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CS5525-BS

16-Bit/20-Bit Multi-Range ADC with 4-Bit Latch

Cirrus Logic 

CS5525 CS5526

Analog Input

Figure 7 illustrates a block diagram of the analog in-

put signal path inside the CS5525/26. The front end

consists of a chopper-stabilized instrumentation am-

plifier with 20X gain and a programmable gain sec-

tion. The instrumentation amplifier is powered from

VA+ and from the NBV (Negative Bias Voltage) pin

allowing the CS5525/26 to be operated in either of

two analog input configurations. The NBV pin can

be biased to a negative voltage between -1.8 V and

-2.5 V, or tied to AGND. The choice of the operating

mode for the NBV voltage depends upon the input

signal and its common mode voltage.

For the 25 mV, 55 mV, and 100 mV input ranges, the

input signals to AIN+ and AIN- are amplified by the

20X instrumentation amplifier. For ground refer-

enced signals with magnitudes less then 100 mV, the

NBV pin should be biased with -1.8 V to -2.5 V. If

NBV is tied between -1.8 V and -2.5 V, the (Com-

mon Mode + Signal) input on AIN+ and AIN- must

stay between -0.150 V and 0.950 V to ensure prop-

er operation. Alternatively, NBV can be tied to

AGND where the input (Common Mode + Signal)

on AIN+ and AIN- must stay between 1.85 V and

2.65 V to ensure that the amplifier operates prop-

erly.

For the 1 V, 2.5 V, and 5 V input ranges, the instru-

mentation amplifier is bypassed and the input sig-

nals are directly connected to the Programmable

Gain block. With NBV tied between -1.8 V and

-2.5 V, the (Common Mode + Signal) input on

AIN+ and AIN- must stay between NBV and VA+.

Alternatively, NBV can be tied to AGND where

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

AIN- pins can span the entire range between

AGND and VA+.

The CS5525/26 can accommodate full scale ranges

other than 25 mV, 55 mV, 100 mV, 1 V, 2.5 V and

5 V by performing a system calibration within the

limits specified. See the Calibration section for

more details. Another way to change the full scale

range is to increase or to decrease the voltage refer-

ence to other than 2.5 V. See the Voltage Refer-

ence 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’ table on page 3). Additionally, the

differential 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

VREF+ VREF-

AIN +

AIN -

NBV

14

D iffere ntial 4 th

D igital F ilter

X20

Program m able

order delta-

G ain

sigm a m odulator

Figure 7. Block Diagram of Analog Signal Path

DS202F3

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