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參數資料
| 型號： | AD7729AR |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 模擬信號調理 |
| 英文描述： | Dual Sigma-Delta ADC with Auxiliary DAC |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PDSO28 |
| 封裝： | SOIC-28 |
| 文件頁數： | 10/16頁 |
| 文件大?。?/td> | 142K |
| 代理商： | AD7729AR |
AD7729
–10–
REV. 0
V
BIAS
+ V
REF
/2
V
BIAS
V
BIAS
– V
REF
/2
V
IRxN
QRxN
IRxP
QRxP
10 ... 00
00 ... 00
ADC CODE
01 ... 11
Figure 11. ADC Transfer Function for Differential Operation
V
BIAS
+ V
REF
V
BIAS
V
BIAS
– V
REF
V
IRxN
QRxN
IRxP
QRxP
10 ... 00
00 ... 00
ADC CODE
01 ... 11
Figure 12. ADC Transfer Function for Single-Ended
Operation
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
S
/2), the noise energy contained in the
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).
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.
QUANTIZATION
NOISE
BAND OF
INTEREST
F
S
/2
3.25MHz
a) Effect of High Oversampling Ratio
NOISE
SHAPING
BAND OF
INTEREST
F
S
/2
3.25MHz
b) Use of Noise Shaping to Further Improve
SNR
DIGITAL FILTER
CUTOFF FREQUENCY = 100kHz
BAND OF
INTEREST
F
S
/2
3.25MHz
c) Use of Digital Filtering to Remove the Out-
of-Band Quantization Noise
Figure 13.
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.
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