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CS5372

Low-power, High-performance ∆Σ Modulators

Cirrus Logic 

CS5371 CS5372

3. MODULATOR PERFORMANCE

Figures 4 and 5 illustrate the spectral performance

of the CS5371/72 modulators when combined with

the CS5376A or CS5378 digital filter. The plots

were created from ten averaged 1024 point FFTs.

3.1. Full-scale Signal Performance

Figure 4 illustrates the full-scale signal perfor-

mance of the CS5371/72 modulators and digital fil-

ter using a 31.25 Hz input signal and a 1000 SPS

output word rate. The outstanding full-scale signal

characteristics of the CS5371/72 modulators are

shown, with no harmonic components exceeding -

120 dB. Analysis of this data set yields a signal-to-

noise ratio (SNR) of 124.0 dB and a signal-to-dis-

tortion ratio (SDR) of 119.0 dB. Note that the full-

scale signal peak in Figure 4 shows a slightly re-

duced amplitude due to spectral smearing associ-

ated with the FFT windowing function, and is a

purely digital phenomenon.

3.2. Noise Performance

Figure 5 illustrates the noise performance of the

CS5371/72 modulators and digital filter using a

31.25 Hz -24 dB input signal at a 1000 SPS output

word rate. The outstanding noise characteristics of

the CS5371/72 modulators are shown, with the av-

eraged noise components consistently below the

-150 dB level. Analysis of this data set yields a dy-

namic range of 124.7 dB. Note that the 0.7 dB

variation between the signal-to-noise calculation in

Figure 4 and the dynamic range calculation in Fig-

ure 5 is not modulator dependent and results from

jitter in the test signal generator when producing a

full-scale output, as evidenced by the skirt sur-

rounding the signal peak below the -140 dB level in

Figure 4.

4. SIGNAL INPUTS

The CS5371/72 modulators use a switched capac-

itor architecture for the analog signal inputs, which

has increased jitter tolerance compared with con-

tinuous time signal input stages.

4.1. Differential Inputs - INR+/-, INF+/-

The analog signal inputs are differential and use

four pins: INR+, INR-, INF+, and INF-. Two inputs,

INR+ and INF+, are connected to the positive half

of the differential signal, while two inputs, INR- and

INF-, are connected to the negative half. The INR+

and INR- pins are switched capacitor ‘rough

charge’ inputs that pre-charge the internal sam-

pling capacitor before it is connected to the INF+

and INF- fine input pins.

The full-scale analog signal span is defined by the

voltage applied across the VREF+ and VREF-

pins. A 2.5 volt reference input sets full-scale sig-

nals as 5 volts peak-to-peak, fully differential. Dif-

ferential inputs increase the dynamic range of

0

-20

-40

-60

-80

-100

-120

-140

-160

-180

-200

0

S/N = 124.0 dB

S/D = 119.0 dB

50 100 150 200 250 300 350 400 450 500

Hz

Figure 4. 1024 Point FFT plot with a 31.25 Hz

input at Full-scale, ten averages

DS255F3

0

-20

-40

-60

-80

-100

-120

-140

-160

-180

-200

0

Dynamic Range = 124.7 dB

50 100 150 200 250 300 350 400 450 500

Hz

Figure 5. 1024 Point FFT plot with a 31.25 Hz

input at -24 dB, ten averages

9

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