Hello--
To preserve bandwidth and slew rate I've selected a high speed op amp with low wide-band voltage noise and current noise. The 1/f noise is only 7nV/Hz^(1/2) at 10 Hz, which makes the part suitable for lower frequency signals extending to DC.
However, the input offset voltage is typically 40 microvolts (300 microvolts max), which may need to be nulled since the op amp is being used in a non-inverting configuration with high voltage gain.
On my schematic, I have the op amp wired up in a non-inverting configuration. At the negative input, the feedback resistor is connected to the output and another resistor is connected to ground. I've also connected another resistor (with a value of 1 Megaohm) to this negative input. The terminal of the resistor that is not connected to the negative input is connected to the output of a 16-bit buffered DAC.
The idea is that the DAC will be used to adjust the voltage at the negative input, thereby nulling the offset voltage without using a trim pot. After shunting the positive input of the op amp to GND via a CMOS switch and load resistor, I'll use a microcontroller to sample an ADC connected to the op amp output and a minimization algorithm to select the proper voltage required to reduce the input offset.
However, I'm concerned about whether the 1 Megaohm resistor will inject significant noise into the feedback loop. Although I know how to calculate the noise density created by a very large 1 Megaohm resistor, I am wondering if this noise will be amplified by the op amp feedback.
If so, then is there a way to reduce this noise? Might an RC filter be an applicable way to go? How do you reduce noise created by a mechanical trim pot?