Answer 6
This circuit implements the same basic delta-sigma converter, except that now the feedback is inverted in the digital domain and applied to a common summing node along with the input voltage. The duty cycle of the digital output pin now represents the complement of the input voltage, centered around the value of V_REF.

There are three advantages to using this circuit.

First, the inputs to the: comparator are fixed, so there are no issues relating to exceeding its common-mode input range.

Second, input scaling can be adjusted by varying the ratio of R1 to R2. The input range can even exceed the supply rails of the microcontroller.

Third, the center of the input voltage range can be adjusted by adjusting V_REF, within the common-mode limits of the comparator.

The disadvantages of this circuit are that it requires one extra resistor and has a lower impedance (equal to R1).

The source impedance (i.e., the potentiometer) must be at least an order of magnitude lower to avoid significant errors.

Contributor: Dave Tweed

Published May 2003