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LMC6081 and Unity Gain

Luke Beardslee
Prodigy 60 points
Other Parts Discussed in Thread: LMC6081, LMC6061, LMC6041, LMC6442, TLV2461

Hi,

            I've been testing a photodiode integrator circuit with some different op-amps. The circuit integrates the charge from the photodiode for a period of time and then the feedback capacitor is reset using a relay. I'm having an issue where during the reset the output voltage oscillates. Is the LMC6081 designed for unity gain stability? One thing I was thinking of trying would be to include a larger resistor (currenly 8 ohm) in the discharge circuit to see if this increases the gain during the capacitor reset. Any other suggestions on where this issue could be coming from?

I'm considering using the LMC6041 or LMC6061 because both of these have low input referred current noise and also have a lower GBW than the LM6081. My circuit operates near DC so I think that using a lower bandwidth op-amp could help from a noise/interference perspective. Does anyone have any experience using either of these in an integrator/photodiode circuit. Can anyone suggest any other op-amps that might work well for this application? Thanks!

  • 0
    TI__Mastermind 49926 points
    Hi Luke,

    Yes. The LMC6xxx series, except for the LMC6442, are all unity gain stable.

    However, they are sensitive to capacitive loading on the output - as noted in the datasheet Apps section and cap load graphs. It should be able to drive up to 50pF easily, but after 100pF, it may oscillate. Be sure to include the capacitance of any output load (any cables? Filters?). Usually a resistor is added in series with the output to isolate a capacitive load (~200-500 ohms). The follower configuration is the most unstable configuration - which does not help you!

    When the reset relay directly shorts the -IN to the output, this can place any input capacitance directly on the output. Photodiodes can have a large amount of capacitance - depending on the diode area. It could also be on the edge of oscillation due to output loading, and the reset just pushes it over the edge. We would need to see your circuit to determine any issues.

    You can try placing a resistor (~200 ohms) in series with the integrating capacitor. This will isolate the capacitance from the output during reset. Place the resistor on the output side of the cap/relay.

    Note that the lower power devices will have higher voltage noise - so you may want to see if saving a fractions of a pA of noise current is worth the increase in the voltage noise.

    Regards,
  • 0 in reply to Paul Grohe
    Prodigy 60 points
    Hi Paul,
    Thanks for your response. The capacitance across the terminals of the photodiode is 700 pF, so looking at the stability vs. capacitance curves in the datasheet this may in fact be the issue that I'm having.

    Can you suggest a non-low power op-amp with similar current noise, but lower voltage noise to the LMC6081? Thanks!

    -Luke
  • 0 in reply to Luke Beardslee
    TI__Mastermind 49926 points

    Hi Luke,

    Yeah..that 700pF could be the issue. Try adding the series resistor.

    From what I see, the TLV2461 has similar current noise and lower voltage noise (11nV vs 22nV).

    Since you stated this is a near-DC app, then the low frequency noise is important. Be sure to compare the noise graphs at 1-10Hz, and not just the front page noise number (which is usually at the 1KHz "flatband" freq). Where the 1/f corner resides can change the values of the 1 and 10Hz numbers between devices.

    Regards,