Cirrus Logic AN31 - Manual (Page 10)

12.5 mV (considered the nominal value for this
particular selection of PGA gain and VREF
voltage) can be gain calibrated for input signals
from 20 % less than or 20 % greater than the
nominal 12.5 mV value. In other words, the gain
can be calibrated with an input as low as 10 mV
or as high as 15 mV when the nominal value is
set for 12.5 mV. The nominal input can be
changed by changing the PGA gain or by
changing the divider resistors for the excitation
voltage. The converter can accept a VREF input
voltage of any value between 2.0 to 3.8 V.
The CS5516 and CS5520 can be operated on
any clock frequency from 1.0 MHz to 5.0 MHz.
the digital filter will give greater than 90 dB of
attenuation to 50 and 60 Hz line interference if
the input clock is 4.096 mHz or less. With a
4.096 MHz clock into the converter it will
output conversion words at a 50 Hz rate. For
optimal filtering it is desirable to average output
words from the converter. If ten output words
are averaged, the noise bandwidth is reduced to
about 2.5 Hz.
The CS5516 and CS5520 support either dc or ac
bridge excitation.
AC-EXCITED OR CHOPPED SIGNAL
BRIDGE CIRCUITS
When measuring low level signals,
measurement performance can be enhanced if
the signal is "chopped". Chopper amplifiers are
commonly used to minimize amplifier offset
drift. The disadvantage of chopper amplifiers is
that they are generally manufactured using
CMOS technology and have higher thermal
noise than bipolar amplifiers. Chopper amplifiers
have low offset drift and the 1/f noise of the
amplifier tends to be averaged out due to
chopping. Still, CMOS integrated circuit chopper
amplifiers tend to have noise performance
somewhere between 45 nV/
√
Hz to
250 nV/
√
Hz. This noise limits the measurement
performance when used in a high resolution
bridge digitizer. Another limitation of a chopper
amplifier is that it corrects only its own offset
errors and does not correct offsets or parasitic
thermocouples external to itself, including those
created when its own package leads connect to
the circuit card traces.
There are several other approaches to chopping
the signal which can be used to enhance
performance. Circuits will illustrate a number of
these approaches. Some chop the signal after it
is output from the bridge. Others actually switch
the polarity to the bridge itself. Either method
can be used to remove amplifier offsets and
parasitic thermocouple effects. Switching the
bridge has the advantage that it enables any
nonratiometric offset of the bridge to be
removed. But caution is advised; some silicon
gages can be damaged if the excitation supply is
reversed. Check with the gage manufacturer to
determine if a silicon gage bridge can be used
with AC excitation.
Switching the bridge may not be practical in
applications which have very long cables due to
the large cable capacitance.
Bridge with Digital Offset Correction and
Kelvin Reference Sensing.
One method of getting the lower noise of bipolar
amplifiers and achieve good offset stability is to
use digital offset correction. Figure 9 illustrates a
circuit in which the input of the amplifier stage
is periodically shorted and the offset measured
with the A/D. The digital code is then used to
correct readings from the converter when the
signal is being measured. The schematic shows
only the circuitry for one channel (the CS5504
has two input channels). Only one half of the
DG303 is used per channel. LT1007 op amps are
used for their low noise; but a dual LT1013 or
quad LT1014 could be used if higher noise is
acceptable. The LT1013 and LT1014 are capable
of measuring signals with an input range which
Bridge Transducer Digitizer Circuits
10 AN31REV3