Using AD7690 to measure positive voltage range


I am using the AD7690 SAR ADC to measure an analog voltage within the range of 0V to 5V. According to the datasheet, this particular ADC can be used to digitize a "true diferential analog input range," which is

+/- VREF voltage. However, it appears that the IN+ and IN- inputs of this ADC are only rated for 0V to +VREF and +VREF to 0V. Because the inputs are differential, they are driven by an amplifier such as the AD7690 which produces two scaled and phase-shifted outputs.

Table 7 on page 13 of the AD7690 datasheet shows output codes in 2's complement format for a VREF of +5V, and a bi-polar input voltage of +/- 5V.

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Because the inputs of this ADC can only permit positive voltages, it is apparent that a signal swinging between +/-5V must be scaled around a common-mode voltage of 2.5V.

It appears that a bipolar voltage swinging between -5V and +5V must be scaled to swing between +5V and 0V.

So in Table 7 of the datasheet, a midscale voltage of 0V really corresponds to an ADC input voltage of 2.5V.

I believe that it is possible to use the AD7690 ADC to measure only a positive voltage which swings between 0V and 5V. This seems trivial.

I reason that an ADC negative output code corresponds to a voltage less than VREF/2, which is the midpoint. For a VREF of 5.0V, does this mean that

(1) Voltages less than VREF/2 are negative? (2) Voltages greater than VREF/2 are positive?


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Nicholas Kinar
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