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  • operational_amplifier
  • opamp
  • oscillation
  • stability
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Operational amplifier stability question

Former Member
Former Member over 14 years ago

I'm designing a portable biological signal acquisition system for my licence project and while designing the mixed signal board, a potential problem came in my mind: is it stable ?

I'm using an opamp (OPA350) as a buffer for the reference voltage (I'm using the reference voltage as a virtual ground). For every instrumentation amplifier I connected a capacitor between 5V (the positive supply rail) and reference, and one between analog ground (the negative supply rail with respect to the virtual ground). Is this circuit prone to oscillation because of these capacitors ?

 

Thanks !

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  • lpetacchi
    0 lpetacchi over 14 years ago

    Just connect capacitors to AGND (get rid of C31..34).

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  • Former Member
    0 Former Member over 14 years ago in reply to lpetacchi

    I have connected the capacitors between +5VA and AGND but I am concerned about RF interference that might be picked up by the reference voltage distribution line (it is about 60 mm long, as it can be seen from the attached file).

    This application is a highly sensitive one, since I will be working with EEG signals as small as 100uV, that can be easily contaminated by electromagnetic interference.

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  • lpetacchi
    0 lpetacchi over 14 years ago in reply to Former Member

    Use small series resistors and bypass to AGND with caps near INA333AIDGKRINA333AIDGKR pin. You don't need to buffer the reference, if it is connected only to these pins.

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  • Former Member
    0 Former Member over 14 years ago in reply to lpetacchi

    I'll leave some space on the PCB for series resistors and I'll experiment both with them and without them. Thank you for the suggestion.

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  • michaelwylie
    0 michaelwylie over 8 years ago in reply to Former Member

    If you are really concerned you can make the board 4 layers and keep sensitive lines between 2 ground planes, essentially shielding the trace.

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  • dougw
    0 dougw over 8 years ago

    The ref opamp could be quite unstable with this capacitance load.

    image

    A small resistor on the output (in series) of the opamp is a common way to improve stability with high capacitance loads.

    A resistive load can also help.

    Here are some other methods:

    http://www.analog.com/library/analogdialogue/archives/38-06/capacitive_loading.pdf

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