CS5541 datasheet(11/26 Pages) CIRRUS

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CS5541

DS500PP1

2.2 Voltage Reference Input

The differential voltage between VREF+ and

VREF- sets the nominal full scale input range of

the converter. For a single-ended reference voltage,

the reference output is connected to the VREF+ pin

of the CS5541 and the ground reference is connect-

ed to the VREF- pin. Note that the differential ref-

erence voltage can be from 0.1 V to ((VA+)-

(VA-)). The noise-free resolution of a single sam-

ple from the ADC is directly proportional to the

voltage reference as depicted in Figures 6 and 7.

Note:

When a lower reference voltage is used, the

resulting code widths are smaller. Since the

output codes exhibit more changing codes for

a fixed amount of noise, the converter

appears noisier.

2.2.1 Voltage Reference Input Model

Figure 8 illustrates the input models for the VREF

pins. It includes a coarse/fine charge buffer which

reduces the dynamic current demand on the exter-

nal reference. The reference’s buffer is designed to

accommodate rail-to-rail (common-mode plus sig-

nal) input voltages. Typical CVF (sampling) cur-

rent is about 20 nA (MCLK = 32.768 kHz; see

Figure 8).

00.511.5

2.5

VREF (V)

Figure 6. Typical Noise-Free Resolution

vs. Voltage Reference

One-Time Cal, 4 Cycle Settling, 50/60 Hz Reject

Noise-Free Res. = log2 (Bipolar Span/6.6*RMS Noise)

0.5

1.5

2.5

VREF (V)

Figure 7. Typical Noise-Free Resolution

vs. Voltage Reference

Continuous Cal, 1Cycle Settling, 50/60 Hz Reject

Noise-Free Res. = log2 (Bipolar Span/6.6*RMS Noise)

VREF

C = 12 pF

f = 2*MCLK = 65.536 kHz

φ Coarse

φ Fine

n = CV osf

os ≤ 25mV

Figure 8. Input model for VREF+ and VREF- pins.

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