Cirrus Logic AN31 - Manual (Page 18)

Switched Bridge with CS5504 Using
±
5 V
Analog Supplies
This circuit in Figure 12 is basically identical to
the previous circuit, but is configured to run
from
±
5 V on the analog supplies.
CDB5516/20 Evaluation Board Circuit
The CDB5516 and CDB5520 evaluation boards
use the circuit in Figure 13. The CS5516 (16-bit)
and CS5520 (20-bit) converters are optimized
for bridge measurement applications. The
evaluation board comes with software which
runs on a PC-compatible computer. The
evaluation board includes a microcontroller
which communicates with the PC via the RS-232
serial port. The software allows the user to read
and write all of the registers inside the
CS5516/20 converter, perform conversions, save
conversion data to a file, and perform some
noise statistics on the captured data.
The CS5516 and CS5520 support both
dc-excited bridges and ac-excited bridges.
Figure 14 illustrates the benefit of AC excitation.
In one of the plots in Figure 14, the CS5520
converter was set up for a bipolar input span of
±
12.5 mV and dc bridge excitation.
Conversions were performed with a zero input
signal from the bridge and data was collected for
a one hour time interval. One LSB of the
CS5520 was equivalent to about 25 nV. The data
collected indicates that over the one hour period
the average value of the data drifted as much as
1.25
µ
V, or about 50 counts. The drift is due to
parasitic thermocouples in the components or
wiring of the board. The evaluation board was
open to the air. The data illustrates that the
cycling of the air conditioner induced thermal
gradients across the circuitry, changing the
voltage effects of the parasitic thermocouples in
the circuitry. The second plot in Figure 14
illustrates the stability of the data when the
converter is set up for the same operating
conditions, but with ac bridge excitation. The
plot illustrates the normal thermal noise of the
circuit but the average value remains stable over
time.
The CS5516 (16-bit) and CS5520 (20-bit) A/D
converters include an instrumentation amplifier
with X25 gain, a PGA (programmable gain
amplifier) with gains of 1, 2, 4, and 8, and a four
bit DAC which can trim out offset up to
±
200%
of the full scale signal magnitude. The input
span can be adjusted by changing either the
magnitude of the voltage at the VREF pins of
the converter or by changing the PGA gain.
In the circuit shown in Figure 13, the bridge is
excited with a 1 kHz square wave from the
MIC4428 (or the Micrel MIC4425) driver. The
driver outputs about
±
5 V. The 1 kHz drive
signal is output from the BX2 pin of the
CS5520. Control bits in a configuration register
inside the chip have been set to select internal ac
excitation with a frequency of 1 kHz (XIN =
4.096 MHz). The converter is designed to
perform synchronous detection on the AIN and
VREF input signals when operated in the ac
excitation mode. This means that the converter
measures the signal which is of the same
frequency and phase as the excitation clock
coming from the BX2 pin.
Resistors R1, R2, and R3 divide the excitation
voltage to give a 2.5 V reference signal into the
VREF pins. The input span at the AIN pins of
the converter is determined by dividing the
voltage at the VREF pins by the PGA gain and
the X25 instrumentation amplifier gain. For
example, with 2.5 V into the VREF pins, and the
PGA set to a gain of 8, the input span at the
AIN pins is 2.5/(8 X 25) = 12.5 mV in unipolar
mode or
±
12.5 mV in bipolar mode. The
converter offers several calibration features to
remove offset and to adapt the gain. The
nominal input span of 12.5 mV can be gain
calibrated for input signals within
±
20% of
Bridge Transducer Digitizer Circuits
18 AN31REV3