8.3.9 Modulator Over-Range

The ADS1282-SP modulator is inherently stable, and therefore, has predictable recovery behavior resulting from an input overdrive condition. The modulator does not exhibit self-resetting behavior, which often results in an unstable output data stream.

The ADS1282-SP modulator outputs a 1s density data stream at 90% duty cycle 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 less than 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). 31 additional readings (62 total) are required for completely settled data.

If the inputs are sufficiently overdriven to drive the modulator to full duty cycle, 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 may not always clip the output code. When the input returns to the normal range, the modulator requires up to 12 modulator clock cycles (ƒ_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 over-range, either input is overdriven, exceeding the voltage of either analog supply voltage plus an internal ESD diode drop. The internal diodes begin to conduct and the signal on the input is clipped. When the input overdrive is removed, the diodes recover quickly. Keep in mind that the input current must be limited to 100-mA peak or 10-mA continuous if an overvoltage condition is possible.