pefhany
    Dec 22 at 20:26



















1














To balance the circuit, you need a 499K resistor in series with the (+), pin 3, input. It will cancel any offset and possibly solve your oscillation problem.






share|improve this answer























    Your Answer





    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("mathjaxEditing", function () {
    StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
    StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["\$", "\$"]]);
    });
    });
    }, "mathjax-editing");

    StackExchange.ifUsing("editor", function () {
    return StackExchange.using("schematics", function () {
    StackExchange.schematics.init();
    });
    }, "cicuitlab");

    StackExchange.ready(function() {
    var channelOptions = {
    tags: "".split(" "),
    id: "135"
    };
    initTagRenderer("".split(" "), "".split(" "), channelOptions);

    StackExchange.using("externalEditor", function() {
    // Have to fire editor after snippets, if snippets enabled
    if (StackExchange.settings.snippets.snippetsEnabled) {
    StackExchange.using("snippets", function() {
    createEditor();
    });
    }
    else {
    createEditor();
    }
    });

    function createEditor() {
    StackExchange.prepareEditor({
    heartbeatType: 'answer',
    autoActivateHeartbeat: false,
    convertImagesToLinks: false,
    noModals: true,
    showLowRepImageUploadWarning: true,
    reputationToPostImages: null,
    bindNavPrevention: true,
    postfix: "",
    imageUploader: {
    brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
    contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
    allowUrls: true
    },
    onDemand: true,
    discardSelector: ".discard-answer"
    ,immediatelyShowMarkdownHelp:true
    });


    }
    });














    draft saved

    draft discarded


















    StackExchange.ready(
    function () {
    StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f413450%2fthis-op-amp-buffer-is-oscillating-and-i-cant-figure-out-why%23new-answer', 'question_page');
    }
    );

    Post as a guest















    Required, but never shown

























    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    18














    Chip suppliers are keen that their users avoid common design errors, shown by application examples in their data sheets. This one is addressed by Linear Technology in their data sheet for LTC6241. It also applies to many other opamps:




    The good noise performance of these op amps can be attributed to large
    input devices in the differential pair. Above several hundred
    kilohertz, the input capacitance rises and can cause amplifier
    stability problems if left unchecked. When the feedback around the op
    amp is resistive (RF), a pole will be created with RF, the source
    resistance, source capacitance (RS, CS), and the amplifier input
    capacitance. In low gain configurations and with RF and RS in even the
    kilohm range (Figure 4), this pole can create excess phase shift and
    possibly oscillation. A small capacitor CF in parallel with RF
    eliminates this problem.






    schematic





    simulate this circuit – Schematic created using CircuitLab






    share|improve this answer





















    • As was suggested by glen_geek, I've added a 15pF cap across R1. At the freq. of oscillation (~400KHz) this has an effective impedance of just over 25KOhm. Paralleled with 1MOhm R1 this figure remains almost unchanged. At that freq. the gain is about -0.025 so high freq. get filtered out. The output is now an inverted sine wave, as was expected. Thank you all for your contributions!
      – user733606
      Dec 22 at 15:15












    • At that freq. the gain is about 0.025 so high freq. get filtered out. Can you explain what you mean by that? I thought the gain of this op-amp is (-1). How did it get to 0.025 and why is it affected by the frequency?
      – Eran
      Dec 22 at 15:36










    • @Eran at 400Khz the 15pF cap has an impedance of about 26.5Kohm and R1 almost does not change that figure so the gain that the op-amp has at that freq. is -26.5K / 1M = -0.0265 which is attenuation at that higher freq.. This is compared with the gain at a lower freq. of say 5KHz where the cap has much higher impedance so the gain of the opamp is closer to -1. This is a typical behavior of a low pass filter.
      – user733606
      Dec 22 at 15:49










    • Right! Even though you wrote that, I didn't think about the impedance of the capacitor and resistor in parallel changing the overall gain of the op-amp - I thought the gain was still (-1) because there are two 1M resistors. Thanks!
      – Eran
      Dec 22 at 16:05






    • 1




      +1 One of the CMOS-input parts I've used a lot has a front end that consists of scores of MOSFETs in parallel, arranged in an X-Y array with half the transistors for each input. That way the variations across the wafer are minimized and Vos is minimized. Neither that nor the consequences (high input capacitance) are disclosed in the datasheet despite being aimed at low power applications where high value feedback resistors are common. So maybe TI isn't as keen as LTC was.
      – Spehro Pefhany
      Dec 22 at 20:26
















