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OPA335: input cap requirement for CMRR improvement

Chi Zhang2
Intellectual 1700 points
Part Number: OPA335

Hi Expert, 

my customer is using OPA335, to improve the CMRR, they want to put a cap between in+ and in -, so do we have any limitation for this cap value. because based on previous experience, large cap will make amp has self-oscillating. look forward to your comments. 

BR
Chi
  • 0
    TI__Guru 60610 points

    Your customer may not improve CMRR above what the OPA335 datasheet graph on page 5 shows.  Also, the worst thing one may do from stability point of you is to add ANY cap between the input terminals. If you need further help how to prevent deterioration of CMRR, please provide detailed circuit schematic.

  • 0 in reply to Marek Lis
    TI__Intellectual 1700 points

    Hi Marek,

    here is the schematic for OPA335, how to better improve the CMRR, do we need to remove the 10nf cap between in+ and in-?

    BR

    Chi

  • +1 in reply to Chi Zhang2
    TI__Genius 17797 points

    Chi,

    1. Placing a capacitor between the inputs of the amplifier is not a good idea.  It typically leads to stability problems:  Intro to stability.  When a amplifier is not stable that means it oscillates.  Common mode rejection for the OPA335 is really very good (130dB means that common mode signals are reduced by a factor of 3,162,000).  Furthermore, your input signal is single ended and not common mode (i.e. applied to both inputs). 
    2. I suspect your objective is to minimize noise pickup.  The input filter (1k x 0.1uF), is a good way to do that.  I suggest you remove your 10nF capacitor.
    3. Stability of amplifiers is also impacted by output capacitive loading.  In your case you have 0.1uF connected via a 20 ohm resistor.  The 20 ohm resistor will help improve stability but it is not enough.  Below is a simulation of your circuit with the 0.1uF load.  Notice it oscillates.  Changing the isolation resistor to 1k (as opposed to 20 ohm) improves the issue.  Also attached is the  TINA SPICE simulation file.

    opa335 stability.TSC

    I hope this helps.

    Art

  • 0 in reply to Chi Zhang2
    TI__Guru 60610 points

    Adding to Art's comments, in the case of diff amp the 20ohm series output resistor is likewise NOT sufficient to stabilize the circuit - see below 83% overshoot whereas the maximum 25% is recommended.

    But adding just 100ohm series output resistor stabilizes the circuit with the overshoot of around 17% - see below.

    opa335 stability ML.TSC

  • 0 in reply to Marek Lis
    TI__Intellectual 1700 points

    Hi Art and Marek,

    thanks for your so detailed simulation and explanation. 

    customer also do some verification on board, they find placing a capacitor between the inputs of the amplifier didn't show any osc, although in my perspective, it may be a critical state or a fake stable status, which any external parameter will make ringing or large overshooting. I want to double check with you. 

    1. any cap added between the inputs of the amplifier to improve the noise is not recommended, am I correct? because disturb feedback network of the amplifier and make ringring happen in more possibility? 

    2. if we could allow to add a small value of cap without any ringing risk, which is the limitation? <100pF

    BR

    Chi

  • 0 in reply to Chi Zhang2
    TI__Genius 17797 points

    Chi,

    1. I agree that you do not want to place a capacitor between the inputs to reduce noise.  The 0.1uF on the non-inverting input is the recommended way of filtering an amplifier
    2. I would not recommend adding a a capacitor between the inputs.  However, if you did, the important consideration is to balance the input capacitance with a feedback capacitor (Rf x Cf = Rin x Cin).  Nevertheless, I would not recommend adding the capacitor especially a 10nF.
    3. Note: stability issues do not always show sustained oscillations.  Sometimes, you just see very poor settling.  This settling can be impacted by load transient as well as input transients.

    Best regards,

    Art