CS5374 datasheet(18/44 Pages) CIRRUS

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CS5374

DS862PP1

MODULATOR OPERATION

The CS5374 modulators are fourth-order

ΔΣ type

optimized for extremely high-resolution measure-

ment of signals between DC and 2000 Hz. When

combined with the internal differential amplifiers,

the CS4373A test DAC and CS5376A digital filter,

a small, low-power, self-testing, high-accuracy,

multi-channel measurement system results.

The modulators have high dynamic range and low

total harmonic distortion with very low power con-

sumption. They are optimized for extremely high-

resolution measurement of 5 Vp-p or smaller differ-

ential signals. They convert analog input signals

from the differential amplifiers to an oversampled

serial bit stream which is then passed to the digital

filter.

The companion CS5376A digital filter generates

the clock and synchronization inputs for the modu-

lators while receiving the one-bit data and over-

range flag outputs. The digital filter decimates the

modulator’s oversampled output bit stream to a

high-resolution, 24-bit output at the final selected

output word rate.

4.1 Modulator Anti-Alias Filter

The modulator inputs are required to be bandwidth

limited to ensure modulator loop stability and pre-

vent high-frequency signals from aliasing into the

measurement bandwidth. The use of simple, sin-

gle-pole, differential, low-pass RC filters across

the INR± and INF± inputs ensures high-frequency

signals are rejected before they can alias into the

measurement bandwidth.

The approximate –3 dB corner of the input anti-

alias filter is nominally set to the internal analog

sampling rate divided by 64, which itself is a divi-

sion by 4 of the MCLK rate.

Figure 1 on page 6 illustrates the CS5374 amplifi-

er-to-modulator analog connections with input

anti-alias filter components. Filter components on

the rough and fine pins should be identical values

for optimum performance, with the capacitor val-

ues 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 lin-

earity. Using X7R-type capacitors on the fine ana-

log inputs will significantly degrade total harmonic

distortion performance.

Reset, Clock,

and

Synchronization

INR1-

VREF-

MFLAG1

MCLK

MSYNC

4th Order

Modulator

INF1- INF1+INR1+

RST

VREF+

MDATA1

Figure 14. CS5374 Modulator Block Diagram

•

MCLK Frequency = 2.048 MHz

•

Sampling Frequency = MCLK / 4 = 512 kHz

•

–3 dB Filter Corner = Sampling Freq / 64 = 8 kHz

•

RC filter = 1 / [ 2

π x(2xRseries)xCdiff ] ~ 8 kHz

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