Both the buffer or gain stage configuration can be used to directly measure true differential input signals. Measuring single-ended inputs requires additional consideration because the OPA2320 is powered by a unipolar, 5-V supply. Additionally, the OPA2320 requires up to 45 mV of headroom between the output voltage and either supply (V+ or V–). These two limitations restrict the ability of the default amplifier circuit shown in Figure 3-3 to measure single-ended signals referenced to ground. Instead, the amplifier negative input (MUXOUTN) must be level-shifted to a mid-supply voltage. This type of measurement is referred to as pseudo-differential, or a measurement with respect to some fixed voltage above ground.

Implement a pseudo-differential measurement on the ADS1258EVM by shorting the 2.5-V REFOUT voltage to the appropriate channel. In Auto-Scan mode, use jumper JP2 to short REFOUT to AINCOM because AINCOM is used as the negative channel for all single-ended measurements. In Fixed-Channel mode, short REFOUT to whichever analog input is selected as AINN in the MUXSCH register. This short can be applied externally using terminal block J3. Figure 3-4 shows both JP2 and J3.

Figure 3-4 Shorting AINCOM to REFOUT

A pseudo-differential measurement only sets the voltage on the MUXOUTN pin to be within the amplifier output voltage limits. The user must also make sure that the voltage applied to the ADCINP pin is within this same range. To measure signals that are outside of the amplifier output voltage limitations, consider bypassing the external multiplexer loop as described in Section 3.3.