Circuit for small level voltage detection (microvolts)












5












$begingroup$


I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.enter image description here



The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layoutsenter image description here



enter image description here



Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.






low-power medical







share|improve this question









$endgroup$












  • $begingroup$
    First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
    $endgroup$
    – JRE
    18 hours ago






  • 2




    $begingroup$
    And, I think you are going to need a solid ground plane underneath your whole circuit.
    $endgroup$
    – JRE
    18 hours ago










  • $begingroup$
    The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
    $endgroup$
    – DodZi
    17 hours ago










  • $begingroup$
    The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
    $endgroup$
    – JRE
    15 hours ago










  • $begingroup$
    Did you check the impulse response of the filter?
    $endgroup$
    – Peter Smith
    13 hours ago
















5












$begingroup$


I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.enter image description here



The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layoutsenter image description here



enter image description here



Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.






low-power medical







share|improve this question









$endgroup$












  • $begingroup$
    First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
    $endgroup$
    – JRE
    18 hours ago






  • 2




    $begingroup$
    And, I think you are going to need a solid ground plane underneath your whole circuit.
    $endgroup$
    – JRE
    18 hours ago










  • $begingroup$
    The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
    $endgroup$
    – DodZi
    17 hours ago










  • $begingroup$
    The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
    $endgroup$
    – JRE
    15 hours ago










  • $begingroup$
    Did you check the impulse response of the filter?
    $endgroup$
    – Peter Smith
    13 hours ago














5












5








5


1



$begingroup$


I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.enter image description here



The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layoutsenter image description here



enter image description here



Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.






low-power medical







share|improve this question









$endgroup$




I have been working on a project lately which help in the detection myogenic potentials (of the levels of micro-volts).
After studying from several places, I finalized the following test circuit.enter image description here



The initial voltage dividers helps in generating the micro-level test signal from 1V 1.5khz sine wave. Second stage is the instrumentation amplifier, next the second order low pass filter and lastly the non-inverting amplifier.
When this circuit is simulated in TINA from TI, it works as required. But when the same is implemented on the PCB, I am not getting anything close to the Input but a 150-180kHz wave, somewhat in triangular shape.
following are the sch and board layoutsenter image description here



enter image description here



Can anyone suggest how can I go ahead since such small voltages are already very difficult to measure. Thanks in advance.






low-power medical




low-power medical






share|improve this question













share|improve this question











share|improve this question




share|improve this question










asked 18 hours ago









DodZiDodZi

262




262












  • $begingroup$
    First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
    $endgroup$
    – JRE
    18 hours ago






  • 2




    $begingroup$
    And, I think you are going to need a solid ground plane underneath your whole circuit.
    $endgroup$
    – JRE
    18 hours ago










  • $begingroup$
    The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
    $endgroup$
    – DodZi
    17 hours ago










  • $begingroup$
    The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
    $endgroup$
    – JRE
    15 hours ago










  • $begingroup$
    Did you check the impulse response of the filter?
    $endgroup$
    – Peter Smith
    13 hours ago


















  • $begingroup$
    First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
    $endgroup$
    – JRE
    18 hours ago






  • 2




    $begingroup$
    And, I think you are going to need a solid ground plane underneath your whole circuit.
    $endgroup$
    – JRE
    18 hours ago










  • $begingroup$
    The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
    $endgroup$
    – DodZi
    17 hours ago










  • $begingroup$
    The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
    $endgroup$
    – JRE
    15 hours ago










  • $begingroup$
    Did you check the impulse response of the filter?
    $endgroup$
    – Peter Smith
    13 hours ago
















$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
18 hours ago




$begingroup$
First step: break it down. Where does the "triangle wave" first appear (output of which amplifier?)
$endgroup$
– JRE
18 hours ago




2




2




$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
18 hours ago




$begingroup$
And, I think you are going to need a solid ground plane underneath your whole circuit.
$endgroup$
– JRE
18 hours ago












$begingroup$
The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
17 hours ago




$begingroup$
The first triangle is observed at he output of filter circuit. Also, I am guessing that before that, very small signal might be there which the scope is not able to show (at-least my one)
$endgroup$
– DodZi
17 hours ago












$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
15 hours ago




$begingroup$
The filter shouldn't have enough gain (amplification) to pick up something totally hidden and amplify to the point you can see it.
$endgroup$
– JRE
15 hours ago












$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
13 hours ago




$begingroup$
Did you check the impulse response of the filter?
$endgroup$
– Peter Smith
13 hours ago










3 Answers
3






active

oldest

votes


















7












$begingroup$

The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.



