SBAA275A June 2018 – March 2023 ADS1120 , ADS112C04 , ADS112U04 , ADS1147 , ADS1148 , ADS114S06 , ADS114S06B , ADS114S08 , ADS114S08B , ADS1220 , ADS122C04 , ADS122U04 , ADS1247 , ADS1248 , ADS124S06 , ADS124S08 , ADS125H02 , ADS1260 , ADS1261 , ADS1262 , ADS1263
As previously mentioned, IDAC current matching is important. The impact of IDAC current mismatch is small for lead compensation because the additional error is small. However, IDAC current mismatch results in a gain error in the RTD measurement. As an example if IDAC2 is larger than IDAC1 by 1%, the reference would be 0.5% larger than expected, resulting in a 0.5% gain error:
This gain error due to mismatched IDAC current sources can be removed by chopping the IDAC currents. Chopping is achieved by making a measurement and averaging this first measurement with a second measurement after the IDAC currents have been swapped. Starting with the original configuration, the input voltage and reference voltage are given in the following. Equation 34 shows the first measurement, while Equation 35 shows the reference voltage.
If the IDACs are swapped so that IDAC2 is sourced from AIN0, and IDAC1 is sourced from AIN3, the reference voltage stays the same. However, the second measurement now becomes:
Averaging the first input measurement and the second input measurement, the result is:
The resulting ADC measurement is:
Using averaging, the (IIDAC1 + IIDAC2) terms cancel; and if the lead wire resistances are equal, they are cancelled as well:
With averaging, the ADC output code is no longer dependent on IDAC current matching, resulting in a more accurate measurement.
For chopping IDAC currents, set the register values: