Cirrus Logic AN31 - Manual (Page 2)

offset drift, amplifier noise (both thermal and
1/f), amplifier finite open loop gain, and
parasitic thermocouples. Parasitic thermocouples
are introduced any time two dissimilar metals
are connected. For example, using tin-lead solder
to solder a wire to a copper PC trace can
introduce an unwanted thermocouple junction
which changes as much as 3
µ
V/°C when
subjected to temperature gradients.
This application note will introduce some A/D
converter circuits which illustrate a number of
application ideas to the design engineer who
uses bridge transducers. In the AC-excited
bridge section, a number of design ideas will be
introduced which offer very good solutions to
some of the problems encountered in low level
bridge measurement.
DC-EXCITED BRIDGE CIRCUITS
CS5507,8,9 Bridge Transducer Operating
From a Single +5 V Supply, or with the
Analog Supply at +5 V and Digital Supply at
+3.3 V
Figure 1 illustrates the low cost CS5509 16-bit
converter operating from +5 V. The A/D can
operate in either unipolar or bipolar mode and
yields 20 conversions/second when running from
a low cost 32.768 kHz crystal. When operated at
32.768 kHz the digital filter in the converter
notches out 50 and 60 Hz line interference.
The LTC1051 dual chopper amplifier is used as
the bridge amplifier. Bandwidth is limited to
about 3.8 Hz by the 100k and 0.47
µ
F feedback
0.1
100k
0.47
0.47
100k
+5
VA+ VD+
VREF+
VREF-
AIN+
CS5509
16 bits
AIN-
GND
SCLK
SDATA
CONV
CAL
XIN
XOUT
32.768kHz
10k
5.23k
350 350
350 350
5k
1k
3 mV/V
Transducer
3
2
1
6
5
4
7
LTC1051
≈ 3.25 V
100 pF
40.2k
-
5k
+
11 13
3
2
1
16
15
6
14
4
5
9
CS
DRDY
BP/UP
12
8
7
10
Microcontroller
System
100
100
0.1
0.1
931
Optional
Gain
Trim
500
+3.3 V
Regulator
VD+ can be
+5V or +3.3V
Optional
Zero
Trim
0.1
20 Conversions/sec.
Figure 1. CS5507,8,9 Bridge Transducer Operating from a Single +5 V Supply, or with the Analog Supply at +5 V
and Digital Supply at +3.3 V.
Bridge Transducer Digitizer Circuits
2 AN31REV3