Driving AD9240

Our application is driving the AD9240 14 bit 10MHz A/D.This is an instrumentation application so settling time, low noise and low drift are the more critical parameter. The goal is to have switchable gains up to 1000x using multiple stages. The input could be a square wave or pulse mixed with random noise.

Using the parametric search the ADA4895-1 and/or AD8021 appear to be the top choices with limits of 3nV/rtHz, GBP > 130M, The AD8099 and AD829 met those but the VosTC was 10x or more higher than the others.

Suggestions and comments welcome.

.

Note: The AD9240 was selected for compatibility with the test platform. Ultimately one of the newer serial A/Ds will be used, 20MHz or faster so an amplifier chain that can support the higher speeds is of interest.

Scott

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  • Scott,

      In the 1970's in the days of 741s, input bias current was in the microamp range.  So we were

    taught that we should match impedances on the inputs of op amps to reduce the I*R error

    ti IOS*R.  This is not a law that needs to be blindly followed.  I see customers using CMOS op amps

    and JFET op amps with picoamps of bias current still matching impedances.  But 5pA times 1 meg is

    5 uV, so you can leave the resistor out.  Saw an AD8630 with 470k in the feedback of a follower and

    it oscillated at 100 kHz, (112 kHz predicted)

    The offset adjust is a manual adjust, so whether the op amp has 10 mV of offset, or Ibias*R =10 mV,

    it doesn't matter.  You will just move the pot farther.

    So now it's reversed;  don't put the resistor in unless you can justify it.

    For any op amp w/ a gbw >50-100 MHz, you have to have a very clean pcb;  four layer min, bypass caps

    very close to the op amp, all traces/copper removed on all layers underneath the op amp inputs, etc.

    Harry

Reply
  • Scott,

      In the 1970's in the days of 741s, input bias current was in the microamp range.  So we were

    taught that we should match impedances on the inputs of op amps to reduce the I*R error

    ti IOS*R.  This is not a law that needs to be blindly followed.  I see customers using CMOS op amps

    and JFET op amps with picoamps of bias current still matching impedances.  But 5pA times 1 meg is

    5 uV, so you can leave the resistor out.  Saw an AD8630 with 470k in the feedback of a follower and

    it oscillated at 100 kHz, (112 kHz predicted)

    The offset adjust is a manual adjust, so whether the op amp has 10 mV of offset, or Ibias*R =10 mV,

    it doesn't matter.  You will just move the pot farther.

    So now it's reversed;  don't put the resistor in unless you can justify it.

    For any op amp w/ a gbw >50-100 MHz, you have to have a very clean pcb;  four layer min, bypass caps

    very close to the op amp, all traces/copper removed on all layers underneath the op amp inputs, etc.

    Harry

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