Hello again everyone. I finally got around to sourcing all the parts and have started to put this circuit together. I decided to go with the
IN118P instrumentation amplifier and an
OPA227 op amp to buffer the reference input of the IN118P. The output from the IN118P will go into an
MCP3204 12-bit ADC IC that communicates with the PIC via SPI. The IN118P, OPA227, the V
REF input of the MCP3204, and the load cell are all powered by a precision 5V voltage reference, the
MAX6143. The rest of the 5V devices (PIC, LCD, V
DD of the MCP3204 ADC IC, etc) are powered by a non-precision 5V source, the
KF50B.
Anyway, I ran into a problem right away that I can use some help with. I started by setting up a test circuit for the amplifier portion just to check that everything was working correctly. I put the IN118P in a breadboard and connected the inverting, non-inverting, and reference pins to ground. I should mention that this test circuit was powered by a standard 7805 and my bench power supply. There is also a 100nF cap no shown that I placed between V+ and V-. Here’s a schematic:
**broken link removed**
In this configuration, I would have expected the output to be 0V ± any offset. Well, I measured about 35 mV. I have 2 IN118Ps and both of them output the same 35 mV. I also know it's not my multimeter because it reads 0 V when I touch the positive lead to ground.
The data sheet quotes an offset voltage of 50 µV max so I'm not sure why I'm seeing 35 mV. If the 35 mV is just the input offset getting amplified, then I would have expected the output to change when I adjusted the gain pot but nothing happened.
Is there something wrong with my test set up or my understanding of how this circuit is supposed to work? Any guidance you can offer is much appreciated.
Thank you.
P.S. I also attached the most recent schematic of the entire project for reference just in case anyone wants to see it.