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OPA2188: Which one is the true value when test the input bias current?

xiang tao
Prodigy 100 points

Part Number: OPA2188

Tool/software:

Hi, TI's Masters:

I'm trying to test input current of OPA2188, the test topology is shown below:

I use a  portal multi-meter like FLUKE 289C to measure the voltages of A and B point, then divide the value by 10K, then get the input current of OPA2188.

but here comes a strange phenomenon：

case 1: the black probe contact BAT-, and red probe to A and B respectively, then change the input voltage from 100V to 800V, 100V step, the measure data is as followed below:

input voltage/V voltage A/V voltage B/V
100 0.2005 0.2005
200 0.3871 0.3868
300 0.5737 0.5732
400 0.7604 0.7597
500 0.9476 0.9468
600 1.1342 1.1331
700 1.3215 1.3203
800 1.5090 1.5075

I don't understand why the voltage difference between A and  B become more and more with input voltage rising.

and the V(A-B)=1.5mV at 800V input, so the current across 10K is nearly 150nA, which is ten times more than datasheet.

case 2: I tried to measure A and B immediately, black probe to A and red probe to B, this time the V(A-B) is always nearly 10uV regardless of Vin, less than case 1 much a lot.

So which case is the correct measurement method? and why?

thanks a lot!