Maker Pro
Maker Pro

Using AD7690 to measure positive voltage range

N

Nicholas Kinar

Jan 1, 1970
0
Hello--

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.

http://www.analog.com/static/imported-files/data_sheets/AD7690.pdf

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?

Nicholas
 
Top