    18














    Chip suppliers are keen that their users avoid common design errors, shown by application examples in their data sheets. This one is addressed by Linear Technology in their data sheet for LTC6241. It also applies to many other opamps:




    The good noise performance of these op amps can be attributed to large
    input devices in the differential pair. Above several hundred
    kilohertz, the input capacitance rises and can cause amplifier
    stability problems if left unchecked. When the feedback around the op
    amp is resistive (RF), a pole will be created with RF, the source
    resistance, source capacitance (RS, CS), and the amplifier input
    capacitance. In low gain configurations and with RF and RS in even the
    kilohm range (Figure 4), this pole can create excess phase shift and
    possibly oscillation. A small capacitor CF in parallel with RF
    eliminates this problem.






    schematic





    simulate this circuit – Schematic created using CircuitLab






    share|improve this answer





















    • As was suggested by glen_geek, I've added a 15pF cap across R1. At the freq. of oscillation (~400KHz) this has an effective impedance of just over 25KOhm. Paralleled with 1MOhm R1 this figure remains almost unchanged. At that freq. the gain is about -0.025 so high freq. get filtered out. The output is now an inverted sine wave, as was expected. Thank you all for your contributions!
      – user733606
      Dec 22 at 15:15












    • At that freq. the gain is about 0.025 so high freq. get filtered out. Can you explain what you mean by that? I thought the gain of this op-amp is (-1). How did it get to 0.025 and why is it affected by the frequency?
      – Eran
      Dec 22 at 15:36










    • @Eran at 400Khz the 15pF cap has an impedance of about 26.5Kohm and R1 almost does not change that figure so the gain that the op-amp has at that freq. is -26.5K / 1M = -0.0265 which is attenuation at that higher freq.. This is compared with the gain at a lower freq. of say 5KHz where the cap has much higher impedance so the gain of the opamp is closer to -1. This is a typical behavior of a low pass filter.
      – user733606
      Dec 22 at 15:49










    • Right! Even though you wrote that, I didn't think about the impedance of the capacitor and resistor in parallel changing the overall gain of the op-amp - I thought the gain was still (-1) because there are two 1M resistors. Thanks!
      – Eran
      Dec 22 at 16:05






    • 1




      +1 One of the CMOS-input parts I've used a lot has a front end that consists of scores of MOSFETs in parallel, arranged in an X-Y array with half the transistors for each input. That way the variations across the wafer are minimized and Vos is minimized. Neither that nor the consequences (high input capacitance) are disclosed in the datasheet despite being aimed at low power applications where high value feedback resistors are common. So maybe TI isn't as keen as LTC was.
      – Spehro Pefhany
      Dec 22 at 20:26














    18












    18








    18






    Chip suppliers are keen that their users avoid common design errors, shown by application examples in their data sheets. This one is addressed by Linear Technology in their data sheet for LTC6241. It also applies to many other opamps:




    The good noise performance of these op amps can be attributed to large
    input devices in the differential pair. Above several hundred
    kilohertz, the input capacitance rises and can cause amplifier
    stability problems if left unchecked. When the feedback around the op
    amp is resistive (RF), a pole will be created with RF, the source
    resistance, source capacitance (RS, CS), and the amplifier input
    capacitance. In low gain configurations and with RF and RS in even the
    kilohm range (Figure 4), this pole can create excess phase shift and
    possibly oscillation. A small capacitor CF in parallel with RF
    eliminates this problem.






    schematic





    simulate this circuit – Schematic created using CircuitLab






    share|improve this answer












    Chip suppliers are keen that their users avoid common design errors, shown by application examples in their data sheets. This one is addressed by Linear Technology in their data sheet for LTC6241. It also applies to many other opamps:




    The good noise performance of these op amps can be attributed to large
    input devices in the differential pair. Above several hundred
    kilohertz, the input capacitance rises and can cause amplifier
    stability problems if left unchecked. When the feedback around the op
    amp is resistive (RF), a pole will be created with RF, the source
    resistance, source capacitance (RS, CS), and the amplifier input
    capacitance. In low gain configurations and with RF and RS in even the
    kilohm range (Figure 4), this pole can create excess phase shift and
    possibly oscillation. A small capacitor CF in parallel with RF
    eliminates this problem.






    schematic





    simulate this circuit – Schematic created using CircuitLab







    share|improve this answer












    share|improve this answer



    share|improve this answer










    answered Dec 22 at 15:36









    glen_geek

    8,9031916




    8,9031916












    • As was suggested by glen_geek, I've added a 15pF cap across R1. At the freq. of oscillation (~400KHz) this has an effective impedance of just over 25KOhm. Paralleled with 1MOhm R1 this figure remains almost unchanged. At that freq. the gain is about -0.025 so high freq. get filtered out. The output is now an inverted sine wave, as was expected. Thank you all for your contributions!
      – user733606
      Dec 22 at 15:15












