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Composite amplifier using ADA4637-1 and ADA4870-1

I want to build a composite amplifier using the Figure 77 of the datasheet ADA4870-1. It says that the voltage offset is less than 500 uV. I simulate this circuit using LTSpice, when driving ada4637-1 with 0V and monitor the output of ADA4870-1, it is actually 5mV. And the output of the first stage amplifier is ~25 mV.

1. What's wrong with this simulation?

2. Can I achieve better temperature drift (offset and gain) using this composite amplifier architecture?

3. Can I replace ADA4637 with LT6018 or AD8275 for better DC accuracy?


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[edited by: byliu at 1:18 PM (GMT 0) on 15 May 2019]
  • Hi byliu,

    1. Currently, ADA4870 spice model is not properly working with LTspice. The fix is on-going. The offset should be <500 uV.

    2. Yes, the offset and gain drift will be highly dependent on the major loop. The drift of the composite amplifier will be dominated by RF//RG (R5//R6) and the front-end amp (ADA4637). So pick the amplifier with the best gain and offset drift performance.

    3. Yes. Just make sure that the combination is stable and the op-amp on the front-end is in open-loop.

    I hope this helps.



  • 2. Is the front-end amp in a de-compensate state? i.e., the minimal gain of front-end amp should be at least 5 or more. Do I need a small capacitor between Vout of ADA4637 and the inverting pin?

    3. I would like to use AD8675 as the front-end op-amp, where AD8675 was configured with gain=1. Will AD8675 + ADA4870 achieve less temperature drift plus less dc offset?