EDN Access -- 06.09.94 Simple ADC is surprisingly accurat
-- EDN, June 9, 1994
Design Ideas:June 9, 1994
Simple ADC is surprisingly accurate
Mike Walne,
Farnell Instruments, Wetherby, West Yorkshire, UK
The reference voltage in Fig 1a's simple rail-to-rail PWM ADC is the only critical component. The input-voltage range extends from ground to the power rail which also acts as the reference. The circuit is essentially a variation on the classic dual-slope integrator, the slopes being proportional to VIN and VREF-VIN. The circuit works well in µP-based systems that have the spare capacity to perform the mark/space measurement and the necessary calculations.
Assuming that there is no loading, the output voltage is a square wave with amplitude equal to VREF and a mean equal to VIN. Provided that the period is much greater than the digital transition times, the principal source of error is the input offset voltage of the op amp. The accuracy is at least 9 bits (and more likely 12) over the full operating-input and supply-voltage range with no trims. Working with a higher supply rail yields better accuracy. You can increase resolution by lengthening the time for the mark and period.
You can also use the circuit to measure the ratio of two resistors RU and RL as in Fig 1b. The mark/space ratio is then independent of VREF and equal to RL/RU. (DI #1443)
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