I am using a Wheatstone load gauge through an AD8295 op-amp to a SAMD21 uC ADC. The op-amp uses a resistor to set gain. An analog switch is used to change gain. Since the resistors are not exact (G = 49.4k/R + 1) I want measure the overall gain by using the DAC output of the uC. The gains will be stored in EEPROM during programming or memory on startup. Analog.com tech article shows a DAC connected to the +IN of a single ended op-amp to set offset but I am using a differential op-amp, so put a summing junction on the +IN side of the op-amp for the DAC? Should the DAC output be buffered with another op-amp? The offset of the zero load point in the ADC also has to be adjusted to get the maximum resolution. Loads are typically higher in one direction (the application is a shock dyno). By auto changing amplification and offset I can get the maximum resolution of the data. Thanks.
Some additional information - The op-amp used in the Analog.com article is a differential op-amp. The article is here: https://www.analog.com/en/technical-articles/offset-adjust-for-a-differential-op-amp-driving-an-adc.html I am currently using a PGA204 with fixed gain. However, I am changing from a Labjack board to a Feather M0 because of cost and upgrading to WiFi connection to the PC. Ti.com shows using a PGA204 and PGA205 to get programmed gains but the result is not in the ranges that I need. The 1000lb calibrated strain gauge output is 3mV/V with 12V supply giving +/-3.6mV output. The SAMD21 (corrected) works on 3.3V with options for Vcc, 1V or Vref for scaling. For full scale ADC at 3.3V the gain has to be 45.83. Reading full scale at lower forces are multiples of this, hence the analog switch and variable gain op-amp. The resistance of the switch can be taken into account by adjusting the fixed resistor value. However, the gain will not be exact which is why I want to calibrate the amplifier using the DAC. The non-loaded Wheatstone bridge will be connected when the DAC adds the gain calibration offset so the differential op-amp will see this as a signal input. Reading the ADC with the known DAC input should give the actual gain within the resolution of the ADC. The resolution of the ADC can be increased from 12 to 16 bits through oversampling (this cannot be done during runs due to time constraints). Once the actual gains are measured they can be stored in non-volatile memory. Peak loads on a shock are typically ten times higher on compression than rebound. Having a gain for reading +/-500lb to read forces that are +500/-50lb will need to be lower resolution than a gain for 600lb. range. Using the DAC to change the zero load offset will allow higher resolution.