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TLV170: OP TLV170IDBVR input add capacitor

Red Yen
Expert 4310 points
Part Number: TLV170

Hi Support team 

Due to the large ripple noise at output, if a 1000Pf capacitor is added between the input V + V-, the noise can be significantly suppressed.
What is the reason that can explain this solution ??
In addition, any side effect if adding 1000Pf to the input ? Will it cause the circuit or IC to be unstable?

  • 0
    Guru 140200 points

    Hi Red,

    I wouldn't do that because this can erode the phase margin of OPAmp and cause instability. Instead, I would modify the circuit like shown below, provided that the noise and ripple is coming from the inputs of differential amplifier. R5 and C3 could be increased. Also, R2 could be split into two resistors R6 and R9 and a suited low pass filtering cap C5 can be added then.

    Another issue is the signal routing arround the OPAmp. All the signal routing especially at the -input of OPAmp should be kept as short as possible.

    Kai

  • 0
    TI__Genius 15410 points

    Hi Red,

    Kai is correct. Typically a capacitor across the input pins is not recommended because it can degrade the phase margin. 

    It is important to find the source of your noise before making modifications to the circuit. Do you know where the excessive noise is coming from?

    Can you provide scope shots of before and after the 1000pF cap across the input pins?

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    TI__Expert 4310 points

    Hi Tim & Kai 

    thanks for valuable inputs, 

    may i have recommend value of filtering cap C5? 

  • 0 in reply to Red Yen
    Guru 140200 points

    Hi Red,

    hard to say without knowing the complete circuit :-)

    Maybe arround 1nF?

    Kai

  • 0 in reply to Red Yen
    TI__Genius 15410 points

    Hi Red,

    What frequency range is the customer interested in using? As Kai mentioned, we can't just arbitrarily recommend a component value without know anything about the circuit design requirements.

    Thank you,

    Tim Claycomb