Cirrus Logic AN31 - Manual (Page 4)

All of the converters (CS5504-09) can be
operated with a single +5 V supply. All of the
converters can also be operated with +5 V
analog supply and +3.3 V on the digital supply.
If this dual supply arrangement is used, the
digital supply should be derived from the analog
supply to ensure proper operation. Under all
conditions, including start-up, the voltage on the
VA+ pin must be the more positive than any
other pin on the device to ensure proper
substrate biasing of the chip.
CS5507/8 with +10 V Analog Supply and
+5 V Digital Supply
It is common for many weigh scales to be
operated from batteries with a 12 V
automotive-type battery being common. The
CS5504/5/6/7/8 devices can be operated with
higher supply voltage on the analog portion of
the chip than on the digital portion (Note: the
CS5509 is an exception and is specified with an
analog supply of +5 V only). The analog supply
(VA+) must always be the most positive voltage
on the chip to ensure proper operation. Figure 4
illustrates the CS5507 operating from +10 V on
the analog and +5 V on the digital. The bridge is
excited with the +10 V and resistors are used to
divide this excitation supply to obtain a
ratiometric voltage reference of about 3.33 V for
the converter. The circuit is designed to operate
with the A/D in bipolar mode to yield more
µ
V/count. The A/D is set-up for an input span of
±
3.33 V. A 200k pull down resistor forces a
0.047
100
75k
100
x332
0.047
0.047
75k
453
+10
+
VA+ VD+
VREF+
VREF-
AIN+
CS5508
16 or
20 bits
AIN-
VREFOUT
VA- DGND
SCLK
SDATA
CONV
M/SLP
CAL
XIN
System
Microcontroller
+5
+5 0.1
+5Volt
Regulator
0.33
DRDY
CS
BP/UP
-
350 Ω Bridge
2mV/V
≈ 3.33 V
5k
200k
+5
LT1007
LT1007
+5
10k
1k
Optional
Gain
Trim
20k
200k
1k
3.57k
Coarse
Offset
Resistor
Optional
Fine
Offset
Adjust
1
2
3
4
6
7
8
10
11
12
13
14
15 16
17
18
19
20
+10
+10
CS5507
162kHz
100 Conversions/sec
Figure 4. CS5507/8 with +10V Analog Supply and +5 V Digital Supply.
Bridge Transducer Digitizer Circuits
4 AN31REV3