How to make a simple transimpedance amp

Hi there - on a board I am redesigning I need a transimpedance amplifier that takes a 0-2ma input and outputs -10 to 10V. I don't care if -10 corresponds with 0ma or 2ma, but I do of course need linearity. The 0-2ma signal is being generated by a DAC onboard a Silicon Labs C8051F360

formatting link
That is being powered by +3.3V. The DAC's output can go from 0-2.1V. If it weren't for that limitation this would be easy! I should mention my op-amps are being powerd by +-12V.

So - is there any way to accomplish this with a single op-amp? I have a quad op-amp on board that I'm using 3/4 of the op-amps on already, so it'd be really nice if I didn't have to throw another chip on there, but I'll survive either way. To do it with two op-amps I could simply run the current through a transimpedance amp with the non- inverting terminal grounded, and then do a summing amp with some voltage reference to get it into the right range. But I'd really prefer to stick with just one op-amp.

Thanks!

-Michael

Reply to
Michael
Loading thread data ...

Well, the simple method would be to wire it as a transimpedance amp with a 10k feedback resistor, and then sink 1 mA from the summing junction to the negative supply, or a negative voltage reference if one is available. I'd probably want to use a diode in series with the DAC current output, and another one from the summing junction to ground (anode grounded) just to make sure I didn't blow up my MCU if I lost one of the analog supplies.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

I must be missing something, this is basic stuff. You want 0-2mA to give

-10 to +10V. That's a 20V swing, so an inverting opamp with a feedback resistor of 10k will do the job. But that's 0 to -20V, so you need to offset it by 10V, or 1mA at the input. You've got -12V, so a 12k from the summing junction to that gives you the offset you want.

The 0-2ma signal is being generated by a DAC onboard a

So why is it 0-2mA?

Reply to
Paul Burke

Hi Phil - your idea seems just about right. Somehow I hadn't really thought about diverting current like that.

One thing that has been bothering me: Would it be necessary for me to put a resistor inline between the microcontroller DAC output and the summing junction? If I didn't, the DAC output would have 0V at it's output. I do not know if this would bother a DAC or not.

Thanks,

-Michael

Reply to
Michael

Is it a VOLTAGE or CURRENT ouput DAC?

Reply to
Paul Burke

Nope - I think I was the one missing something :). Your suggestion seems exactly right.

It is a current output DAC.

-Michael

Reply to
Michael

That would depend on the DAC, but I'd think it would be odd for a single-supply device to be bothered by that. In any case, the protection diodes I suggested would move the DAC output voltage up to about 0.7V.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.