AD7722ASZ Analog Devices Inc, AD7722ASZ Datasheet - Page 14

• Manufacturer Part Number: AD7722ASZ
• Description: ADC Single Delta-Sigma 195KSPS 16-Bit Parallel/Serial 44-Pin MQFP
• Manufacturer: Analog Devices Inc

Specifications of AD7722ASZ

• Package: 44MQFP
• Resolution: 16 Bit
• Sampling Rate: 195 KSPS
• Architecture: Delta-Sigma
• Number Of Analog Inputs: 1
• Digital Interface Type: Parallel|Serial (3-Wire)
• Input Type: Voltage
• Polarity Of Input Voltage: Unipolar|Bipolar
• Number Of Bits: 16
• Sampling Rate (per Second): 220k
• Data Interface: Serial, Parallel
• Number Of Converters: 1
• Power Dissipation (max): 375mW
• Voltage Supply Source: Analog and Digital
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 44-QFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

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
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
Due to the high oversampling rate, which spreads the quantization
noise from 0 to f
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
filter to f
data rate, f
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.
CLKIN
BAND OF INTEREST
BAND OF INTEREST
BAND OF INTEREST
S
, is a little over twice the signal bandwidth, which
/64 at the output of the filter. The AD7722 output
CLKIN
/2, the noise energy contained in the band
Figure 9. Σ-∆ ADC
QUANTIZATION NOISE
DIGITAL FILTER CUTOFF FREQUENCY
WHICH EQUALS 97.65kHz (12.5MHz)
NOISE SHAPING
a.
b.
c.
CLKIN
at the input of the
f
f
f
CLKIN
CLKIN
CLKIN
CLKIN
/2
/2
/2
CLKIN
.
. The
–14–
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
filter that samples the output of the first filter at f
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
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
At these frequencies, there are frequency bands ± f
(f
of n × f
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
a single-pole filter will provide 36 dB of attenuation at f
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.
CLKIN
3 dB
Figure 10b. Frequency Response of Antialias Filter
0dB
is the –3 dB bandwidth of the digital filter) on either side
/128) one-half its output rate. With a clock frequency
CLKIN
Figure 10a. Digital Filter Frequency Response
0dB
DATA RATE
where noise passes unattenuated to the output.
f
OUTPUT
CLKIN
/ 64
CLKIN
1f
ANTIALIAS FILTER
. The second filter is a 151-tap half-band
CLKIN
RESPONSE
2f
CLKIN
CLKIN
f
CLKIN
where n = 1, 2, 3. . . .
3f
CLKIN
REQUIRED
ATTENUATION
CLKIN
3 dB
CLKIN
CLKIN
/32 and
REV. B
wide
/64,
.