The data sheet quotes an offset voltage of 50 µV max so I'm not sure why I'm seeing 35 mV. I also know it's not my multimeter because it reads 0 V when I touch the positive lead to ground. I have 2 IN118Ps and both of them output the same 35 mV. In this configuration, I would have expected the output to be 0V ± any offset. There is also a 100nF cap no shown that I placed between V+ and V. I should mention that this test circuit was powered by a standard 7805 and my bench power supply. I put the IN118P in a breadboard and connected the inverting, non-inverting, and reference pins to ground. I started by setting up a test circuit for the amplifier portion just to check that everything was working correctly. 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.Īnyway, I ran into a problem right away that I can use some help with. 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 output from the IN118P will go into an MCP3204 12-bit ADC IC that communicates with the PIC via SPI. I decided to go with the IN118P instrumentation amplifier and an OPA227 op amp to buffer the reference input of the IN118P. I finally got around to sourcing all the parts and have started to put this circuit together. Or, maybe I misinterpreted the document and I'm stuck with my 4-wire load cell.ĮDIT: I just found this IC and despite the fact that Digikey want about $300 for one of them, it looks promising. What a bummer, I was just starting to understand the 4-wire circuit. If I went that route, I'd likely spend a lot of time learning to use the IC and while I'm not apprehensive about putting in the time to learn something new, the schedule I've been given to finish this project may not allow for that. I did find 2 ICs with 6-wire interface examples but they look pretty complicated. I already did some preliminary digging on interface circuits for 6-wire load cells and that info seems to be less readily available compared to 4-wire interface circuits. I'm not sure if I'm interpreting it correctly but can I cut the cable for my 4-wire load cell and then convert it into a 6-wire one by splicing a -Sense line into the +Excitation and +Sense into the -Excitation? I suspect that's where I originally read the statement about never cutting a 4-wire load cell cable. I had already stumbled across the load cell primer document whilst surfing the interwebs.