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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: 5V Reference Voltage for AD7707 AD-Converter??
Date: Tue, 10 Sep 2002 04:40:58 GMT
NNTP-Posting-Date: Mon, 09 Sep 2002 21:40:58 PDT
Mario Trams wrote:
> I'm planning to use an Analog Devices AD7707 AD-converter for measuring
> the output of a sensor that is swinging around 2.5V. The sensor is
> ratiometric to the supply voltage (5V nominally).
> Application notes for the sensor state that the sensor supply voltage
> should be used directly as reference voltage for the AD-converter.
> This eliminates some noise in the measurement caused by minor changes
> in the supply voltage (because the sensor is ratiometric).
> Anyways, the problem is that the data sheet of the AD7707 states
> that a reference input of 2.5V shall be used "for specified
> It is left unclear how much we are "off the specified performance"
> with a 5V reference (at least, 5V are permitted by the datasheet).
> I would be very grateful, if anybody could tell me his experience with
> the "performance" issue or can give a short comment how to deal in best
> way with the specified 2.5V reference while respecting the ratiometric
> design (OpAmps, resistive voltage dividers, ... ?).
> Thank you very much,
I finally got around to looking into the AD7707. I did not download the
full data sheet but I looked at its architecture to see if there was
anything about it that might make what you want to do difficult. There
The idea of using the sensor supply as the reference works well with SAR
ADCs as long as they are implemented with multiplying DACs. For example,
if you need to convert (A-B)/(A+B) with a more conventional ADC, A-B
goes to the input and A+B goes to the reference. This is essentially the
arithmetic you need with your sensor. You need output/supply which is
The AD7707 appears to be able to do this but it is not perfectly
straight forward. The AD7707 has a single ended full swing of 0 to +2.5
on the low level inputs when operated on 5 volts. The limitation on this
will be the dynamic range of the internal buffer and the PGA. It would
appear that your best approach would be to use a by 2 voltage divider on
the sensor output to get within the AD7707 input range. This is at most
the resistive divider and two buffers depending on input and output
impedances. Similarly, you divide the sensor supply by 2 probably using
a single buffer to obtain the reference. All of this assumes that the
conditions I'm recommending will end up with the PGA set at a gain of
Another suggestion is to treat the reference as I suggested above and
use one of the high level inputs for the sensor and set the PGA so that
the attenuation times the gain is 1/2.
The idea is that division by 2 is optimum for getting the reference from
the supply so you need to use the same division on the sensor output to
perform the desired normalization in the ADC itself.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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