CS5371A-ISZ(2006) View Datasheet(PDF) - Cirrus Logic

Part Name

Description

MFG CO.

CS5371A-ISZ
(Rev.:2006)

Low-power, High-performance ∆Σ Modulators

Cirrus Logic 

CS5371A CS5372A

4.3 Anti-alias Filter

The modulator inputs are required to be band-

width limited to ensure modulator loop stability

and prevent high-frequency signals from alias-

ing into the measurement bandwidth. The use

of simple, single-pole, differential, low-pass

RC filters across the INR± and INF± inputs en-

sures high-frequency signals are rejected be-

fore they can alias into the measurement

bandwidth.

The CS3301A / CS3302A differential amplifi-

ers are designed with separate rough and fine

analog outputs (OUTR±, OUTF±) that match

the modulator rough and fine inputs (INR±,

INF±). External anti-alias series resistors and

external differential capacitors are required to

create the anti-alias RC filters.

The approximate -3 dB corner of the input anti-

alias filter is nominally set to the internal ana-

log sampling rate divided by 64, which itself is

a division by 4 of the MCLK rate.

• MCLK Frequency = 2.048 MHz

• Sampling Frequency = MCLK / 4 = 512 kHz

• -3 dB Filter Corner = Sampling Freq / 64 = 8 kHz

• RC filter = 1 / [ 2π x (2 x Rseries) x Cdiff ] ~ 8 kHz

Figure 9 on page 14 illustrates the CS5371A and

CS5372A modulator analog connections with

input anti-alias filter components. Filter com-

ponents on the rough and fine pins should be

identical values for optimum performance, with

the capacitor values a minimum of 0.02 µF.

The rough input can use either X7R- or C0G-

type capacitors, while the fine input requires

C0G-type capacitors for optimal linearity. Us-

ing X7R-type capacitors on the fine analog in-

puts will significantly degrade total harmonic

distortion performance.

4.4 Analog Differential Signals

Differential analog signals into the CS5371A

and CS5372A consist of two halves with equal

but opposite magnitude varying about a com-

mon mode voltage. A full-scale, 5 VP-P, differ-

ential signal centered on a -0.15 V common

mode voltage will have:

SIG+ = -0.15 V + 1.25 V = +1.1 V

SIG- = -0.15 V - 1.25 V = -1.4 V

SIG+ is +2.5 V relative to SIG-

For the opposite case:

SIG+ = -0.15 V - 1.25 V = -1.4 V

SIG- = -0.15 V + 1.25 V = +1.1 V

SIG+ is -2.5 V relative to SIG-

So the total swing for SIG+ relative to SIG- is

(+2.5 V) – (-2.5 V) = 5 Vp-p differential. A simi-

lar calculation can be done for SIG- relative to

SIG+.

It’s important to note that a 5 Vp-p differential

signal centered on a -0.15 V common mode

voltage never exceeds +1.1 V with respect to

ground and never drops below -1.4 V with re-

spect to ground on either half. By definition,

differential voltages are measured with re-

spect to the opposite half, not relative to

ground. A voltmeter differentially measuring

between SIG+ and SIG- in the above example

would correctly read 1.767 Vrms, or 5 Vp-p.

18

DS748F1

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