How to make a current buffer

I don't have a clear picture of the problem but OPA861 comes to mind. D from BC

Michael snipped-for-privacy@gmail.com posted to sci.electronics.design:

For most of the parts i know of both the DAC and the opamp are toast. Throw them away and put in new ones. Then look up Widlar current mirror.

I haven't even soldered it together yet - I'm aiming to avoid toasting.

And I don't see why the op-amp would get damaged by this.

-Michael

Hi - hopefully my crude ASCII art helps?

|\\

Supply to the op-amp is +-12V.

I'll take a look at the OPA861.

-Michael

My goal is to get a -10V to 10V output with a 0-2ma input from a DAC. I could care less if 0ma = -10V or if 0ma = 10V. I just need a linear transimpedance amp with that output range. The op-amp is supplies with

Thanks!

-Michael

If my mental picture is correct, all you should have to check for is "compliance" on your current output. IOW, can it handle 10V from an external source, and still provide the current output?

If the "10V source to the NITOTOA" was a mistake, then get hacking! ;-)

Cheers! Rich

That obviously won't work.

Tie the +input to GND and add a resistor from the neg input to a negative reference voltage that will shift the output to +10V when the DAC outputs no current.

View in a fixed-width font such as Courier.

. . . . 20V . | Vout= - --- x (Iin-1mA) . +10 + 2mA . | \\ . | \\ . ^ | \\ = - 10K x (Iin-1mA) . | | \\ . | \\ . Vout -+------+-----+--> Iin mA . |0 1\\ 2 . | | \\ | . ^ | \\ . | \\ | . | \\ . -10 + - - - - - - . | . . . . . Basic Inverting Summer: . . . . . . 0V . Iin >-------------+--[10k]--. . | | . | |\\ | . Vbias>---[Rb]------+-|-\\ | Vbias . | >----+-> Vout = - 10K x (Iin+ -----) . .-|+/ Rb . | |/ . | . --- . 0V . . Vbias . Make ----- = - 1mA for required transfer function. . Rb . . Like so: . . . 0V . Iin >----+--------+--[10k]--. . | | | . | | |\\ | . [10k] '-|-\\ | . | | >----+-> Vout= - 10K x (Iin-1mA) . | .-|+/ . --- | |/ . -10V | . --- . 0V . . . . . - or - . . . . . . 0V . Iin >----+--------+--[10k]--. . | | | . | | |\\ | . [12k] '-|-\\ | . | | >----+-> Vout= - 10K x (Iin-1mA) . | .-|+/ . --- | |/ . -12V | . --- . 0V . . . . - or - . . . . Any number of combinations can be used. The amplifier offset . . voltage will be developed at the (-) node and this will intro- . . duce an error of Voffset/Rb into the 1mA current subtracted . . from Iin. .

Michael snipped-for-privacy@gmail.com posted to sci.electronics.design:

I misunderstood your proposed circuit. Fred Bartoli's changes are correct. Just remember that many op amps have differential limiting diodes on the inputs. The path through the op amp inputs could force too much current through the inputs cause a meltdown and connect the output of the DAC to the +10 V source.

Hi Fred - wouldn't some of the current from the current source be able to flow into the negative reference voltage? Thanks,

-Michael

Michael a écrit :

Don't understand what you mean... The opamp's minus input is always at GND voltage thanks to the opamp loop gain (not accounting for the opamp offset) and all the DAC current flows into the opamp's feedback resistor.

Draw a diagram and see. That's really simple.