CS5529-AP datasheet(10/30 Pages) CIRRUS

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CS5529

DS246F1

GENERAL DESCRIPTION

The CS5529 is a 16-bit

∆Σ Analog-to-Digital Con-

verter (ADC) which includes coarse/fine charge

buffers, a fourth order

∆Σ modulator, a calibration

microcontroller, eight digital filters which provide

selectable decimation rates, a 6-bit output latch,

and a three-wire serial interface. The ADC is opti-

mized to digitize unipolar or bipolar signals in in-

dustrial applications.

The digital filters provide eight selectable output

word rates (OWRs) of 1.88 Hz, 3.76 Hz, 7.51 Hz,

15.0 Hz, 30.0 Hz, 61.6 Hz, 84.5 Hz, 101.1 Hz when

operated from a 32.768 kHz watch crystal or equiv-

alent clock (output word rates can be increased by

approximately 3X by using 100 kHz clock). The

filters are designed to settle to full accuracy for the

selected output word rate in one conversion. When

operated at word rates of 15 Hz or less (XIN =

32.768 kHz), the filter rejects both 50 Hz and 60 Hz

line interference simultaneously.

Analog Input

The CS5529 provides a nominal 2.5 V input span

when the gain register is 1.0 decimal and the differ-

ential reference voltage between VREF+ and

VREF- is 2.5 V. The gain registers content is used

during calibration to set the gain slope of the

ADC’s transfer function. The differential reference

voltage magnitude and the gain register are two

factors that can be used to scale the nominal 2.5 V

input span. After reset, the gain register defaults to

1.0 decimal. In this case, the external voltage be-

tween the VREF+ pin and the VREF- pin sets the

ADC’s nominal full scale input span to 2.5 V. If a

user want to modify the input span, either the gain

to be changed. For example, if a 1.25 V reference is

used in place of the nominal 2.5 V input, the full-

scale span is cut in half. To achieve the same 1.25V

input span, the user could simply use a 2.5 V refer-

ence and modify the gain register to 2.0 decimal.

Note that to keep from saturating the analog front

end, the input span must stay at or below 1.5 times

the reference voltage. This corresponds to a gain

is used.

Note:

When a smaller 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.

Calibration can also affect the ADC’s full scale

span because system gain calibration can be used to

increase or decrease the full scale span of the

ADC’s transfer functions. At its limit, the input full

scale can be reduced to the point in which the gain

occur when the ADC is gain calibrated with an in-

put signal less than or equal to approximately 1/4 of

its nominal full scale, if the ADC does not have in-

trinsic gain error). Calibration and its effects on the

analog input span is detailed in a later section of the

data sheet.

Analog Input Model

Figure 1 illustrates the input models for the AIN

pins. The model includes a coarse/fine charge buff-

er which reduces the dynamic current demands

from the signal source. The buffer is designed to

accommodate rail to rail (common-mode plus sig-

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

rent is about 16nA (XIN = 32.768 kHz, see Figure

1). Application Note 30, “Switched-Capacitor A/D

Input Structures”, details various input architec-

tures.

Voltage Reference Input Model

Figure 2 illustrates the input models for the VREF

pins. It includes a coarse/fine charge buffer which

reduces the dynamic current demand of the exter-

nal reference. Typical CVF (sampling) current is

about 8nA (XIN = 32.768 kHz, see Figure 2).

The reference’s buffer is designed to accommodate

rail-to-rail (common-mode plus signal) input volt-

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