AD7764 datasheet(16/32 Pages) AD

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AD7764

Rev. 0 | Page 16 of 32

THEORY OF OPERATION

The AD7764 features an on-chip fully differential amplifier to

feed the Σ-Δ modulator pins , an on-chip reference buffer, and

a FIR filter block to perform the required digital filtering of the

Σ-Δ modulator output. Using this Σ-Δ conversion technique

with the added digital filtering, the analog input is converted to

an equivalent digital word.

Σ-Δ MODULATION AND DIGITAL FILTERING

The input waveform applied to the modulator is sampled and

an equivalent digital word is output to the digital filter at a rate

equal to ICLK. By employing oversampling, the quantization

noise is spread across a wide bandwidth from 0 to f

ICLK. This

means that the noise energy contained in the signal band of

interest is reduced (see Figure 29). To further reduce the

quantization noise, a high-order modulator is employed to

shape the noise spectrum so that most of the noise energy is

shifted out of the signal band (see Figure 30).

QUANTIZATION NOISE

fICLK/2

BAND OF INTEREST

Figure 29. Σ-Δ ADC, Quantization Noise

fICLK/2

NOISE SHAPING

BAND OF INTEREST

Figure 30. Σ-Δ ADC, Noise Shaping

fICLK/2

BAND OF INTEREST

DIGITAL FILTER CUTOFF FREQUENCY

Figure 31. Σ-Δ ADC, Digital Filter Cutoff Frequency

The digital filtering that follows the modulator removes the

large out-of-band quantization noise (see Figure 31) while also

reducing the data rate from f

ICLK at the input of the filter to

ICLK/64 or less at the output of the filter, depending on the

decimation rate used.

The AD7764 employs three FIR filters in series. By using

different combinations of decimation ratios, data can be

obtained from the AD7764 at three data rates.

The first filter receives data from the modulator at ICLK

MHz

where it is decimated 4× to output data at (ICLK/4)

MHz. The

second filter allows the decimation rate to be chosen from

8× to 32×.

The digital filtering on the AD7764 provides full-band filtering.

This means that its stop-band attenuation occurs at the Nyquist

frequency (ODR/2). This feature provides increased protection

against aliasing of sampled frequencies that lie above the

Nyquist rate (ODR/2). The filter gives maximum attenuation at

the Nyquist rate (see Figure 32). This means that it attenuates all

possible alias frequencies by 110 dB. The frequency response in

Figure 32 occurs when the AD7764 is operated with a 40 MHz

MCLK in the decimate 64× mode. Note that the first stop-band

frequency occurs at Nyquist. The frequency response of the

filter scales with both the decimation rate chosen and the MCLK

frequency applied.

The third filter has a fixed decimation rate of 2×. Table 6 shows

some characteristics of the digital filtering where ICLK =

MCLK/2. The group delay of the filter is defined to be the delay

to the center of the impulse response and is equal to the compu-

tation plus the filter delays. The delay until valid data is available

(the FILTER-SETTLE status bit is set) is approximately twice

the filter delay plus the computation delay. This is listed in

terms of MCLK periods in Table 6.

–160

–140

–120

–100

–80

–60

–40

–20

250

200

150

100

FREQUENCY (kHz)

PASS-BAND RIPPLE = 0.05dB

–0.1dB FREQUENCY = 125.1kHz

–3dB FREQUENCY = 128kHz

STOP BAND = 156.25kHz

Figure 32. Filter Frequency Response (312.5 kHz ODR)

Table 6. Configuration with Default Filter

ICLK

Frequency

Decimation

Rate

Data State

Computation

Delay

Filter Delay

SYNC to

FILTER-SETTLE

Pass-Band

Bandwidth

Output Data Rate

(ODR)

20 MHz

64×

Fully filtered

2.25 μs

87.6 μs

7122 × tMCLK

125 kHz

312.5 kHz

20 MHz

128×

Fully filtered

3.1 μs

174 μs

14217 × tMCLK

62.5 kHz

156.25 kHz

20 MHz

256×

Fully filtered

4.65 μs

346.8 μs

27895 × tMCLK

31.25 kHz

78.125 kHz

12.288 MHz

64×

Fully filtered

3.66 μs

142.6 μs

7122 × tMCLK

76.8 kHz

192 kHz

12.288 MHz

128×

Fully filtered

5.05 μs

283.2 μs

14217 × tMCLK

38.4 kHz

96 kHz

12.288 MHz

256×

Fully filtered

7.57 μs

564.5 μs

27895 × tMCLK

19.2 kHz

48 kHz

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