The ADS1258 multiplexer has optional differential output connections (MUXOUTx) that enable a single signal-conditioning circuit to be used for all analog inputs. The default signal conditioning circuit is a differential unity-gain buffer (OPA2320) between each MUXOUTx and ADCINx pin. A low-pass filter is connected to the output of the buffer. As shown in Figure 3-3, capacitor C20 is used to bypass the ADC sampling currents and resistors R25 and R35 are used to isolate the op-amp outputs from the filter capacitor.

Figure 3-3 ADS1258EVM Configurable Multiplexer Loop With a Signal-Conditioning Circuit

The signal-conditioning circuit shown in Figure 3-3 can also be configured into a differential-input, differential-output gain stage by changing resistors R24, R30, and R31 as per Equation 1.

Importantly, Equation 1 assumes that R31 = R24.

As an example, replacing R24 and R31 with 10-kΩ resistors and R30 with a 2.2-kΩ resistor creates a gain of 10 V/V between the MUXOUTx and ADCINx pins. Choose R24, R30, and R31 to be size 0603 resistors with a tolerance ≤0.1% to maintain system accuracy.

Enable the multiplexer loop by setting BYPAS = 1 in the CONFIG0 register. To bypass the multiplexer loop, set BYPAS = 0. One important caveat to the multiplexer bypass option is that the signal-conditioning circuit is only disconnected from the ADCINx pins. In other words, the signal-conditioning circuit is still connected to the MUXOUTx pins regardless of the state of the BYPAS bit.