    • At that freq. the gain is about 0.025 so high freq. get filtered out. Can you explain what you mean by that? I thought the gain of this op-amp is (-1). How did it get to 0.025 and why is it affected by the frequency?
      – Eran
      Dec 22 at 15:36










    • @Eran at 400Khz the 15pF cap has an impedance of about 26.5Kohm and R1 almost does not change that figure so the gain that the op-amp has at that freq. is -26.5K / 1M = -0.0265 which is attenuation at that higher freq.. This is compared with the gain at a lower freq. of say 5KHz where the cap has much higher impedance so the gain of the opamp is closer to -1. This is a typical behavior of a low pass filter.
      – user733606
      Dec 22 at 15:49










    • Right! Even though you wrote that, I didn't think about the impedance of the capacitor and resistor in parallel changing the overall gain of the op-amp - I thought the gain was still (-1) because there are two 1M resistors. Thanks!
      – Eran
      Dec 22 at 16:05






    • 1




      +1 One of the CMOS-input parts I've used a lot has a front end that consists of scores of MOSFETs in parallel, arranged in an X-Y array with half the transistors for each input. That way the variations across the wafer are minimized and Vos is minimized. Neither that nor the consequences (high input capacitance) are disclosed in the datasheet despite being aimed at low power applications where high value feedback resistors are common. So maybe TI isn't as keen as LTC was.
      – Spehro Pefhany
      Dec 22 at 20:26


















    • As was suggested by glen_geek, I've added a 15pF cap across R1. At the freq. of oscillation (~400KHz) this has an effective impedance of just over 25KOhm. Paralleled with 1MOhm R1 this figure remains almost unchanged. At that freq. the gain is about -0.025 so high freq. get filtered out. The output is now an inverted sine wave, as was expected. Thank you all for your contributions!
      – user733606
      Dec 22 at 15:15












    • At that freq. the gain is about 0.025 so high freq. get filtered out. Can you explain what you mean by that? I thought the gain of this op-amp is (-1). How did it get to 0.025 and why is it affected by the frequency?
      – Eran
      Dec 22 at 15:36










    • @Eran at 400Khz the 15pF cap has an impedance of about 26.5Kohm and R1 almost does not change that figure so the gain that the op-amp has at that freq. is -26.5K / 1M = -0.0265 which is attenuation at that higher freq.. This is compared with the gain at a lower freq. of say 5KHz where the cap has much higher impedance so the gain of the opamp is closer to -1. This is a typical behavior of a low pass filter.
      – user733606
      Dec 22 at 15:49










    • Right! Even though you wrote that, I didn't think about the impedance of the capacitor and resistor in parallel changing the overall gain of the op-amp - I thought the gain was still (-1) because there are two 1M resistors. Thanks!
      – Eran
      Dec 22 at 16:05






    • 1




      +1 One of the CMOS-input parts I've used a lot has a front end that consists of scores of MOSFETs in parallel, arranged in an X-Y array with half the transistors for each input. That way the variations across the wafer are minimized and Vos is minimized. Neither that nor the consequences (high input capacitance) are disclosed in the datasheet despite being aimed at low power applications where high value feedback resistors are common. So maybe TI isn't as keen as LTC was.
      – Spehro Pefhany
      Dec 22 at 20:26
















    As was suggested by glen_geek, I've added a 15pF cap across R1. At the freq. of oscillation (~400KHz) this has an effective impedance of just over 25KOhm. Paralleled with 1MOhm R1 this figure remains almost unchanged. At that freq. the gain is about -0.025 so high freq. get filtered out. The output is now an inverted sine wave, as was expected. Thank you all for your contributions!
    – user733606
    Dec 22 at 15:15






    As was suggested by glen_geek, I've added a 15pF cap across R1. At the freq. of oscillation (~400KHz) this has an effective impedance of just over 25KOhm. Paralleled with 1MOhm R1 this figure remains almost unchanged. At that freq. the gain is about -0.025 so high freq. get filtered out. The output is now an inverted sine wave, as was expected. Thank you all for your contributions!
    – user733606
    Dec 22 at 15:15














    At that freq. the gain is about 0.025 so high freq. get filtered out. Can you explain what you mean by that? I thought the gain of this op-amp is (-1). How did it get to 0.025 and why is it affected by the frequency?
    – Eran
    Dec 22 at 15:36




