Plecto
Full Member level 5
Hi. I'm trying to make an ammeter that can measure at least uA and hopefully hundreds of nA, but I feel that I'm on thin ice. I've chosen a 18-bit ADC called MCP3421 (**broken link removed**), but because of it's sample capacitor it was advised to use an input buffer. I bought a low input offset voltage op-amp called AD8628 (https://www.analog.com/static/imported-files/data_sheets/AD8628_8629_8630.pdf) in hope that it will suffice. The datasheet brags about it's low drift, bias current and offset voltage so I hope it was a good choice. The op-amp has a typical offset voltage of 1uV and this makes me wonder. Unless I have a very large shunt resistor, 1uV of offset could seriously damage the measurements, wouldn't it? Also, the offset voltage is a voltage seen at the input of the op-amp when the input voltage is in reality 0V so the higher the gain, the higher the output voltage caused by the offset voltage, right? Does this mean that I would ideally want a gain of 1? I also assume that the board layout is important, but I'm not sure which paths that should be as short and thick as possible. I have now designed the board so that the shunt resistor, R3 and op-amp ground are as close as possible, there is also a huge copper pad going from this point to the negative input of the ADC (about 4.8mm). The issue is the path going from the output of the op-amp to the + input of the ADC, this path is 21mm long (currently 16mil thick). I'm not sure if this will have any consequences as I can't really see it getting a voltage drop of significance. Here's a schematic of the op-amp and adc:
I thought about using the op-amp in a differential configuration, but is this really necessary if all the grounds are connected tightly together? I would really like some advice on this project![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
![21c5hmx.jpg](https://i44.tinypic.com/21c5hmx.jpg)
I thought about using the op-amp in a differential configuration, but is this really necessary if all the grounds are connected tightly together? I would really like some advice on this project