CS5504 datasheet(13/24 Pages) CIRRUS

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Converter Performance

The CS5504 A/D converter has excellent linear-

ity performance.

Calibration minimizes the

errors in offset and gain.

The CS5504 device

has no missing code performance to 20-bits.

The converter achieves Common Mode Rejec-

tion (CMR) at dc of 105 dB typical, and CMR at

50 and 60 Hz of 120 dB typical.

The CS5504 can experience some drift as tem-

p e ra tu re

c ha n ge s .

Th e

CS5 504

uses

chopper-stabilized techniques to minimize drift.

Measurement errors due to offset or gain drift

can be eliminated at any time by recalibrating

the converter.

Analog Input Impedance Considerations

The analog input of the CS5504 can be modeled

as illustrated in Figure 4 (the model ignores the

multiplexer switch resistance). Capacitors (15 pF

each) are used to dynamically sample each of

the inputs (AIN+ and AIN-). Every half XIN cy-

cle the switch alternately connects the capacitor

to the output of the buffer and then directly to

the AIN pin. Whenever the sample capacitor is

switched from the output of the buffer to the

AIN pin, a small packet of charge (a dynamic

demand of current) is required from the input

source to settle the voltage of the sample capaci-

tor to its final value. The voltage on the output

of the buffer may differ up to 100 mV from the

actual input voltage due to the offset voltage of

the buffer. Timing allows one half of a XIN

clock cycle for the voltage on the sample capaci-

tor to settle to its final value.

An equation for the maximum acceptable source

resistance is derived.

Rsmax =

−1

2XIN

(15pF + CEXT ) ln









Ve +

15pF

(100mv)

(15pF + CEXT )









This equation assumes that the offset voltage of

the buffer is 100 mV, which is the worst case.

The value of Ve is the maximum error voltage

which is acceptable. CEXT is the combination

of any external or stray capacitance.

For a maximum error voltage (Ve) of 600 nV in

the CS5504 (1/4LSB at 20-bits), the above equa-

tion indicates that when operating from a

32.768 kHz XIN, source resistances up to 84 k

Ω

in the CS5504 are acceptable in the absence of

external capacitance (CEXT = 0).

The VREF+ and VREF- inputs have nearly the

same structure as the AIN+ and AIN- inputs.

Therefore, the discussion on analog input imped-

ance applies to the voltage reference inputs as

well.

Digital Filter Characteristics

The digital filter in the CS5504 is the combina-

tion of a comb filter and a low pass filter. The

comb filter has zeros in its transfer function

which are optimally placed to reject line interfer-

ence frequencies (50 and 60 Hz and their

multiples) when the CS5504 is clocked at

15 pF

Vos ≤ 100 mV

Internal

Bias

Voltage

15 pF

AIN+

AIN-

Figure 4. Analog Input Model

CS5504

DS126F1

CS5504

DS126F2

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