Cirrus Logic CS5528 - Manual (Page 15)

CS5521/22/23/24/28
DS317F8 15
1.1.3 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 Calibration 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 Voltage Reference 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 satu-
rate in either its input stage or output stage. To pre-
vent saturation, the absolute voltages on AIN+ and
AIN- must stay within the limits specified (refer to
the Analog Input section). Additionally, the differ-
ential output voltage of the amplifier must not ex-
ceed 2.8 V. The equation
ABS(VIN + VOS) x 20 = 2.8 V
defines the differential output limit, where
VIN = (AIN+) - (AIN-)
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 per-
cent 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 is
being used properly. For example, in the 55 mV
range, to determine the nominal input voltage to the
modulator, divide VREF (2.5 V) by the Gain Fac-
tor (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.
1.1.4 Measuring Voltages Higher than 5 V
Some systems require the measurement of voltages
greater than 5 V. The input current of the instru-
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.
Input Range (1)
Max. Differential Output
20X Amplifier
VREF Gain Factor
Δ-Σ Nominal (1)
Differential Input
Δ-Σ (1)
Max. Input
± 25 mV
2.8 V
(2) 2.5V 5 ± 0.5 V ± 0.75 V
± 55 mV
2.8 V (2) 2.5V 2.272727...
± 1.1 V ± 1.65 V
± 100 mV
2.8 V
(2) 2.5V 1.25 ± 2.0 V ± 3.0 V
± 1.0 V
- 2.5V 2.5
± 1.0 V ± 1.5 V
± 2.5 V - 2.5V 1.0 ± 2.5 V ± 5.0 V
± 5.0 V - 2.5V 0.5 ± 5.0 V 0V, VA+
Table 1. Relationship between Full Scale Input, Gain Factors, and Internal Analog
Signal Limitations