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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

2.1.1 Instrumentation Amplifier

The instrumentation amplifier is chopper stabilized

and is activated any time conversions are performed

with the low level input ranges, ≤100 mV. The am-

plifier is powered from VA+ and from the NBV

(Negative Bias Voltage) pin allowing the

CS5521/22/23/24/28 to be operated in either of two

analog input configurations. The NBV pin can be bi-

ased to a negative voltage between -1.8 V and -

2.5 V, or tied to AGND (for the CS5528, NBV has

to be between -1.8 V and -2.5 V for the ranges below

100 mV when the amplifier is engaged). The com-

mon-mode plus signal range of the instrumentation

amplifier is 1.85 V to 2.65 V with NBV grounded.

The common-mode plus signal range of the instru-

mentation amplifier is -0.150 V to 0.950 V with

NBV between -1.8 V to -2.5 V. Whether NBV is

tied between -1.8 V and -2.5 V or tied to AGND,

the (Common Mode + Signal) input on AIN+ and

AIN- must stay between NBV and VA+.

Figure 5 illustrates an analog input model for the

ADCs when the instrumentation amplifier is en-

gaged. The CVF (sampling) input current for each

of the analog input pins depends on the CFS1 and

CFS0 (Chop Frequency Select) bits in the configu-

ration register (see Configuration Register for de-

tails). Note that the CVF current is lowest with the

CFS bits in their default states (cleared to logic 0s).

Further note that the CVF current into the instru-

mentation amplifier is less than 300 pA over -40°C

to +85°C. Note that Figure 5 is for input current

modeling only. For physical input capacitance see

‘Input Capacitance’ specification under ANALOG

CHARACTERISTICS. Also refer to Applications

Note AN30 “Switched-Capacitor A/D Converter

Input Structures” for more details on input models

and input sampling currents.

Note: Residual noise appears in the converter’s baseband for

output word rates greater than 61.6 Hz if the CFS bits

are logic 0 (chop clock = 256 Hz). For word rates of

30 Hz and lower, 256 Hz chopping is recommended,

and for 61.6 Hz, 84.5 Hz and 101.1 Hz filters, 4096 Hz

chopping is recommended.

2.1.2 Coarse/Fine Charge Buffers

The unity gain buffers are activated any time conver-

sions are performed with the high level inputs rang-

es, 1 V, 2.5 V, and 5 V. The unity gain buffers are

designed to accommodate rail to rail input signals.

The common-mode plus signal range for the unity

gain buffer amplifier is NBV to VA+.

Typical CVF (sampling) current for the unity gain

buffer amplifiers is about 10 nA

(XIN = 32.768 kHz, see Figure 6).

25 mV, 55 mV, and 100 mV Ranges

AIN

C = 48 pF

Vos ≤ 25 mV

in = fVos C

CFS1/CFS0 = 00, f = 256 Hz

CFS1/CFS0 = 01, f = 4096 Hz

CFS1/CFS0 = 10, f = 16.384 kHz

CFS1/CFS0 = 11, f = 1024 Hz

1 V, 2.5 V and 5 V Ranges

φ1 Fine

AIN

Vos ≤ 25 mV

in = fVos C

φ 1 Coarse

C = 20 pF

f = 32.768 kHz

Figure 5. Input Models for AIN+ and AIN- pins,

≤(100 mV Input Ranges

14

Figure 6. Input Models for AIN+ and AIN- pins,

>100 mV input ranges

DS317F2

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