AD73360L datasheet(21/32 Pages) AD

21 / 32 page

REV. 0

AD73360L

–21–

The sampling rate can be varied by programming the Decimation

Rate Divider settings in CRB. For a DMCLK of 16.384 MHz

sample rates of 64 kHz, 32 kHz, 16 kHz and 8 kHz are available.

Figure 21 shows the final spectral response of a signal sampled

at 8 kHz using the maximum oversampling rate.

FREQUENCY – kHz

–20

–100

–140

–120

–40

–60

–80

SNR = 78dB (DC TO 4kHz)

Figure 21. FFT (ADC 8 kHz Internally Decimated from

64 kHz)

It is possible to generate lower sample rates through reducing

the oversampling ratio by programming the DMCLK Rate

Divider Settings in CRB (MCD2-MCD1). This will have the

effect of spreading the quantization noise over a lesser band-

width resulting in a degradation of dynamic performance.

Figure 22 shows a FFT plot of a signal sampled at 8 kHz rate

produced by reducing the DMCLK Rate.

FREQUENCY – kHz

–20

–100

–140

–120

–40

–60

–80

SNR = 72.2dB (DC TO fS/2)

Figure 22. FFT (ADC 8 kHz Sampling with Reduced

DMCLK Rate)

1/2

74HC74

CLK

DSP CONTROL

TO SE

MCLK

SE SIGNAL SYNCHRONIZED

TO MCLK

1/2

74HC74

CLK

DSP CONTROL

RESET

MCLK

RESET SIGNAL SYNCHRONIZED

TO MCLK

Figure 19. SE and

RESET Sync Circuit for Cascaded

Operation

PERFORMANCE

As the AD73360L is designed to provide high-performance,

low-cost conversion, it is important to understand the means by

which this high performance can be achieved in a typical appli-

cation. This section will, by means of spectral graphs, outline

the typical performance of the device and highlight some of the

options available to users in achieving their desired sample rate,

either directly in the device or by doing some post-processing in

the DSP, while also showing the advantages and disadvantages

of the different approaches.

Encoder Section

The encoder section samples at DMCLK/256, which gives a

64 kHz output rate for DMCLK equal to 16.384 MHz. The

noise-shaping of the sigma-delta modulator also depends on the

frequency at which it is clocked, which means that the best

dynamic performance in a particular bandwidth is achieved by

oversampling at the highest possible rate. If we assume that the

signals of interest are in the bandwidth of dc–4 kHz, then sam-

pling at 64 kHz gives a spectral response which ensures good

SNR performance in that bandwidth, as shown in Figure 20.

FREQUENCY – kHz

–20

816

–100

–140

–120

–40

–60

–80

SNR = 59.0dB (DC TO fS/2)

SNR = 78.2dB (DC TO 4kHz)

Figure 20. FFT (ADC 64 kHz Sampling)

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