Datasheet

AD7764
Rev. A | 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
f
ICLK
/2
BAND OF INTEREST
06518-012
Figure 29. Σ-Δ ADC, Quantization Noise
f
ICLK
/2
NOISE SHAPING
BAND OF INTEREST
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Figure 30. Σ-Δ ADC, Noise Shaping
f
ICLK
/2
BAND OF INTEREST
DIGITAL FILTER CUTOFF FREQUENCY
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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
f
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 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.
0
–160
–140
–120
–100
–80
–60
–40
–20
0 30025020015010050
AMPLITUDE (dB)
FREQUENCY (kHz)
PASS-BAND RIPPLE = 0.05dB
–0.1dB FREQUENCY = 125.1kHz
–3dB FREQUENCY = 128kHz
STOP BAND = 156.25kHz
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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
Pass-Band
Bandwidth
to
FI LT ER -SETTLE
Output Data Rate
(ODR)
20 MHz 64× Fully filtered 2.25 µs 87.6 µs 7122 × t
MCLK
125 kHz 312.5 kHz
20 MHz 128× Fully filtered 3.1 µs 174 µs 14217 × t
MCLK
62.5 kHz 156.25 kHz
20 MHz 256× Fully filtered 4.65 µs 346.8 µs 27895 × t
MCLK
31.25 kHz 78.125 kHz
12.288 MHz 64× Fully filtered 3.66 µs 142.6 µs 7122 × t
MCLK
76.8 kHz 192 kHz
12.288 MHz 128× Fully filtered 5.05 µs 283.2 µs 14217 × t
MCLK
38.4 kHz 96 kHz
12.288 MHz 256× Fully filtered 7.57 µs 564.5 µs 27895 × t
MCLK
19.2 kHz 48 kHz