Design Ideas: March 3, 1994

The infinite-hold circuit in Fig 1 automatically zeros out long-term drift from an instrument or a sensor. In operation, a control system (not shown) periodically takes the instrument or sensor off line and applies a known stimulus. The known stimulus generates a baseline-output signal.

The back-to-back ADCs and DACs constitute an infinite S/H circuit. The end-of-conversion signal from the ADC writes the new baseline output into the DAC's latches and kicks off the DAC's conversion cycle. Together, therefore, the DAC's latches and conversion cycles memorize the new baseline-output signal.

Then, when the control system puts the instrument or sensor back on line, the summing junction automatically subtracts out the new baseline signal.

The biggest hurdle overcome in this design was the unexpected and difficult task of finding an ADC/DAC pair having compatible signal lines. The National ADC1225 and DAC1210 in the figure are one such pair. EDN BBS /DI_SIG #1374