CS5521 datasheet(14/56 Pages) CIRRUS

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CS5521/22/23/24/28

DS317F8

1.1.1 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)

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 Sps if the CFS bits

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

30 Sps and lower, 256 Sps chopping is recommended,

and for 61.6 Sps, 84.5 Sps and 101.1 Sps word rate set-

tings, 4096 Hz chopping is recommended.

1.1.2 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

about

10 nA

(XIN = 32.768 kHz, see Figure 6).

AIN

25 m V , 5 5 m V , a nd 10 0 m V R a nge s

C= 48 p F

CF S 1 /C F S 0 = 00 , f = 2 5 6 Hz

CF S 1 /C F S 0 = 01 , f = 4 0 9 6 H z

C F S 1 /C F S 0 = 10 , f = 1 6 .38 4 k H z

CF S 1 /C F S 0 = 11 , f = 1 0 2 4 H z

≤ 25 m V

i= fV

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

≤100

mV Input Ranges

AIN

C= 2 0 p F

f= 3 2 .7 6 8 kH z

φ Coarse

φ Fine

≤ 25 m V

i= fV

1 V , 2 .5 V an d 5 V R a n ges

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

>100

mV input ranges

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