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CS5521
(Rev.:2000)

16-Bit or 24-Bit, 2/4/8-Channel ADCs with PGIA

Cirrus Logic 

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 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 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

Analog Input 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-)

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-

Input Range(1)

± 25 mV

± 55 mV

± 100 mV

± 1.0 V

± 2.5 V

± 5.0 V

Max. Differential Output

20X Amplifier

2.8 V (2)

2.8 V (2)

2.8 V (2)

-

-

-

VREF

2.5V

2.5V

2.5V

2.5V

2.5V

2.5V

Gain Factor

5

2.272727...

1.25

2.5

1.0

0.5

∆-Σ Nominal(1)

Differential Input

± 0.5 V

± 1.1 V

± 2.0 V

± 1.0 V

± 2.5 V

± 5.0 V

Table 1. Relationship between Full Scale Input, Gain Factors, and Internal Analog

Signal Limitations

∆-Σ(1)

Max. Input

± 0.75 V

± 1.65 V

± 3.0 V

± 1.5 V

± 5.0 V

0V, VA+

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

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