Other Parts Discussed in Thread: ADS1256, , OPA2189, OPA350, OPA192, OPA2192, OPA320, REF7025, TMUX1108
I was redirected here from the Data converters forum for help with building a calibration board for a handheld instrument that consists of a 24-bit ADS1256 sigma-delta ADC and a 12-bit MCU SAR ADC. The instrument has a 2.5V reference.
The instrument has three connectors with pins that lead into these two types of ADC inputs. These inputs will in some instances be used for single-ended and in others for differential measurements. Some, but not all, signal paths go via analog MUXes. The 12-bit MCU SAR ADC inputs might become 'enhanced' with an external differential PGA. In other words, there are differences in grounding and signal paths that probably calls for system calibration (and not only the ADS1256 self calibration).
Calibration will require a set of reference voltages that are slightly less than full-scale for each range, single-ended and differential. I'm currently thinking along the line of letting most of the calibration board reference voltages be generated from a 2.5V reference. It will also use a 4.096V reference for ADS1256 unbuffered full-scale calibration.
I suppose that the only practical design is to use a number of resistor dividers each with a unity-gain OPAMP buffer. I am currently planning to use a 16:2 MUX from the reference voltages and then a 2:16 MUX to the instrument's connectors. I'm also planning to use three mechanical relays for switching GND to each of the connectors. Switching the MUXes and relays will be controlled by an MCU (that is controlled by a PC GUI).
1. I assume that about 10k ohm over each resistor divider is a good compromise between low load on the 2.5V reference and low source resistance. Is this a reasonable assumption for four such dividers in parallel fed by a voltage reference or an OPAMP unity-gain buffer? (1 mA load in total.)
2. Could you recommend a suitable OPAMP unity-gain buffer for the resistor dividers? If I understand correctly, such OPAMPs must be designed to work with high input impedance, which rules out some zero-drift, low-noise OPAMPs? I am currently looking at OPA189/OPA2189, but are there other ones better suited for this?
3. Should you always incorporate a feedback resistor for the unity-gain buffer? Would 100 to 330 ohm be good as a rule of thumb or should it match the input impedance from the resistor divider?
4. Should there be an RC network at the output of each unity-gain buffer?
5. For the instrument's 2.5V reference, it is sometimes a good idea to use an OPAMP buffer. The OPA350 is commonly used as an example, e.g. in the ADS1256 EVM, but are there newer, higher-performance OPAMPs, that are currently recommended? The REF70xx series is not yet available and the REF60xx is not a preferred choice.
Please don't redirect me back to the Data converters forum.