The ADS1262 and ADS1263 incorporate two, integrated, matched current sources (IDAC1, IDAC2). The current sources provide excitation current to resistive temperature devices (RTDs), thermistors, diodes, and other sensors that require constant current biasing. These devices also contain an internal IDAC multiplexer that provides connection of IDAC1 or IDAC2 to one of the 11 analog pins (AIN0 to AINCOM). The IDACs can be programmed over these current ranges: 50 μA, 100 μA, 250 μA, 500 μA, 750 μA, 1000 μA, 1500 μA, 2000 μA, 2500 μA, and 3000 μA. Figure 9-25 details the IDAC connection. The IDAC switches shown in the diagram are used in the IDAC rotation mode.
The internal reference must be enabled for IDAC operation. Take care not to exceed the compliance voltage of the IDACs. In other words, the voltage on the input pin must not exceed VAVDD – 1.1 V; otherwise, the specified accuracy of the IDAC current is not met.
The IDAC currents track the internal reference voltage. As a result of using the same reference voltage for IDAC1 and IDAC2, the current sources are matched. Matched performance is important for applications such as hardware compensated, 3-wire RTDs. IDAC to IDAC mismatch can be improved further by use of the IDAC rotation mode. The rotation mode automatically swaps the IDAC1 and IDAC2 connections of alternate conversions. The ADC averages the alternate conversions to eliminate IDAC mismatch. IDAC rotation can be performed manually by the user (by alternating the IDAC pin connections) or by the IDAC automatic rotation mode. The IDAC rotation sequence is shown as follows:
The sequence repeats for all succeeding conversions.
In rotation mode, the ADC provides a time delay to allow for settling after the IDAC pin connections are alternated. Note IDAC switching transients may interact with external components that may require additional time to settle. Additional settling time are provided by bits DELAY[3:0] in the MODE0 register. The total delay time results in a reduction of the nominal data rate (see the Section 9.4.2 section). Nevertheless, the existing frequency response nulls provided by the digital filter remain unchanged.