Cs5516 – Cirrus Logic AN31 User Manual

Page 20

decade divider (74HC4017) can be followed by
a binary 2

divider (74HC4020) to yield a

25 Hz excitation frequency. The converter will
output conversion words at a 50 Hz rate, or two
output words for each one cycle of the bridge
excitation. The 25 Hz excitation reduces the
switching frequency of the bridge so the circuit
spends more time measuring and less time
settling. This will improve measurement
performance, but multiple output words (an even
number of them) must be averaged to ensure

equal samples for both polarities of the
excitation clock. Figure 15 illustrates this
circuit. Note that the details on connecting the
clock divider chips have not been shown to
simplify the schematic.

CS5516/CS5520 with AC-Excitation
Controlled by a Microcontroller

If the load cell cables are very long, the
capacitance may be so large that the circuit
cannot settle and yield an accurate result with
the 25 Hz circuit. Another option exist. Rather
than use the counters in Figure 15 to control the
BX1 signal and the drive polarity, one can use a
microcontroller output line. With the converter
set up in the external excitation mode, the
microcontroller can control the polarity of the

100k

10k

4.096 MHz

BX2

XIN

XOUT

TP0610

0.1

VREF-

VREF+

7.5k

-5

301

4.7 nF

AIN+

AGND1

AGND2

AIN-

MICREL

MIC4428

Bridge

Sync

25X

Gain

Block

1,2,4,8

Calibration

BX1

470pF

74HC4017

74HC4020

25Hz

CS5520

16 or 20 bits

301

4.7 nF

CS5516

See Text

for another

control option.

25 Conversions/sec averaging required

Figure 15. CS5516 with External 25 Hz AC Excitation.

Bridge Transducer Digitizer Circuits

AN31REV3