ZHCSIU4A September 2018 – August 2019 ADS1284
PRODUCTION DATA.
The modulator is inherently stable, and therefore, has predictable recovery behavior resulting from an input overdrive condition. The modulator does not exhibit self-reset cycles, which often results in an unstable output data stream. The ADS1284 modulator outputs a data stream with 90% duty cycle of ones-to-zeroes density with the positive full-scale input signal applied (10% duty cycle with the negative full-scale signal). If the input is overdriven to exceed 10% or 90% modulation, but not saturated, the modulator remains stable and continues to output the 1s density data stream. The digital filter may or may not clip the output codes to +FS or –FS, depending on the duration of the overdrive. When the input returns to the normal range from a long-duration overdrive (worst case), the modulator returns immediately to the normal range, but the group delay of the digital filter delays the return of the conversion data to within the linear range (31 readings for linear phase FIR). An additional 31 readings (62 total) are required for completely settled data.
If the inputs are overdriven sufficiently to drive the modulator to full duty cycle (that is, all 1s or all 0s), the modulator is saturated. The digital output code may clip to +FS or –FS, again depending on the duration of the overdrive. A small-duration overdrive may not always clip the output code. When the input returns to the normal range, the modulator requires up to 12 modulator clock cycles (fMOD) to exit saturation and return to linear operation. The digital filter requires an additional 62 conversions for fully-settled data (linear-phase FIR).
In the extreme case of input overrange (where either overdriven input exceeds the voltage of the analog supply voltage plus the input protection diode drop), the protection diodes begin to conduct, thus clipping the input signal. When the input overdrive is removed, the diodes recover quickly. Make sure to limit the input current to 10 mA (continuous duty) if an overvoltage input signals are possible.