ZHCSHW1C March 2018 – January 2019 ADS1260 , ADS1261
PRODUCTION DATA.
The PGA and modulator are chopper-stabilized at high frequency in order to reduce offset voltage, offset voltage drift and 1/f noise. The offset and noise artifacts are modulated to high frequency and are removed by the digital filter. Although chopper stabilization is designed to remove all offset, a small offset voltage may remain. The optional global chop mode removes the remaining offset errors, providing exceptional offset voltage drift performance.
Chop mode alternates the signal polarity of consecutive conversions. The ADC subtracts consecutive, alternate-phase conversions to yield the final conversion data. The result of subtraction removes the offset.
As shown in Figure 73, the internal chop switch reverses the signal after the input multiplexer. VOFS models the internal offset voltage. The operational sequence of chop mode is as follows:
Conversion C1: VAINP – VAINN – VOFS → First conversion withheld after start
Conversion C2: VAINN – VAINP – VOFS → Output 1 = (C1 – C2) / 2 = VAINP – VAINN
Conversion C3: VAINP – VAINN – VOFS → Output 2 = (C3 – C2) / 2 = VAINP – VAINN
The sequence repeats for all conversions. Because of the internal mathematical operations, the chop mode data rate is reduced. The chop mode data rate is proportional to the order of the sinc filter. Referring to Table 8, the new data rate is equal to 1 / latency values and the first conversion latency is 2 × latency values. Because of the two-point data averaging arising from the mathematical operations, noise is reduced by √2. For chop mode, divide the noise data values shown in Table 1 by √2 to derive the new noise performance data. The null frequencies of the digital filter are not changed in chop-mode operation. However, new null frequencies appear at multiples of fDATA / 2.