8.3.3.1.1 Modulator Overrange

The ADS1283 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 ADS1283 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 past 90% modulation, but below 100% modulation (10% and 0% for negative overdrive, respectively), 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 result 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 sufficiently overdriven to drive the modulator to full duty cycle (that is, all 1s or all 0s), the modulator enters a stable saturated state. The digital output code may clip to +FS or –FS, again depending on the duration. A small-duration overdrive condition may not always clip the output code. When the input returns to the normal range, the modulator requires up to 12 modulator clock cycles (f_MOD) to exit saturation and return to the linear region. 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 an internal ESD diode drop), the internal 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 condition is possible.