SBAS949A February 2020 – February 2021 ADS131M06
The ADS131M06 incorporates a global-chop mode option to reduce offset error and offset drift inherent to the device due to mismatch in the internal circuitry to very low levels. When global-chop mode is enabled by setting the GC_EN bit in the GLOBAL_CHOP_CFG register, the device uses the conversion results from two consecutive internal conversions taken with opposite input polarity to cancel the device offset voltage. Conversion n is taken with normal input polarity. The device then reverses the internal input polarity for conversion n + 1. The average of two consecutive conversions (n and n + 1, n + 1 and n + 2 and so on) yields the final offset compensated result.
Figure 8-15 shows a block diagram of the global-chop mode implementation. The combined PGA and ADC internal offset voltage is modeled as VOFS. 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 conversion period in global-chop mode differs from the conversion time when global-chop mode is disabled (tDATA = OSR x tMOD). Figure 8-16 shows the conversion timing for an ADC channel using global-chop mode.
Every time the device swaps the input polarity, the digital filter is reset. The ADC then always takes three internal conversions to produce one settled global-chop conversion result.
The ADS131M06 provides a programmable delay (tGC_DLY) between the end of the previous conversion period and the beginning of the subsequent conversion period after the input polarity is swapped. This delay is to allow for external input circuitry to settle because the chopping switches interface directly with the analog inputs. The GC_DLY[3:0] bits in the GLOBAL_CHOP_CFG register configure the delay after chopping the inputs. The global-chop delay is selected in terms of modulator clock periods from 2 to 65,536 x tMOD.
The effective conversion period in global-chop mode follows Equation 8. A DRDY falling edge is generated each time a new global-chop conversion becomes available to the host.
The conversion process of all ADC channels in global-chop mode is restarted in the following two conditions so that all channels start sampling at the same time:
The conversion period of the first conversion after the ADC channels have been reset is considerably longer than the conversion period of all subsequent conversions mentioned in Equation 8, because the device first needs to perform two fully settled internal conversions with the input polarity swapped. The conversion period for the first conversion in global-chop mode follows Equation 9.
Using global-chop mode reduces the ADC noise shown in Table 7-1 at a given OSR by a factor of √2 because two consecutive internal conversions are averaged to yield one global-chop conversion result. The DC test signal cannot be measured in global-chop mode.
Phase calibration is automatically disabled in global-chop mode.