I previously had configured a PIC (16F1704) to use the internal FVR for ADC reference, with gain of the FVR set to 2x, i.e. 2.048V reference. I've since added another signal to be measured, but the signal spans 0..900mV only. I'm tempted to change the FVR's gain to 1x while reading this channel to get one bit more resolution.

The code therefore looks like:

  1. Set FVR's gain to 2x
  2. Read ADC channel 1:
    1. Change ADC channel to 1
    2. Perform ADC conversion
  3. Read ADC channel 2:
    1. Change ADC channel to 2
    2. Perform ADC conversion
  4. Read ADC channel 3:
    1. Change ADC channel to 3
    2. Perform ADC conversion
  5. Set FVR's gain to 1x
  6. Read ADC channel 4:
    1. Change ADC channel to 4
    2. Perform ADC conversion
  7. Repeat

Now, while changing the ADC channels it's recommended to introduce a small delay before you start the ADC conversion, so the input capacitor of the ADC can settle. I've been using 5µs delay (after each channel change) as recommended by a Microchip tutorial. Is there an equivalent delay associated with changing the FVR gain? I haven't found a recommendation on that, nor a datasheet value on the Independent Gain Amplifiers' settling times.

  • \$\begingroup\$ What does the data sheet say? \$\endgroup\$ – Andy aka Mar 15 '18 at 13:12
  • \$\begingroup\$ It doesn't say anything, unfortunately \$\endgroup\$ – anrieff Mar 15 '18 at 13:13

enter image description here The datasheet only provides this figure for the FVR stabilization period, though this one implies only when coming out of RESET or exiting sleep mode.

Irregardless, I would use this as a starting point.

  • 1
    \$\begingroup\$ This is for the LF device, mine is F. The F devices are somehow "always stabilized", but I think this refers to the base FVR voltage (1.024V), nothing is mentioned regarding the gain amplifiers. \$\endgroup\$ – anrieff Mar 15 '18 at 13:47
  • \$\begingroup\$ "When either FVR Buffer1 or FVR Buffer2 is enabled, then the buffer amplifier circuits require 30 μs to stabilize. This stabilization time is still required when the FVR buffer is in operation." \$\endgroup\$ – Leon Heller Mar 15 '18 at 14:08

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