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CS5371-BS

Low-Power High-Performance ∆Σ Modulators

Cirrus Logic 

CS5371/CS5372

3. MODULATOR PERFORMANCE

Figures 4 and 5 illustrate the spectral performance

of the CS5371/CS5372 modulators when com-

bined with the CS5376 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/CS5372 modulators and

CS5376 digital filter using a 31.25 Hz input signal

and a 1000 sps output word rate. The outstanding

full-scale signal characteristics of the

CS5371/CS5372 modulators are shown, with no

harmonic components exceeding -120 dB. Analy-

sis of this data set yields a signal-to-noise ratio

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

(SDR) of 119.0 dB. Note that the full-scale signal

peak in Figure 4 shows a slightly reduced ampli-

tude due to spectral smearing associated with the

FFT windowing function, and is a purely digital

phenomenon.

3.2. Noise Performance

Figure 5 illustrates the noise performance of the

CS5371/CS5372 modulators and CS5376 digital

filter using a 31.25 Hz -24 dB input signal and a

1000 sps output word rate. The outstanding noise

characteristics of the CS5371/CS5372 modulators

are shown, with the averaged noise components

consistently below the -150 dB level. Analysis of

this data set yields a dynamic 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 Figure 5 is not modulator de-

pendent and results from jitter in the test signal

generator when producing a full scale output, as ev-

idenced by the skirt surrounding the signal peak be-

low the -140 dB level in Figure 4.

4. SIGNAL INPUTS

The CS5371/CS5372 modulators use a switched

capacitor architecture for the analog signal inputs,

which has increased jitter tolerance relative to 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-. The posi-

tive inputs, INR+ and INF+, are connected to the

positive half of the differential signal, while the

negative inputs, INR- and INF-, are connected to

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

DS255PP2

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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