I'm working with Microchip MCP6N16-001 instrumentation amplifier to amplify a signal of 0-1.5V with high input impedance by a factor of two. The configuration is very basic, as per datasheet typical circuits. Picture of the schematic below.
The problem is the amplifier seems to have a way lower differential input impedance (Everything is DC, so call it resistance) then expected - datasheets specifies 10^13 Ohms (table 1-1) and input bias / offset currents in the range of a couple 100pA. What I'm measuring is about 3.3Mohm and a current into the inputs around 0.2uA, depending on the input voltage. That's about 1000x discrepancy, looking at currents!
How I'm measuring this? I supply 1V from an (isolated / floating in relation to GND and VDD) adjustable power supply through a 1M resistor into the input pins. The output in this case is at about 1.4V instead of the expected 2V. Shorting the 1M resistor, output jumps to the expected 2V. I've done the input impedance measurement a few different ways, such as measuring the voltage drop on the 1M resistor (taking into account the multimeters 10M internal impedance) and calculating from there. I've also done some measurements with a SMU, looking at the differential input current. The measured input impedance always seems to end up at 3.3Mohm.
I've also done a common mode input impedance measurements (see schematic) which comes out as expected - I'm measuring about 160pA, using multimeter's 10M impedance as a shunt at about 3V of common mode voltage. Not sure how reliable this measurement is, but seems to be in the right range. The common mode leakage is low throughout the whole range, until the protection diodes kick in, above the VDD.
I'm measuring the same differential input impedance (within a few %) on all 6 channels on my board. I've also soldered up a dead-bug style adapter to test a fresh, never reflowed amplifier on a breadboard with minimal external components (see schematic note) - exactly the same results.
Flux residue leakage and esd / overheat damage come to mind but damaging 7 different ICs to have exatly the same differential input impedance? With one IC being soldered in a totaly different way? I don't think so.
IC's are from Mouser, so they should be legit.
I've opened a Support case at Microchip about this - their latest response (after about three weeks of waiting...) was that their internal team has not yet had the time to check this out, but the support guy I'm talking to did some tests and got similar results to mine. Their recomendation is to "work around the experimentally determined value".
Any ideas what's going on?