# choosing op-amp

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#### arhi

##### Member
Need to choose the right op-amp for the job.... I have following voltage's available

+5, +30, -30, var+, var- (var going 0-30±)

I'm using op-amp to amplify voltage across resistor (0R1) and get it to 0-5V scale (the max I is 5A so this makes the math easy).

I used the op-amp like this:

Code:
 GND ---[ 20M ]-\
|  |\
|  |  \
IN1 -----[ 2M ]----| + \
IN2 -----[ 2M ]----| - /------ OUT
|  |  /     |
|  |/       |
\--[ 20M ]--/
20/2 = 10 so the OUT should be (IN1-IN2) * 10

I use the same thing to get the variable voltage to go from 0-3V only reversing the 2M and 20M and setting IN2 to 0 (or IN1 depending if I'm getting the positive or the negative voltage)

All this works in simulation with TL082 / TL084 powered from ±30V but I'm not sure it will work in real life. The datasheet show that I can power it using 30V but that difference between IN1 and IN2 cannot be more then 15V (and I will have 30V in extreme case)... This mean that in first case where difference between IN1 and IN2 is below 1V I can use the TL082 but that for 0-30V difference I have to forget about the opamp solution and go with voltage divider. I initially did it with voltage divider but the negative part was giving me troubles pushing 0- -30 to 0-5V ain't easy .. I managed to make it swing from 2.5-0 (2.5V for 0 on input and 0V for -30V on input) but I like the op-amp solution better...

I believe you got the question, as TL082 seams unusable in this case, anyone have a suggestion what opamp will get the job done? (pdip case if possible but soic will do, 2 or 4 opamps in the same box)

the 4 outputs are fed to uC that then display the voltage on the + rail, voltage on the - rail, current on the + rail and current on the - rail.

I attached what I did so far if the question was not clear enough

EDIT: I believe purpose of the circuit is clear, I have 24 + 24V AC 6A and 6V AC 1A, I will be using LT1033 and LT1083 variable regulators on a heatsink + BT05T or some similar 1A 5V regulator for the uC part. There will be some uC (some 18F is on the pic attm as that was the only one in ISIS with 12bit ADC, some other 12bit ADC will probably replace him, if I do not decide that 10bit ADC and 16F will do the trick), I'd like the circuit to be as accurate as possible, so I will use 1% resistors .... but speed is irrelevant, 10sps or 1ksps or 10ksps or 100ksps don't matter ... I'd probably do between 5 and 10 samples per second...

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#### ericgibbs

##### Well-Known Member
hi arhi,

Get the datasheets for the Texas OPA445 and OPA547 opa's.

#### Roff

##### Well-Known Member
AD628 will handle your input range. You would have to use lower supply voltages.

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