Cirrus Logic CS5508 - Manual (Page 16)

External reference voltages can range from 1.0
volt minimum to 3.6 volts maximum. The com-
mon mode voltage range of the external
reference can allow the reference to lie at any
voltage between the VA+ and VA- supply rails.
Figures 5 and 6 illustrate how the CS5505/6/7/8
converters are connected for external and for in-
ternal voltage reference use, respectively.
Analog Input Range
The analog input range is set by the magnitude
of the voltage between the VREF+ and VREF-
pins. In unipolar mode the input range will equal
the magnitude of the voltage reference. In bipo-
lar mode the input voltage range will equate to
plus and minus the magnitude of the voltage ref-
erence. While the voltage reference can be as
great as 3.6 volts, its common mode voltage can
be any value as long as the reference inputs
VREF+ and VREF- stay within the supply volt-
ages for the A/D. The differential input voltage
can also have any common mode value as long
as the maximum signal magnitude stays within
the supply voltages.
The A/D converter is intended to measure dc or
low frequency inputs. It is designed to yield ac-
curate conversions even with noise exceeding
the input voltage range as long as the spectral
components of this noise will be filtered out by
the digital filter. For example, with a 3.0 volt
reference in unipolar mode, the converter will
accurately convert an input dc signal up to
3.0 volts with up to 15% overrange for 60 Hz
noise. A 3.0 volt dc signal could have a 60 Hz
component which is 0.5 volts above the maxi-
mum input of 3.0 (3.5 volts peak; 3.0 volts dc
plus 0.5 volts peak noise) and still accurately
convert the input signal (XIN = 32.768 kHz).
This assumes that the signal plus noise ampli-
tude stays within the supply voltages.
The CS5505/6/7/8 converters output data in bi-
nary format when converting unipolar signals
and in offset binary format when converting bi-
polar signals. Table 2 outlines the output coding
for the 16-bit CS5505/7 and the 20-bit CS5506/8
in both unipolar and bipolar measurement
modes.
CS5505 and CS5507 (16 Bit) CS5506 and CS5508 (20 Bit)
Unipolar Input
Voltage
Output
Codes
Bipolar Input
Voltage
Unipolar Input
Voltage
Output
Codes
Bipolar Input
Voltage
>(VREF - 1.5 LSB) FFFF >(VREF - 1.5 LSB) >(VREF - 1.5 LSB) FFFFF >(VREF - 1.5 LSB)
VREF - 1.5 LSB FFFF
FFFE
VREF - 1.5 LSB VREF - 1.5 LSB FFFFF
FFFFE VREF - 1.5 LSB
VREF/2 - 0.5 LSB 8000
7FFF
-0.5 LSB VREF/2 - 0.5 LSB 80000
7FFFF
-0.5 LSB
+0.5 LSB 0001
0000 -VREF + 0.5 LSB +0.5 LSB 00001
00000 -VREF + 0.5 LSB
<(+0.5 LSB) 0000 <(-VREF + 0.5 LSB) <(+0.5 LSB) 00000 <(-VREF + 0.5 LSB)
Note: VREF = (VREF+) - (VREF-); Table excludes common mode voltage on the signal and reference inputs.
Table 2. Output Coding
-VA
+VA
0.1 µF
CS5505/6/7/8
VA+
VREF+
VREF-
VREFOUT
VA-
Figure 6. Internal Reference Connections
CS5505/6/7/8
16 DS59F7
CS5505/6/7/8
16 DS59F7