Other Parts Discussed in Thread: TLC2272, REF2033, OPA196
I have to condition a low frequency (1Hz - 150Hz) signal that fluctuates within 3.5V max above and below signal ground. The purpose of the conditioning is to convert the signal to 0 to 3.3V so that it may be read by the ADC on my microcontroller. My power supply rails are +/-5V split.
I have two problems: (a) Make sure that the signal to the ADC has the maximum dynamic range of 0 to 3.3V but doesn't go outside the 'no damage' range -0.5V to 5.0V; (b) offset the signal so that I can adjust the zero input level to be 1.65V.
I have shown my attempted solution below. This must be a common problem so I'm wondering if there's a better way to do it? In particular, is there a "standard" way to generate a stable 1.65V that can sink and source 200µA current? Is TLE2141 (low noise) a good choice of op amp for the 1.65V buffer?
Circuit description: U3A permits scaling of the input up to a gain of 4. The maximum output from U3A will be 10V peak-to-peak. U4 is a differential amplifier that scales down this input to with an attenuation of 3.3V/10V (as you can see, in simulation, a 6.86Vpk-pk input gives a 3.21Vpk-pk at the output). the purpose of U5 is to supply a bias voltage of 1.65V with a bit of adjustability to trim out any system offset error. The bias is independent of the gain of U4.
Final question. I've gone for a non-inverting amplifier for U3. Is there any advantage of using an inverting amplifier in that position instead? I'm trying to minimise noise and maintain stability.
Thank you.