Cirrus Logic AN31 - Manual (Page 23)

the 5 mV span at a fifty samples per second
update rate. The CS5520 should be used if
higher resolution is desired at the 50 Hz update
rate. Averaging 25 samples will yield an output
with an effective 135,000 noise-free counts at
two updates per second; this on a 5 mV signal
span. The AIN ratiometric calibration register
inside the converter can be used to add or
subtract offset from the signal and give some
counts for zero weight underflow (if used in
unipolar mode) or some counts for full scale
overrange (if bipolar mode is used). Averaging
as many as fifty output words may be desirable
in some applications where mechanical vibration
is a problem.
CS5520 and an AC-Excited 1.9 mV/V Weigh
Platform
Figure 17 illustrates another very high resolution
digitizer. A GSE model 4444 "floating beam"
platform is used as the weigh bridge. The model
4444 has a full scale capacity of 100 pounds and
a sensitivity of 1.9 mV/V. The full-scale output
signal from the bridge is (1.9 mV/V) X 9.5 volts
excitation or about 18 mV. The 18 mV output
signal is amplified by two LT1115 amplifiers
configured as a high input impedance buffer
amplifier with a fixed gain of 8. When the 18
mV signal is amplified by 8 it yields an input
signal to the converter slightly above the
nominal value determined by the voltage
reference. The calibration features of the CS5520
enable it to accommodate input spans which are
as much as 20 % above or 20 % below the
nominal value set by the reference voltage.
Vishay resistors (R4-R6) are used in the buffer
amplifier to maintain a stable gain over
temperature. The LT1115 was chosen for its low
noise while sustaining a loop gain greater than
one million. With a X8 closed loop gain, an
open loop gain of 138 dB must be maintained.
The operational amplifier must maintain its high
open loop gain with reduced supply voltages (
±
5
V) and with environmental temperature changes.
A loop gain greater than one million ensures that
gain stability will be dictated by the gain-setting
resistors and not by limited loop gain. Offset
voltage, offset drift, bias current, and bias
current drift are unimportant when ac excitation
is used as these errors are modulated out-of-band
and filtered out by the digital filter inside the
CS5520. Thermal noise at the excitation
frequency remains as the limitation to achieving
high dynamic range. Although the LT1115 is a
very low noise amplifier, the noise in the
digitizer circuit is actually dominated by noise
referred to the buffer amplifier’s input from the
A/D. (Note that a lower cost amplifier such as
the LT1007 can be used with only a minor
increase (5%) in peak-to-peak noise). The effects
of the thermal noise can be reduced by averaging
output conversion words. With the digitizer
using the LT1115s for optimum performance,
you can capture output conversion words from
the digitizer and examine the noise content in the
50 Hz conversion words. You should capture at
least 1000 conversion words from the CS5520 to
have a large enough sample to minimize
statistical uncertainty. The input to the digitizer
should be held at a stable value while the
conversion words are captured. Once the
samples are captured, a frequency distribution of
the samples is computed and plotted.
Spreadsheets such as Lotus or Quattro can be
used to compute and plot the frequency
distribution of the data. Figure 18 illustrates the
histogram of 1000 50 Hz output samples from
the digitizer of Figure 17. The histogram
illustrates that the 50 Hz output words from the
converter have a peak-to-peak noise amplitude
which is less than 6 LSBs (least significant bits)
99% of the time. The noise in the output codes
has a Gaussian characteristic and therefore
averaging can be used to reduce its value.
Averaging samples which include Gaussian
noise will reduce the noise amplitude in
proportion to the square root of the number of
samples which are averaged together. The post
processor computes an average of 50 CS5520
output words to yield a post-filtered output word
Bridge Transducer Digitizer Circuits
AN31REV3 23