REF problem of INA155UA

Hello,

Can you please provide a schematic?  I am interested in the supply voltages and input common-mode voltage (Vcm) because I believe that you are violating the Vcm range of the device.  Here are links to a few articles that may help in the meantime:

http://www.edn.com/design/analog/4437848/2/Instrumentation-amplifier-VCM-vs-VOUT-plots--Part-1

http://www.edn.com/design/analog/4437922/Instrumentation-amplifier-VCM-vs--VOUT-plots--part-2

http://www.edn.com/design/analog/4438001/Instrumentation-amplifier-VCM-vs--VOUT-plots--Part-3

Hello:

       I use it to test PD current. Below is the schematic. For the REF, in order to decrease the input resistance,  I have use REF 3112 to instead the design of below now. But it still not works.

Hello,

Here are some suggestions and observations:

1) The common-mode voltage of your input is near ground (pin 2 is connected to ground).
2) Using a single 5V supply, the common mode range of the device (listed in the data sheet) is 300mV<Vcm<5.2V (assuming Vs=5.5V).
3) Therefore you are violating the input common-mode voltage specification. I suggest you supply the device with a dual supply (+/-2.5V).
4) Note that the output swing depends on Vcm and Vref, as shown by the input common-mode voltage vs. Vref and Vout graphs on page 8 of the data sheet.
5) The reference pin should be driven by a low impedance connection (not a voltage divider) as discussed in the Applications Information section in the data sheet. You should buffer the voltage with an op amp or, as you already stated, use a device like the REF3112.

Finally, I'm concerned about the magnitude of the voltage your trying to measure (which is the product of your sense resistor and PD current). You must ensure that this voltage is not dominated by either the input bias current of the instrumentation amplifier or the device's offset voltage. For example, the INA155 maximum offset voltage is +/-1mV and the input bias current is +/-10pA.