Current Sense Opamp Circuit Problem

Use an instrumentation amp (or instrumentation amp configuration) and measure the difference across the sense resistor. Multiply by some suitable factor.

If you aren't too picky, you could use this:

3.0V | | | 10k | 100k 0.6V ___ | ___ o-------------o---o--|___|----------------)--------|___|----. | | | | | | |\\| | .-. .---------------------|-\\ | | | 0.1 ohm | >----------------' | | .----------------|+/ '-' | |/| | | . | | | o--------' | | | | | | LOAD GND

Vout = 0.6 - Iload

(created by AACircuit v1.28.6 beta 04/19/05

Regards, Bob Monsen

I may be viewing that circuit wrong, but if you have both sides of the sense resistor going to the opamp, how will it work since both nodes at the opamps input can't be the same? (due to the drop across the resistor). It won't be able to force the nodes to same value, so it will act as a comparator.....

The other problem with using an instrument amp is that I can't seem to find one with a low input bias current. I need this amp to be able to have a gain of >=3D 5 @ 2MHz to be useful, and the only Instrument Amp with a GBP that high has a input bias current larger than the current I am trying to measure, which will no doubt throw the measurement off quite a bit.

The nice thing with the configuration I linked to is that bias current is sub microamp.

Sorry, I drew the feedback is in the wrong place. Put it between the

10k and 100k resistor.

Here is an LTSpice simulation I did for this circuit. It uses a subtractor, so the output voltage is equal to the current draw through the sense resistor:

Version 4 SHEET 1 880 680 WIRE -112 -32 -256 -32 WIRE 80 -32 -112 -32 WIRE 448 -32 80 -32 WIRE -112 16 -112 -32 WIRE -112 144 -112 96 WIRE -64 144 -112 144 WIRE 32 144 16 144 WIRE 112 144 32 144 WIRE 240 144 192 144 WIRE -112 160 -112 144 WIRE 400 192 368 192 WIRE 528 192 480 192 WIRE -256 208 -256 -32 WIRE 80 224 80 -32 WIRE 32 240 32 144 WIRE 48 240 32 240 WIRE 448 240 448 -32 WIRE 240 256 240 144 WIRE 240 256 112 256 WIRE 272 256 240 256 WIRE 368 256 368 192 WIRE 368 256 352 256 WIRE 416 256 368 256 WIRE -112 272 -112 240 WIRE 48 272 -112 272 WIRE 528 272 528 192 WIRE 528 272 480 272 WIRE -112 336 -112 272 WIRE 272 336 -112 336 WIRE 368 336 352 336 WIRE 416 336 416 288 WIRE 416 336 368 336 WIRE -112 352 -112 336 WIRE 368 368 368 336 WIRE -256 464 -256 288 WIRE -240 464 -256 464 WIRE -112 464 -112 432 WIRE -112 464 -240 464 WIRE 80 464 80 288 WIRE 80 464 -112 464 WIRE 368 464 368 448 WIRE 368 464 80 464 WIRE 448 464 448 304 WIRE 448 464 368 464 FLAG -240 464 0 SYMBOL res -96 256 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R1 SYMATTR Value 0.1 SYMBOL current -112 352 R0 WINDOW 0 24 88 Left 0 WINDOW 3 24 0 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 24 28 Left 0 SYMATTR InstName I1 SYMATTR Value 10mA SYMATTR SpiceLine load SYMBOL voltage -256 192 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 3 SYMBOL Opamps\\\\LT1218 80 192 R0 SYMATTR InstName U1 SYMBOL voltage -112 0 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value 2.4V SYMBOL res 32 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R2 SYMATTR Value 10k SYMBOL res 208 128 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R3 SYMATTR Value 100k SYMBOL Opamps\\\\LT1218 448 208 R0 SYMATTR InstName U2 SYMBOL res 496 176 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R4 SYMATTR Value 100k SYMBOL res 368 240 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R5 SYMATTR Value 100k SYMBOL res 368 320 R90 WINDOW 0 0 56 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R6 SYMATTR Value 100k SYMBOL res 384 464 R180 WINDOW 0 36 76 Left 0 WINDOW 3 36 40 Left 0 SYMATTR InstName R7 SYMATTR Value 100k TEXT -256 488 Left 0 !.dc I1 0 1 1m

the message I just posted has an LTSpice simulation, where there are two opamps. I realized after posting that you can do the function with a single opamp, since the first one just inverts and generates 10x gain.

So, it looks like this:

3V -----------------------------------. | 0.6V --o-------------. | | | | | | | | | | .-. .-. | 0.1R | | 100k | | 1MEG | | | | | ___ | '-' '-' .--|___|---)------. | | | | | | ___ | | |\\| | o-----|___|---)-----o--------|-\\ | | | | >-----o---output | 100k o--------------|+/ | | |/| | | | .---o---. .-. | | | | | 1MEG | | LOAD | | | | | | '-' | '---o---' | | | | | | | | GND ---o-------------o----------------' (created by AACircuit v1.28.6 beta 04/19/05

The opamp should be a single supply variety.

Regards, Bob Monsen