Cirrus Logic AN31 - Manual (Page 3)

elements of the amplifier stage. Note that an
instrumentation amplifier is not needed because
the A/D input is fully differential. The dual
amplifier functions as a differential in,
differential out amplifier. The circuit yields
about 9000 noise-free counts when measuring
unipolar signals. Averaging 10 samples increases
this to about 28,500 noise-free counts.
"Noise-free counts" means full scale signal
divided by six times the rms noise. Noise-free
counts is good figure of merit for comparing
A/D converters used in dc measurement
applications. There is more discussion on this
topic at the end of the application note.
The circuit illustrated uses a 3 mV/V transducer
excited with +5 V for a full scale transducer
output of 15 mV. The transducer output is
amplified with a gain of about 216 to yield
3.25 V full scale. A dual stage amplifier, as
shown in Figure 2 may be preferred to minimize
errors due to limited loop gain. The A/D is
operated in bipolar mode to achieve more
µ
V/LSB. The reference voltage for the converter
is derived from the +5 V excitation voltage. The
measurement remains ratiometric should the
+5 V excitation change.
Figure 1 includes potentiometers for offset and
gain adjustment, as do a number of other circuits
in this application note. Many system designers
prefer to eliminate potentiometers and do all
offset and gain correction in software. To
achieve this in some of the circuits may require
changes to gain stages or voltage references, but
potentiometers are shown for all the engineers
who are more comfortable with screwdrivers
than software.
The CS5509 in Figure 1 can run as fast as 200
conversions per second if operated with a
330kHz external clock. Figure 3 shows an RC
gate oscillator which can produce stable
frequencies, or a CMOS 555 timer can be used.
The gate oscillator can be operated from either a
+5 or +3.3 V supply and maintains fairly good
frequency stability over temperature.
+5
10k
1k
10k
20k
20k
4.32k
x216
Figure 2. Dual Stage Amplifier
0.1
+3.3 to +5
R C 2
R 1
10
output
74HC04
3
R
≈
R 2 1
f
≈ 1.44
2 (R + R ) C
1 2
f
162kHz
200kHz
330kHz
C
330pF
330pF
330pF
R
10k
8.2k
5k
1
R
3.4k
2.7k
1.6k
2
Figure 3. Temperature-Stable Gate Oscillator for +5 or +3.3 Volts.
Bridge Transducer Digitizer Circuits
AN31REV3 3