CS5521 datasheet(15/56 Pages) CIRRUS

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CS5521/22/23/24/28

DS317F8

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

± 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

2.5V

0.5

± 5.0 V

0V, VA+

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

Signal Limitations

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