CS5525 CS5526

DS202F5

15

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 percent

to 70 percent 1’s density) is determined by the

VREF voltage divided by the Gain Factor. See Ta-

ble 3 to determine if the CS5525/26 are being used

properly. For example, in the 55 mV range to de-

termine the nominal input voltage to the modulator,

divide VREF (2.5 V) by the Gain Factor (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 3 is based upon

a VREF = 2.5 V. For other values of VREF, the val-

ues in Table 3 must be scaled accordingly.

Figure’s 8 and 9 illustrate the input models for the

AIN and VREF pins. The dynamic input current for

each of the pins can be determined from the models

shown and is dependent upon the setting of the CFS

(Chop Frequency Select) bit. The effective input

impedance for the AIN+ and AIN- pins remains

constant for the three low level measurement rang-

es (25 mV, 55 mV, and 100 mV). The input current

is lowest with the CFS bit cleared to logic 0.

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

output word rates greater than 60 Sps if CFS is logic 0. By set-

ting CFS to logic 1, the amplifier’s chop frequency chops at

32768 Hz eliminating the residual noise, but increasing the

current. Note that C=48pF is for input current modeling only.

For physical input capacitance see ‘Input Capacitance’ spec-

ification under ‘Analog Characteristics’ on page 3.

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.

Input Range(1)

Max. Differential Output

20X Amplifier

VREF

Gain Factor

∆-Σ Nominal(1)

Differential Input

∆-Σ(1)

Max. Input

± 25 mV

2.8 V (2)

2.5V

5

± 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

± 5.0 V

-

2.5V

0.5

± 5.0 V

0V, VA+

Table 3. Relationship between Full Scale Input, Gain Factors, and Internal Analog Signal Limitations

AIN

25m V , 55m V , and 100m V R anges

V

≤ 25m V

i = fV

C

os

os

n

C = 48pF

C F S = 0 , f = 2 5 6 H z

C F S = 1 , f = 3 2 .7 6 8 kH z

AIN +

1V , 2 .5 V , and 5V R a nges

C = 32pF

i = [(V

) - (V

)] fC

n

AIN +

A IN -

AIN -

f = 32.768 kH z

Figure 8. Input models for AIN+ and AIN- pins

VR E F +

C = 16pF

VR E F -

i

= [(V R E F + ) - (V R E F -)] fC

n

f = 32.768 k H z

Figure 9. Input model for VREF+ and VREF- pins.