CS5521 datasheet(16/56 Pages) CIRRUS

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CS5521/22/23/24/28

DS317F2

mentation amplifier, typically 100 pA, is low

enough to permit large external resistors to divide

down a large external signal without significant

loading. Figure 7 illustrates an example circuit. Re-

fer to Applications Note 158 for more details on

high voltage (>5 V) measurement.

2.1.5 Voltage Reference

The CS5521/22/23/24/28 are specified for opera-

tion with a 2.5 V reference voltage between the

VREF+ and VREF- pins of the device. For a single-

ended reference voltage, such as the LT1019-2.5,

the reference voltage is input into the VREF+ pin

of the converter and the VREF- pin is grounded.

The differential voltage between the VREF+ and

VREF- can be any voltage from 1.0 V up to VA+,

however, the VREF+ cannot go above VA+ and the

VREF- pin can not go below NBV.

Figure 8 illustrates the input models for the VREF

pins. The dynamic input current for each of the pins

can be determined from the models shown.

2.2 Overview of ADC Register Structure

and Operating Modes

The CS5521/22/23/24/28 ADCs have an on-chip

controller, which includes a number of user-acces-

sible registers. The registers are used to hold offset

and gain calibration results, configure the chip’s

operating modes, hold conversion instructions, and

to store conversion data words. Figure 9 depicts a

block diagram of the on-chip controller’s internal

registers for the CS5523/24.

Each of the converters has 24-bit registers to func-

tion as offset and gain calibration registers for each

channel. The converters with two channels have

two offset and two gain calibration registers, the

converters with four channels have four offset and

four gain calibration registers, and the eight chan-

nel converter has eight offset and eight gain cali-

bration registers. These registers hold calibration

results. The contents of these registers can be read

or written by the user. This allows calibration data

to be off-loaded into an external EEPROM. The

user can also manipulate the contents of these reg-

isters to modify the offset or the gain slope of the

converter.

The converters include a 24-bit configuration reg-

ister of which 17 of the bits are used for setting op-

tions such as the conversion mode, operating power

options, setting the chop clock rate of the instru-

mentation amplifier, and providing a number of

flags which indicate converter operation.

Voltage

Divider

PGIA set for

+ 100 mV

±10V

Charge Pump

Regulator

∆Σ ADC

PGIA

+5 V

2.5 V

VA+

VREF+

VREF-

VD+

NBV

≈ -2.1 V

µF

0.033

µF

CPD

0.1

µF

Ω

0.1

µF

1N4148

BAT85

Charge Pump

Circuitry

DGND

chop clock = 256 Hz

10 K

Ω

1 M

Ω

Figure 7. Input Ranges Greater than 5 V

VREF

C = 10pF

f = 32.768 kHz

φ Fine

≤ 25mV

i = fV

φ Coarse

Figure 8. Input Model for VREF+ and VREF- Pins

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