MCP6N11 voltage reference and output problem

Question

I am currently trying to setup an instrumentation amplifier and I used MCP6N11. Basically I'm just trying to test the instruamp to output a certain voltage gain before I connect it to my actual circuit. I created a wheatstone bridge with an output of 2.54 and 3 volts at each leg. Therefore I have 0.46 volts difference If I input it to the intru amp. I just wanted to amplify the output at around 1 volt, therefore I need a gain of 2.

Alright, I set-up the instru amp using the standard circuit based on the datasheet of mcp6n11. I choose 5k for RF and Rg resistor which is also based on the datasheet to have a gain of 2. Now the problem is I am not getting any valid output that is near on my desired gain..I tried to conect the Vref to ground then the ouput is 0, when in Vdd the output is 5volts, when I try to connect it to a 2.5 voltage then it would just output a 2.5... I cant really figure out what is wrong, can any one help me how to properly setup the circuit, should I just leave the Vref hanging? I also tried to disconnect both Vinputs and I'm getting the same result. Here is the schematic

https://kg4vlw.bl3302.livefilestore.com/y2pKKagie1d4aOL-3hLpd0Dlehoh4RG8f4qeQSCCL3gAe_bgA4fLAghwMIrlpHeoxoOClvQ3Hv31gPp2NRt4BQY0swEf5RWn1JQg31ou_zbVWZ4p18dxjp1oLS5KvZl1RYYNyRUWcUSGpuvGfJUAHIz-A/schematic.png?psid=1

Note: I read some similar post like this but it wasnt confirmed if the problem was solved or how.

Thank you in advance

Answer 1

You have 3V applied to the inverting input and 2.5V applied to the non-inverting input. Thus, your output should be -1V with a gain of 2 (assuming reference grounded). It should work properly if the amplifier has a reasonable negative supply (say -5V). However, you show no negative supply, so the amplifier cannot work- the best it can do is get within some tens of mV of the negative supply, as shown in the datasheet.

If you connect the reference to a stiff voltage of 2.5V then it should output 2.5V minus 1.0V or 1.5V with the bridge connected. However you'll need something like a voltage divider plus an op-amp buffer to get the stiff 2.5V supply, a simple voltage divider will not cut it.

It's really not clear what you are trying to achieve with this. In the particular case here you can simply ground the reference input, swap the inverting and non-inverting inputs and get 1V output, however if you need to handle the bridge being unbalanced in either direction you'll need to bias the output by setting Vref to something other than 0V.