Alright, I'm young, I'm a student, so most of my knowledge is textbook rather than real world so I have a question for someone who has more experience.

This concerns a +-10V analog input that I need to cram down between +-225mV. The way I see it, I have two options. A precision voltage divider, or an Op-Amp. I put aside the divider idea because I'd have to build my own and fine tuning a set of SMT resistors doesn't sound like a fun afternoon. That leaves me with the op-amp.

This will be a LF, if not DC, application so frequency response isn't really an issue for me. I realize that I'll have to have an extremely low noise output signal to properly convert a +-10V control signal to something an A/D can actually make sense out of, and I can take all the board fabrication concerns, output offset compensation, drift compensation, PS decoupling, and overall device shielding. Even after all that, is such a low op-amp gain even feasible? I know on paper it is but does anyone have any experience with ultra-low gain circuits?

Regards

 

The farther you look into op-amps, the more you will realize that making an amplifier with a gain less than one always comes back to using resistors to lower the voltage. No matter how complicated you make it, you will always wind up in the same place. Get out your fancy Ohm's Law calculator and start with the resistor process.

 

As far as the divider goes, think of an added trim pot to get that fine adjustment. Fixed resistors just won't do as well. Then you can buffer the mv level signal with a high impedance op amp buffer.

 

I would think a precision voltage divider would be more accurate than an op-amp anyway.

With the divider only the resistors can drift... with the op-amp the op-amp itself can drift along with all the resistors.

 

You'd be looking at an inverting configuration amp, as the minimum gain of the non-inverting topology is 1. Even so, it's generally not advisable as the stability margins get very small below unity gain - even if you're operating at DC this will make the circuit very prone to excessive excitation from noise. So agreed, resistive potdown is the way to go. Use a unity gain buffer at the tap if you like to overpower the ADC input bias currents.