ADC input level offset circuit

[dadler]

Hi-

I am working on a project, where I wish to sample the supply voltage to a subcircuit.

The voltage range is 14-16V.  I want to scale and offset this to be sampled by a 0-5V ADC input (I may end up using a 4.096V external reference for the ADC, but the ask is the same regardless). The bandwidth can be extremely low, as I am only interested in slow changes to the supply voltage.

I have come up with the following two circuits:

1) Scale/offset with non-inverting op amp:





2) Use TL431 as regulated offset voltage (could also scale to full range with another op amp):






The tempco of the TL431 is pretty terrible, but the gain required by the first circuit makes me uneasy from an accuracy/noise perspective.

Which of these is better? Is there a better way of doing this?

Thanks

[dadler]

Thanks for the insight. What you have proposed isn't far off from what I have configured in my layout right now (May replace BAT54 with BAV99 due to leakage)



Although it seems you've added a couple of caps for additional filtering/transients?

According to the datasheet, the TL431 isn't stable with a capacitive load--but I could not get the circuit above to oscillate, even with a wide range of load capacitance. I also breadboarded a version with capacitance across the TL431 as you have done in your circuit, and it was stable as well.