CS5521/22/23/24/28
2.1.3 Analog Input Span Considerations
The CS5521/22/23/24/28 is designed to measure
full scale ranges of 25 mV, 55 mV, 100 mV, 1 V,
2.5 V and 5 V. Other full scale values can be ac-
commodated by performing a system calibration
within the limits specified. See the
sec-
tion for more details. Another way to change the
full scale range is to increase or to decrease the
voltage reference to a voltage other than 2.5 . See
the
section for more details.
Three factors set the operating limits for the input
span. They include: instrumentation amplifier satu-
ration, modulator 1’s density, and a lower reference
voltage. When the 25 mV, 55 mV or 100 mV range
is selected, the input signal (including the common
mode voltage and the amplifier offset voltage)
must not cause the 20X amplifier to saturate in ei-
ther its input stage or output stage. To prevent sat-
uration the absolute voltages on AIN+ and AIN-
must stay within the limits specified (refer to the
section). Additionally, the differen-
tial output voltage of the amplifier must not exceed
2.8 V. The equation
ABS(VIN + VOS) x 20 = 2.8 V
defines the differential output limit, where
VIN = (AIN+) - (AIN-)
Input Range
±
25 mV
±
55 mV
±
100 mV
±
1.0 V
±
2.5 V
±
5.0 V
Max. Differential Output
20X Amplifier
2.8 V
2.8 V
2.8 V
-
-
-
VREF
2.5V
2.5V
2.5V
2.5V
2.5V
2.5V
is the differential input voltage and VOS is the ab-
solute maximum offset voltage for the instrumenta-
tion amplifier (VOS will not exceed 40 mV). If the
differential output voltage from the amplifier ex-
ceeds 2.8 V, the amplifier may saturate, which will
cause a measurement error.
The input voltage into the modulator must not
cause the modulator to exceed a low of 20 percent
or a high of 80 percent 1's density. The nominal full
scale input span of the modulator (from 30 percent
to 70 percent 1’s density) is determined by the
VREF voltage divided by the Gain Factor. See
Table 1 to determine if the CS5521/22/23/24/28
are being used properly. For example, in the
55 mV range, to determine the nominal input volt-
age to the modulator, divide VREF (2.5 V) by the
Gain Factor (2.2727).
When a smaller voltage reference is used, the re-
sulting code widths are smaller causing the con-
verter output codes to exhibit more changing codes
for a fixed amount of noise. Table 1 is based upon
a VREF = 2.5 V. For other values of VREF, the
values in Table 1 must be scaled accordingly.
2.1.4 Measuring Voltages Higher than 5 V
Some systems require the measurement of voltages
greater than 5 V. The input current of the instru-
Gain Factor
5
2.272727...
1.25
2.5
1.0
0.5
∆-Σ
Nominal
Differential Input
±
0.5 V
±
1.1 V
±
2.0 V
±
1.0 V
±
2.5 V
±
5.0 V
∆-Σ
Max. Input
±
0.75 V
±
1.65 V
±
3.0 V
±
1.5 V
±
5.0 V
0V, VA+
Table 1. Relationship between Full Scale Input, Gain Factors, and Internal Analog
Signal Limitations
Note:
1. The converter’s actual input range, the delta-sigma’s nominal full scale input, and the delta-sigma’s
maximum full scale input all scale directly with the value of the voltage reference. The values in the
table assume a 2.5 V VREF voltage.
2. The 2.8 V limit at the output of the 20X amplifier is the differential output voltage.
DS317F2
15
CIRRUS [ CIRRUS LOGIC ]
