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From: Mario Trams
Subject: Re: 5V Reference Voltage for AD7707 AD-Converter??
Date: Tue, 10 Sep 2002 17:48:15 +0200
Organization: TU Chemnitz
References: <3D7CB0A6.9C1F804E@informatik.tu-chemnitz.de> <3D7D77DE.1C671621@webaccess.net>
NNTP-Posting-Date: 10 Sep 2002 15:41:03 GMT
X-Mailer: Mozilla 4.79 [en] (X11; U; Linux 2.4.18-5 i686)
thank you very much for your advices. Further comments below.
Chuck Simmons wrote:
> Mario Trams wrote:
> > ...
> > 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
> > performance".
> > It is left unclear how much we are "off the specified performance"
> > with a 5V reference (at least, 5V are permitted by the datasheet).
> > ...
> 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
That's what I thought as well. Regarding the _two_ buffers you
are they really required? (I'm sorry for this probably stupid question,
but I'm not _that_ expert in analog issues...)
> 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.
From an economical point of view using the high-level input of the
seems to be the best way as the external buffer (or two...) could be
saved. Only the reference voltage had to be halved from the sensor
However, the noise of the high-level input appears to be somewhat higher
than for the low-level inputs.
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