applefat
Newbie level 2
Hello, we've developed our own current measurement circuit for use with a large battery pack. There are several issues. A sketch of the circuit is shown--the dotted box is the shunt measurement board, which includes an ADC (not shown).
The digital output (14 bit resolution with 8x PGA active) always seems to be fixed at a small value (corresponding to a few mV) no matter how much or how little current is flowing through the shunt. Originally I was thinking we could use the 1k pot to balance the resistor ladder to account for resistance uncertainty to the shunt (100k 1% is still a rather large error for a 0.5mOhm shunt). Now it's clear a person's hand is not steady enough and instead it's probably better to clean up in software.
Something I was suspecting was that the ADC input (MCP3234, which has 3.2pf switched capacitor) is changing the voltage at the sense node because the ladder is fairly high-impedence. So I put a large 47uF cap on the inputs; it seems to work a bit better on the bench but not in the system. The only thing I can come up with is that there may be a voltage difference between the ADC ground and the pack ground (but it is differential measurement so should not matter??)
Any recommendations to improve the circuit? Right now hacks are preferred. In retrospect I should have just used INA148 opamp frontend and called it a day.
Thanks so much
The digital output (14 bit resolution with 8x PGA active) always seems to be fixed at a small value (corresponding to a few mV) no matter how much or how little current is flowing through the shunt. Originally I was thinking we could use the 1k pot to balance the resistor ladder to account for resistance uncertainty to the shunt (100k 1% is still a rather large error for a 0.5mOhm shunt). Now it's clear a person's hand is not steady enough and instead it's probably better to clean up in software.
Something I was suspecting was that the ADC input (MCP3234, which has 3.2pf switched capacitor) is changing the voltage at the sense node because the ladder is fairly high-impedence. So I put a large 47uF cap on the inputs; it seems to work a bit better on the bench but not in the system. The only thing I can come up with is that there may be a voltage difference between the ADC ground and the pack ground (but it is differential measurement so should not matter??)
Any recommendations to improve the circuit? Right now hacks are preferred. In retrospect I should have just used INA148 opamp frontend and called it a day.
Thanks so much