ZHCSMK4A september 2022 – july 2023 ADS131B26-Q1
PRODUCTION DATA
The signal chains of ADC1y and ADC3y use a very low-drift, chopper-stabilized PGA and ΣΔ-modulator to provide very low offset error and offset drift. However, a small amount of offset drift remains in normal measurement. For that reason, the ADC1y and ADC3y signal chains incorporate an optional global-chop mode to reduce offset error and offset drift over both temperature and time to exceptionally low levels. When the global-chop mode is enabled by setting the GC13y_EN bit, ADC1y and ADC3y perform two consecutive conversions with alternate input signal polarity to cancel offset error. The first conversion is taken with normal input polarity. The global-chop control logic inverts the input polarity and resets the digital filter for the second conversion. The average of the two conversions yields the final corrected result, removing the offset voltage. Figure 9-19 illustrates a block diagram of the ADC1y global-chop implementation. VOFS models the combined PGA and ADC1y internal offset voltage. Only this device-inherent offset voltage is reduced by global-chop mode. Offset in the external circuitry connected to the analog inputs is not affected by global-chop mode. The GC13y_EN bit enables global-chop mode for both ADC1y and ADC3y.
The operational sequence of global-chop mode is as follows:
The first conversion result (Output 1) after a conversion start is available after ADC1y takes two settled conversions. Because of the sinc3 filter, data of one conversion settles in three conversions cycles. Equation 20 calculates the time required to output the first conversion result after a conversion start.
In continuous-conversion mode with the global-chop mode enabled, subsequent conversions complete in tGC_DATA, as calculated by Equation 21 and shown in Figure 9-20. That means the data rate in global-chop mode is approximately 1/3rd the data rate in normal mode.
Before starting conversions after the input polarity is inverted, ADC1y waits the global-chop delay time, GC13y_DELAY[2:0], to allow for the internal circuitry to settle. In some cases, the programmable global-chop delay time must be increased to allow for settling of external components.
Global-chop mode reduces the ADC1y and ADC3y noise by a factor of √2 because two conversions are averaged. Divide the input-referred noise values in Table 8-1 and Table 8-3 by √2 to derive the noise performance when global-chop mode is enabled.
The notches of the sinc3 filter in Figure 9-3 do not change in global-chop mode. However, additional filter notches appear at multiples of fGC_DATA / 2.