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| unbalanced supply for op-amp in pure DC operation? |
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| Eliminateur:
Here's the issue: i need to measure DC current (with a large dynamic range, i'm measuring from microamps to 10A~) with a 10A/75mV low-side shunt, tested device is battery operated and the testing rig as well(test rig would probably use a 9V battery). So my problem is the op amp selection to amplify the shunt to 5V, all the op amps i have are NOT RtR in/out, all i have at hand are regular FET input amps and the fancies i have is a OP4277 which has a output swing of +-2V to vcc/vss and the common mode input as well, so i can't feed it from the +5Vcc of the digital circuitry if i want the full 5V output. Now, the positive rail is no issue, i can use the +9Vbat and that gives me the clear 5V output, the problem is the ground/negative. I've been toying with using a 555 for negative rail, but that has significant loss leaving me with ~-7.2v for +9V input. ¿what happens to the output if i feed the op amp with +9v/-7.xV?, both circuits grounds are joined (the battery under test and the tester battery)?, for example, under no load, with 0V on the NI input of the amp, ¿what happens to the output? will it stay at 0V or will it go to 0.9Vdc(9-(9+7.2/2)) due to the imbalance?, i've googled around bu can't find a definite answer(some sites say it won't do anything, others that it will) another option i was considering is using two 9v batteries, but ofc they won't have the exact same voltage |
| graybeard:
It will work fine. The rails can be anthing so long as the total meets the minimum, and and does not exceed the maximum voltage rating of the op-amp. Make sure the common mode input range inad output swing will work for the chosen rails. |
| T3sl4co1l:
So you're, uhh, measuring nanovolts with an op-amp? Tim |
| Eliminateur:
--- Quote from: T3sl4co1l on April 14, 2020, 10:41:41 pm ---So you're, uhh, measuring nanovolts with an op-amp? Tim --- End quote --- ¿what's the problem?, i don't think i'll measure down to that scale considering it's all cobbled up together and the ADC resolution won't go so down, at least i must have sub mA accuracy(like i have in my multimeter already in the 10A range) |
| T3sl4co1l:
Alright, fractional mA in the LSBs, is reasonable. You will need a precision uV offset, probably autozero/chopper type, and yes, bipolar rails are a good idea. A typical way to derive split rails from a single battery, is to use a voltage divider and op-amp voltage follower. The amp's output is assigned ground, and the +/-4.5V rails are then used for whatever. Charge pumps are a runner-up, but are very noisy so may not be so attractive here. Tim |
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