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| High side current sense using differential opamp |
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| npelov:
I have differential opamp (1/4 LM324) measuring the current through 8 ohm load (R6) via high side shunt (R1). I started with higher values (I think 3.9k and 15k) of the resistors but I've got 0.6V output offset. I tried to calculate what caused the output offset: Iofs max = 100 nA * 15k = 1.5mV Vofs max = 3mV total offset 4.5mV * gain 4 = 18mV - doesn't match the 600mV. I lowered the values to 750 ohms and 3k, but then I got even higher output voltage without any thing on the input ~ 800mV. I know that input has to be (V+) - 2V, that's why I can't have gain more than 4. With gain of 4 R4 and R5 make divider (by 5) that leaves the In+ 12/5 = 2.4V below V+. minimum voltage should be 3.35*3 = 10.05V, so In+ will be (V+)-2.01V. Since output won't go below 0V the same is true for In-. To measure without current I disconnect R6 (the load) from ground, leaving the shunt connected to V+ Is there anything I'm missing or I killed the opamp somehow. More info: The other 3/4 of the opamp work just fine. I measure battery voltages of 3 cell li-ion battery with gain = 0.5. I measured the voltage directly on their inputs - it's 0.5 to 1.5 mV. However the 4-th amp shows difference 165 mV (last measurement) on it's inputs. 10V on the positive and 10.165V on the negative input. The supply voltage is currently 12.5V With this setup the opamp has to sink 12.5V/3750 ohm = 3.3mA. That's not too high. Minimum sink capability is 10mA. Well in some cases minimum is 5mA, but 3.3 is lower than that. |
| npelov:
Update: Same setup works on low side shunt. Why does it have problem with high side? I tried gain = 1 so the inputs are in the middle of supply voltage and I still get high offset with no current flowing. I also tried another opamp (LM358) - same results. P.S. I messed up the gain calculation. with 3k and 750 it's 3000/750 = 4, (not 5) |
| StillTrying:
"I lowered the values to 750 ohms and 3k, but then I got even higher output voltage without any thing on the input ~ 800mV." That's what the data sheet says, when sinking 3.3mA the minimum voltage the output can go down to is about 0.8V. |
| npelov:
I was jus reading TI datasheet and I saw that at Vo=200mV output current is just 30uA. Then I saw the graph - sinking current vs output voltage. I either need lower current, negative supply or output transistor (mosfet?). Thanks! 2N7002 should be able to go down to 5ohm * 3mA =15 mV. And it has only 50pF max input capacitance. |
| Eka:
--- Quote from: npelov on June 20, 2018, 05:46:57 pm ---Update: Same setup works on low side shunt. Why does it have problem with high side? I tried gain = 1 so the inputs are in the middle of supply voltage and I still get high offset with no current flowing. I also tried another opamp (LM358) - same results. P.S. I messed up the gain calculation. with 3k and 750 it's 3000/750 = 4, (not 5) --- End quote --- The power supplies for the op-amp need to be centered around the voltage at the high side current shunt. So if your high side voltage is 12 volts and your op-amp needs +5 and -5 volts, then you need supplies that are at 18 volts and 7 volts to power the op-amp. If the op-amp also needs to be connected to ground, then it's ground would be connected to 12 volts. Now that the op-amp is powered and operating properly, a circuit to translate the output from being centered around 12VDC to the voltage range the rest of your circuit is is needed. I'm not an op-amp guru so I'll leave that to others. What I do know is this is a common problem and chips have been made to make it much easier to implement a high side current shunt monitor. This is a link to TI's high side current shunt monitors with analog, comparator, and digital outputs. Some even provide integrated shunts. The digital output ones have integrated analog to digital converters, and many have current and voltage monitoring and will do all that is needed to track power use. http://www.ti.com/amplifier-circuit/current-sense/overview.html |
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