Cirrus Logic AN31 - Manual (Page 14)

issued to the converter only after the DG303
switch has been switched to one position long
enough for the buffer amplifier to have settled
on the signal. With the DG303 in one position,
the output of the amplifier will result in a
positive voltage into the converter; when
switched to the other position the output of the
amplifier will be negative into the converter.
The negative reading is then subtracted from the
positive reading and then divided by
two[(+answer - (-answer)]/2. The result will be a
reading of the load cell signal with the offset of
the amplifier removed. For example, let us
assume the circuitry has +332 counts of offset
and the signal from the bridge (the bridge itself
has no offset for illustration purposes) should be
4700 counts. The reading from the converter
with a positive input signal will be 5032 counts;
the reading with the signal reversed will be
-4368 counts. [5032 - (-4368)]/2 = 4700 counts,
which is the answer with the offset averaged out.
Note that dividing by two is really unnecessary
as the number (9400) is representative of the
signal magnitude. The converter can sample at
200 samples per second; performing a
conversion every 5 msec. The converter has two
channels but needs not to measure the
temperature channel very often. The
measurement sequence for channel one is
follows: Switch the DG303 to condition one
(switches 1 and 2 are on, switches 3 and 4 are
off); perform a conversion but throw the data
away as this conversion time is used to allow the
amplifier to settle (the circuit shown takes less
than 4 msec. to settle). Then perform a second
conversion and keep the data. Switch the DG303
to condition two (switches 3 and 4 are on, 1 and
2 are off); perform a conversion but throw away
the data to allow for settling. Then perform a
second conversion, subtract the negative answer
from the previous positive one (from switch
condition one) and divide the answer by two (if
you need the actual answer). Since it will take
four conversion cycles to obtain one averaged
answer, the converter will be able to update at a
50 Hz rate (assuming the temperature channel is not being read). The effects of noise in the
output data can be reduced if words are
averaged. An average of 20 of the final readings
will result in a noise reduction of 4.4 times.
Converting in this fashion will result in a
converter with greater than 150,000 noise-free
counts, and an update rate of about two and a
half times per second. Chopping the signal
lowers the input drift in the amplifier to about
125 nV peak-to-peak under slowly varying
temperature conditions.
Switched Bridge with CS5504 Using +10 V
Analog Supply
The previous circuit achieved offset stability by
chopping the bridge output. In the circuit in
Figure 11 the polarity of the excitation voltage to
the bridge is periodically reversed. Channel one
of the CS5504 is used to measure the amplified
signal from the bridge. The second channel of
the converter is used to measure the magnitude
of the bridge excitation. The bridge excitation is
measured because the driver exhibits some
change in drive output over temperature. The
measurement sequence is as follows. For
notation let the bridge excitation be in position
one when the top of the bridge is +10 V (the
actual voltage will be about 9.5 to 9.8 V
depending upon the driver source impedance).
When switched to this position, the
microcontroller pauses for a short delay (1 msec
or so) before performing a conversion on
channel two to ensure that the circuit has settled.
Once the conversion is performed on channel
two, the data is saved. Then the A0 line to the
converter is switched to select channel one. The
amplifier has settled during the time the
conversion was performed on channel two. A
conversion is performed on channel one and the
data is saved. Then the bridge excitation is
flipped to position two ( the top of the bridge is
grounded). After a 1 msec delay a conversion is
performed on channel two; the negative answer
is subtracted from the previously collected
positive answer from channel two. Then A0 on
Bridge Transducer Digitizer Circuits
14 AN31REV3