Variable gain amplifier with INA128+MCP4251

Hello to everyone, I’m trying to use the two IC in object to create a variable gain amplifier to read values from a wheatstone bridge. To test it , instead of connecting INA 128 pins to the bridge, I connected them to a 1V regulated power source and, even if it works like a charm if I use a common resistor, I have some troubles to make it work with the digital Potentiometer (MCP4251): when I connect even a single pin of the digital potentiometer to the INA the value of its output goes to about 820 and nothing can move it from there (changing the resistance of the digiPot, changing the input voltage, nothing). I’m quite sure that my script controls the digitPot fairly (tried with a LED) and I don’t know what can be the problem. I also tried to shutdown the digiPot and remove its power supply but the output value of the INA doesn’t change. Any help?
This is the circuit I made: SCK, SDI and SDO are connected to SPI headers on an arduino UNO.

Your question doesn't make complete sense to me. What do you mean by the output goes to 820? Not volts, mV maybe?

The INA128 will want a program resistance value between 50K Ohms (2V/V gain) and 5K Ohms, (10000V/V gain). If you have used the MCP4251 with an LED then I suspect that its a lower value MCP4251 ( they come in different Ohm ranges, there is a dash number at the end). Also if you are trying for very high gain then good construction practice is necessary, or the output will be very noisy.

If you tried the MCP4251 with a led you may have damaged that part. The absolute maximum ratings table in the specification shows that 2.5 mA is the max current allowed through the potentiometer pins, its easy to exceed that with leds. So... You probably want a MCP4251-503 (50K) or -103 (10K) but the -503(5K) part will cause too much gain. Also you could check your part with an Ohm meter to make certain that it is still working and that your code sets the value properly.

regards, dougc314

Sorry i omitted that but 820 is what i read from the analog pin in Arduino that is about 4V; the digiPot i'm using id the 100K model and it seems to be working fine with an ohm meter. It's possibile that i exceeded the 2.5 mA current, i actually forgot about it. Do you think that this could have damaged it in a way that is not visibile with the ohm meter? I have to try and see if there is some tension between the potentiometer's pins and ground or something like this. Do you have any suggestion in that sense?

I also tried with another MCP4251 100k with no luck, same with another MCP4251 10k. Does a digital potentiometer need something peculiar to work with an instrumentation amplifier?