If you need that much gain, you need to do it elsewhere.






share|improve this answer









$endgroup$





















    6












    $begingroup$

    Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.



    The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.



    Reduce the gain in that stage to at most 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.






    share|improve this answer











    $endgroup$





















      2












      $begingroup$

      The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.






      share|improve this answer









      $endgroup$













        Your Answer





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        3 Answers
        3






        active

        oldest

        votes








        3 Answers
        3






        active

        oldest

        votes









        active

        oldest

        votes






        active

        oldest

        votes









        7












        $begingroup$

        The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.



        If you need that much gain, you need to do it elsewhere.






        share|improve this answer









        $endgroup$


















          7












          $begingroup$

          The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.



          If you need that much gain, you need to do it elsewhere.






          share|improve this answer









          $endgroup$
















            7












            7








            7





            $begingroup$

            The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.



            If you need that much gain, you need to do it elsewhere.






            share|improve this answer









            $endgroup$



            The opamp in a Sallen-Key filter is supposed to be a unity-gain buffer. Yours has a gain of +3, so it isn't surprising that it's oscillating. Wikipedia talks about this.



            If you need that much gain, you need to do it elsewhere.







            share|improve this answer












            share|improve this answer



            share|improve this answer










            answered 13 hours ago









            Dave Tweed♦Dave Tweed

            118k9145256




            118k9145256

























                6












                $begingroup$

                Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.



                The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.



                Reduce the gain in that stage to at most 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.






                share|improve this answer











                $endgroup$


















                  6












                  $begingroup$

                  Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.



                  The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.



                  Reduce the gain in that stage to at most 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.






                  share|improve this answer











                  $endgroup$
















                    6












                    6








                    6





                    $begingroup$

                    Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.



                    The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.



                    Reduce the gain in that stage to at most 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.






                    share|improve this answer











                    $endgroup$



                    Although it is possible to design a Sallen Key filter with a gain higher than unity, this is rather uncommon for a reason. Any gain in it introduces positive feedback into the structure and leads it towards instability. Particularly when you take the amplifiers own poles into consideration.



                    The OPA177 has a gain bandwidth of ~600kHz, at a gain of 3 you have an unaccounted for pole at ~200kHz in your Salen-Key stage, pretty close to the frequency of oscillation that you are observing.



                    Reduce the gain in that stage to at most 1.5 and recalculate your filter elements. You can start by removing R8 (thus setting the gain to unity) and test what you get.







                    share|improve this answer














                    share|improve this answer



                    share|improve this answer








                    edited 8 hours ago









                    Dave Tweed♦

                    118k9145256




                    118k9145256










                    answered 11 hours ago









                    Edgar BrownEdgar Brown

                    3,794525




                    3,794525























                        2












                        $begingroup$

                        The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.






                        share|improve this answer









                        $endgroup$


















                          2












                          $begingroup$

                          The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.






                          share|improve this answer









                          $endgroup$
















                            2












                            2








                            2





                            $begingroup$

                            The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.






                            share|improve this answer









                            $endgroup$



                            The fact that your circuit is oscillating at such a high frequency suggests very strongly that you have ground/decoupling issues. JRE commented that you need a solid ground plane, and I agree. Admittedly, this means you'll need to get creative about routing -Vcc. Additionally, your schematics do not include the decoupling caps which have clearly used. Please update to show what you have actually used.







                            share|improve this answer












                            share|improve this answer



                            share|improve this answer










                            answered 11 hours ago









                            WhatRoughBeastWhatRoughBeast

                            49.4k22875




                            49.4k22875






























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