I'm looking for the best options for an AFE for a robust and accurate 4-20ma current loop receiver built around a 12-bit A/D integrated into a microcontroller.
I started to build an AFE with a competitor's chips: a specialized high common-mode voltage current measurement instrument amp plus an op-amp for scaling and level shifting. The circuit is acceptable but missing common-mode tolerance on the negative voltage range. The advantage is that each of the chips is multi-channel for reduced board space. I noticed that the circuit bears a strong resemblance to the AD628! However there are limited app notes for the 628 especially with respect to determining accuracy for this application. I'm here to ask for any opinions on what Analog parts might best serve my needs, or if I can get better info on using the AD628 for my needs.
The requirements are:
1) Common mode tolerance while remaining accurate of +/- 120v or greater, or as close as possible to this.
2) Use 80% of the 3.3v, 12-bit A/D FSR (the remaining 20 percent for detecting over-and-under current conditions). This means overall error has to be in the neighborhood of .05% if possible, with Vout range 0-3.3v.
3) Final AFE gain of 0.5-3 to accommodate different load resistances, plus level shifting to remove unused voltage range for 0-4ma.
4) I have a strong preference for single-supply 3.3v but accept that this may have to be given up to get the other specs met.
