REV. B–14–
AD7722
CIRCUIT DESCRIPTION
The AD7722 ADC employs a Σ-∆ conversion technique that
converts the analog input into a digital pulse train. The analog
input is continuously sampled by a switched capacitor modulator
at twice the rate of the clock input frequency, 2 × f
CLKIN
. The
digital data that represents the analog input is in the ones density
of the bit stream at the output of the Σ-∆ modulator. The modu-
lator outputs a bit stream at a data rate equal to f
CLKIN
.
Due to the high oversampling rate, which spreads the quantization
noise from 0 to f
CLKIN
/2, the noise energy contained in the band
of interest is reduced (Figure 9a). To reduce the quantization
noise further, a high order modulator is employed to shape the
noise spectrum so that most of the noise energy is shifted out of
the band of interest (Figure 9b).
The digital filter that follows the modulator provides three main
functions. The filter performs sophisticated averaging on the
1-bit samples from the output of the modulator, while removing
the large out of band quantization noise (Figure 9c). Lastly, the
digital filter reduces the data rate from f
CLKIN
at the input of the
filter to f
CLKIN
/64 at the output of the filter. The AD7722 output
data rate, f
S
, is a little over twice the signal bandwidth, which
guarantees that there is no loss of data in the signal band.
Digital filtering has certain advantages over analog filtering. First,
since digital filtering occurs after the A/D conversion, it can
remove noise injected during the conversion process. Analog
filtering cannot remove noise injected during conversion. Second,
the digital filter combines low pass-band ripple with a steep roll-off
while also maintaining a linear phase response.
BAND OF INTEREST
f
CLKIN
/2
DIGITAL FILTER CUTOFF FREQUENCY
WHICH EQUALS 97.65kHz (12.5MHz)
BAND OF INTEREST
QUANTIZATION NOISE
f
CLKIN
/2
BAND OF INTEREST
f
CLKIN
/2
NOISE SHAPING
a.
b.
c.
Figure 9.
Σ-∆
ADC
The AD7722 employs two finite impulse response (FIR) filters in
series. The first filter is a 384-tap filter that samples the output of
the modulator at f
CLKIN
. The second filter is a 151-tap half-band
filter that samples the output of the first filter at f
CLKIN
/32 and
decimates by 2. The implementation of this filter architecture
results in a filter with a group delay of 42 conversions (84 conver-
sions for settling to a full-scale step).
The digital filter provides 6 dB of attenuation at a frequency
(f
CLKIN
/128) one-half its output rate. With a clock frequency
of 12.5 MHz, the digital filter has a pass-band frequency of
90.625 kHz, a cutoff frequency is 96.92 kHz, and a stop-band
frequency of 104.6875 kHz.
Due to the sampling nature of the digital filter, the filter does not
provide any rejection at integer multiples of its input sampling
frequency. The filter response in Figure 10a shows the unattenu-
ated frequency bands occurring at n × f
CLKIN
where n = 1, 2, 3. . . .
At these frequencies, there are frequency bands ± f
3 dB
wide
(f
3 dB
is the –3 dB bandwidth of the digital filter) on either side
of n × f
CLKIN
where noise passes unattenuated to the output.
Out-of-band signals coincident with any of the filter images are
aliased into the pass band. However, due to the AD7722’s high
oversampling ratio, these bands occupy only a small fraction of
the spectrum, and most broadband noise is filtered. This means
that the antialias filtering requirements in front of the AD7722
are considerably reduced versus a conventional converter with no
on-chip filtering. Figure 10b shows the frequency response of an
antialias filter. With a –3 dB corner frequency set at f
CLKIN
/64,
a single-pole filter will provide 36 dB of attenuation at f
CLKIN
.
Depending on the application, however, it may be necessary to
provide additional antialias filtering prior to the AD7722 to
eliminate unwanted signals from the frequency bands the digital
filter passes. It may also be necessary in some applications to
provide analog filtering in front of the AD7722 to ensure that
differential noise signals outside the band of interest do not
saturate the analog modulator.
1f
CLKIN
0dB
2f
CLKIN
3f
CLKIN
Figure 10a. Digital Filter Frequency Response
OUTPUT
DATA RATE
f
CLKIN
/64
0dB
f
CLKIN
ANTIALIAS FILTER
RESPONSE
REQUIRED
ATTENUATION
Figure 10b. Frequency Response of Antialias Filter
