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?
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?