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From: Mario Trams
Subject: Re: 5V Reference Voltage for AD7707 AD-Converter??
Date: Wed, 11 Sep 2002 14:56:00 +0200
Organization: TU Chemnitz
References: <3D7CB0A6.9C1F804E@informatik.tu-chemnitz.de> <3D7D77DE.1C671621@webaccess.net> <3D7E143F.4EBBAD5E@informatik.tu-chemnitz.de> <3D7E9AFD.FB928FD1@webaccess.net>
NNTP-Posting-Date: 11 Sep 2002 12:48:13 GMT
X-Mailer: Mozilla 4.79 [en] (X11; U; Linux 2.4.18-5 i686)
Chuck Simmons wrote:
> > > 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
> > > one.
> > That's what I thought as well. Regarding the _two_ buffers you
> > mentioned,
> > are they really required? (I'm sorry for this probably stupid question,
> > but I'm not _that_ expert in analog issues...)
> I don't know the exact specifications of the sensor or the AD7707. For
> the AD7707 I went by the block diagram. If your sensor has low output
> impedance and the AD7707 has high input inpedance on the direct (low
> level) inputs, no buffers are needed there.
The sensor has an output impedance of 100Ohm.
If I'm understanding the AD7707 datasheet correctly, with an external
capacitance of 0pF an external resistance of 368kOhm results in no
gain error (for a gain of 1). With a (probably more realistic)
of 50pF the external resistance is around 90kOhm.
From my point of understanding this means, the output impedance of the
sensor including a voltage divider shall not exceed 90kOhm if we assume
50pF capacitance. For instance a divider via two 10kOhm resistors would
result in an output impedance of 10kOhm+100Ohm.
This all is valid for the unbuffered mode of the AD7707.
Additionally, an input buffer can be enabled allowing much higher
> > > 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
> > AD7707
> > 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
> > supply.
> > However, the noise of the high-level input appears to be somewhat higher
> > than for the low-level inputs.
> Going back to the reference. The device may have a specification on
> current in the reference input or an impedance. If the reference input
> is specified as high impedance, it is internally buffered so you can use
> a voltage divider directly. If not, a single opamp with low offset drift
> can be used as a non-inverting buffer for the reference. There are
> several good ones available from AD, Burr-Brown (TI) and some others.
To quote the datasheet:
Both reference inputs provide a high impedance, dynamic load
similar to the analog inputs in unbuffered mode. The maximum
dc input leakage current is +/- 1 nA over temperature, and source
resistance may result in gain errors on the part. In this case, the
sampling switch resistance is 5 kOhm typ and the reference capaci-
tor (CREF) varies with gain.
So a simple voltage divider might be enough. I will experiment a
> Since the ADC is up to a 16 bit converter, your LSB is about 76
> microvolts with a divide by 2. Certain care should be taken in wiring
> for such voltage levels otherwise some of the resolution will be lost.
> Thermal voltages at interconnects are a particular problem. Use gold
> contacts or like metal contacts in any connectors (gold is to be
> preferred). Device sockets must be of the highest quality if used.
> Solder joints should be inspected carefully to make sure there are no
> cold joints or other partial contact problems. Run a separate ground
> wire for the ADC from the sensor so that power supply current is not in
> the measurement ground. That should get you close to best performance
> from the device.
These things are clear anyways. Sensor and ADC will be mounted very
close thogether on a multilayer PCB with appropriate ground planes.
So from this point of view there should be (hopefully) no problems.
Thanks again for your comments!
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