AD7729 Analog Devices, AD7729 Datasheet - Page 10

• Manufacturer Part Number: AD7729
• Description: 3 V, Dual Sigma-Delta ADC with Auxiliary DAC
• Manufacturer: Analog Devices

Specifications of AD7729

• Resolution (bits): 15bit
• # Chan: 2
• Sample Rate: 13MSPS
• Interface: Ser
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p
• Adc Architecture: Sigma-Delta
• Pkg Type: SOIC,SOP

AD7729
Sigma-Delta ADC
The AD7729 receive channels employ a sigma-delta conversion
technique, which provides a high-resolution 15-bit output for
both I and Q channels with system filtering being implemented
on-chip.
The output of the switched-capacitor filter is continuously
sampled at 6.5 MHz (master clock/2), by a charge-balanced
modulator, and is converted into a digital pulse train whose
duty cycle contains the digital information. Due to the high
oversampling rate, which spreads the quantization noise from
0 MHz to 3.25 MHz (F
band of interest is reduced (Figure 13a). To reduce the quanti-
zation noise still 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 13b).
Figure 11. ADC Transfer Function for Differential Operation
Figure 12. ADC Transfer Function for Single-Ended
Operation
V
V
BIAS
V
BIAS
V
BIAS
BIAS
+ V
– V
+ V
– V
V
V
REF
REF
BIAS
BIAS
REF
REF
/2
/2
10 ... 00
10 ... 00
S
/2), the noise energy contained in the
IRxN
QRxN
IRxN
QRxN
IRxP
QRxP
IRxP
QRxP
ADC CODE
ADC CODE
00 ... 00
00 ... 00
01 ... 11
01 ... 11
–10–
The digital filter that follows the modulator removes the large
out-of-band quantization noise (Figure 13c), while converting
the digital pulse train into parallel 15-bit-wide binary data. The
15-bit I and Q data, which is in twos complement format, is
made available via a serial port.
Digital Filter
The digital filters used in the AD7729 receive section carry out
two important functions. Firstly, they remove the out-of-band
quantization noise which is shaped by the analog modulator.
Secondly, they are also designed to perform system level filter-
ing, providing excellent rejection of the neighboring channels.
Digital filtering has certain advantages over analog filtering.
Firstly, since digital filtering occurs after the A/D conversion
process, it can remove noise injected during the conversion
process. Analog filtering cannot do this. Secondly, the digital
filter combines low passband ripple with a steep roll-off, while
also maintaining a linear phase response. This is very difficult to
achieve with analog filters.
However, analog filtering can remove noise superimposed on
the analog signal before it reaches the ADC. Digital filtering
cannot do this and noise peaks riding on signals near full-scale
have the potential to saturate the analog modulator, even
though the average value of the signal is within limits. To allevi-
ate this problem, the AD7729 has overrange headroom built
into the sigma-delta modulator and digital filter which allows
overrange excursions of 100 mV.
a) Effect of High Oversampling Ratio
b) Use of Noise Shaping to Further Improve
c) Use of Digital Filtering to Remove the Out-
BAND OF
INTEREST
BAND OF
INTEREST
BAND OF
INTEREST
of-Band Quantization Noise
SNR
QUANTIZATION
NOISE
SHAPING
DIGITAL FILTER
CUTOFF FREQUENCY = 100kHz
NOISE
Figure 13.
3.25MHz
3.25MHz
3.25MHz
F
F
F
S
S
S
/2
/2
/2
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