The ADS1263 includes an auxiliary, 24-bit, delta-sigma ADC (ADC2). ADC2 operation is independent of ADC1, with independent selections of input channel, reference voltage, sample rate, and channel gain. All input configurations (channel select, IDAC, level shift, sensor bias) are available to ADC2. Use ADC2 to perform main channel (ADC1) cross-checking measurements (for example, diagnostics purposes and redundant channel measurements), system background measurements, or temperature compensation of the primary sensor (such as thermocouple cold junction compensation). Using data rates of 10, 100, and 400 SPS for both ADCs, ADC2 performs virtual parallel conversions with ADC1 on the same input channel.
As shown in Figure 9-29, the ADC2 consists of an input signal multiplexer followed by a high-impedance PGA. The input multiplexer has the same functionality as the ADC1 input multiplexer. The sensor bias current source or a 10-MΩ bias resistor can be connected to the multiplexer output. Connect the sensor bias to either of the ADCs. ADC2 provides gains of 1, 2, 4 ,8, 16, 32, 64, and 128. Depending on the gain settings, ADC2 gains are either implemented in the PGA or in the modulator. For gains of 1, 2, and 4, the PGA is bypassed and the gain is performed in the modulator. For gains of 8, 16, 32, 64, and 128, the modulator gain is fixed at gain = 4, and the additional gains are performed in the PGA.
The PGA drives an inherently-stable, second-order, delta-sigma modulator. The modulator output data are filtered and down-sampled by a programmable decimation digital filter. The digital filter provides data rates of 10, 100, 400, or 800 SPS with 24-bit resolution. A calibration block follows the digital filter. The calibration block consists of 16-bit offset correction and 16-bit, full-scale correction registers. The ADC2 reference multiplexer selects from one of three external reference input pairs, the analog power supply, or the internal reference. The reference input is buffered to minimize errors caused by external circuit loading.