    At that freq. the gain is about 0.025 so high freq. get filtered out. Can you explain what you mean by that? I thought the gain of this op-amp is (-1). How did it get to 0.025 and why is it affected by the frequency?
    – Eran
    Dec 22 at 15:36












    @Eran at 400Khz the 15pF cap has an impedance of about 26.5Kohm and R1 almost does not change that figure so the gain that the op-amp has at that freq. is -26.5K / 1M = -0.0265 which is attenuation at that higher freq.. This is compared with the gain at a lower freq. of say 5KHz where the cap has much higher impedance so the gain of the opamp is closer to -1. This is a typical behavior of a low pass filter.
    – user733606
    Dec 22 at 15:49




    @Eran at 400Khz the 15pF cap has an impedance of about 26.5Kohm and R1 almost does not change that figure so the gain that the op-amp has at that freq. is -26.5K / 1M = -0.0265 which is attenuation at that higher freq.. This is compared with the gain at a lower freq. of say 5KHz where the cap has much higher impedance so the gain of the opamp is closer to -1. This is a typical behavior of a low pass filter.
    – user733606
    Dec 22 at 15:49












    Right! Even though you wrote that, I didn't think about the impedance of the capacitor and resistor in parallel changing the overall gain of the op-amp - I thought the gain was still (-1) because there are two 1M resistors. Thanks!
    – Eran
    Dec 22 at 16:05




    Right! Even though you wrote that, I didn't think about the impedance of the capacitor and resistor in parallel changing the overall gain of the op-amp - I thought the gain was still (-1) because there are two 1M resistors. Thanks!
    – Eran
    Dec 22 at 16:05




    1




    1




    +1 One of the CMOS-input parts I've used a lot has a front end that consists of scores of MOSFETs in parallel, arranged in an X-Y array with half the transistors for each input. That way the variations across the wafer are minimized and Vos is minimized. Neither that nor the consequences (high input capacitance) are disclosed in the datasheet despite being aimed at low power applications where high value feedback resistors are common. So maybe TI isn't as keen as LTC was.
    – Spehro Pefhany
    Dec 22 at 20:26




    +1 One of the CMOS-input parts I've used a lot has a front end that consists of scores of MOSFETs in parallel, arranged in an X-Y array with half the transistors for each input. That way the variations across the wafer are minimized and Vos is minimized. Neither that nor the consequences (high input capacitance) are disclosed in the datasheet despite being aimed at low power applications where high value feedback resistors are common. So maybe TI isn't as keen as LTC was.
    – Spehro Pefhany
    Dec 22 at 20:26













    1














    To balance the circuit, you need a 499K resistor in series with the (+), pin 3, input. It will cancel any offset and possibly solve your oscillation problem.






    share|improve this answer




























      1














      To balance the circuit, you need a 499K resistor in series with the (+), pin 3, input. It will cancel any offset and possibly solve your oscillation problem.






      share|improve this answer


























        1












        1








        1






        To balance the circuit, you need a 499K resistor in series with the (+), pin 3, input. It will cancel any offset and possibly solve your oscillation problem.






        share|improve this answer














        To balance the circuit, you need a 499K resistor in series with the (+), pin 3, input. It will cancel any offset and possibly solve your oscillation problem.







        share|improve this answer














        share|improve this answer



        share|improve this answer








        answered Dec 23 at 14:01


























        community wiki





        Dan_LXI































            draft saved

            draft discarded




















































            Thanks for contributing an answer to Electrical Engineering Stack Exchange!


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            Use MathJax to format equations. MathJax reference.


            To learn more, see our tips on writing great answers.





            Some of your past answers have not been well-received, and you're in danger of being blocked from answering.


            Please pay close attention to the following guidance:


            • Please be sure to answer the question. Provide details and share your research!

            But avoid



            • Asking for help, clarification, or responding to other answers.

            • Making statements based on opinion; back them up with references or personal experience.


            To learn more, see our tips on writing great answers.




            draft saved


            draft discarded














            StackExchange.ready(
            function () {
            StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f413450%2fthis-op-amp-buffer-is-oscillating-and-i-cant-figure-out-why%23new-answer', 'question_page');
            }
            );

            Post as a guest















            Required, but never shown





















































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown

































            Required, but never shown














            Required, but never shown












            Required, but never shown







            Required, but never shown







            Popular posts from this blog

            數位音樂下載

            When can things happen in Etherscan, such as the picture below?

            格利澤436b