Other Parts Discussed in Thread: ADS122C04
I’m considering an upgrade to your ADS122C04 part, for access to higher PGA gain settings. I’d like to use the function of reading the external reference voltage VREFP, through the “system monitor” function with input mux set to 1100.
However I’m concerned about this statement in the data sheet (highlighted):
…
8.3.9 System Monitor
The device provides some means for monitoring the analog power supply and the external voltage reference. To
select a monitoring voltage, the internal multiplexer (MUX[3:0]) must be configured accordingly in the
configuration register. The device automatically bypasses the PGA and sets the gain to 1, irrespective of the
configuration register settings when the monitoring feature is used. The system monitor function only provides a
coarse result and is not meant to be a precision measurement.
When measuring the analog power supply (MUX[3:0] = 1101), the resulting conversion is approximately (AVDD –
AVSS) / 4. The device uses the internal 2.048-V reference for the measurement regardless of what reference
source is selected in the configuration register (VREF[1:0]).
When monitoring the external reference voltage source (MUX[3:0] = 1100), the result is approximately (V(REFP) –
V(REFN)) / 4. The device automatically uses the internal reference for the measurement.
I understand that the resolution will be limited somewhat due to the /4 ratio; but is there another consideration that will make this measurement of the (VREFP - VREFN) less precise than any other input? Specifically, other than the approximation of the /4 ratio (initial gain tolerance), is this measurement going to have higher noise, or higher temperature drift, than the other inputs?
This part should help our design with an overall reduction in component count, if we can expect stability of at least < 50 μVRMS on the reading of the VREFP input. A wider tolerance on the gain (ADC counts per volt) is not a concern, as long as it is stable.
I’d appreciate any reassurance / cautions your applications team can provide.