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參數資料
| 型號： | AD7677 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | 16-Bit, 1 LSB INL, 1 MSPS Differential ADC |
| 中文描述： | 16位，1 LSB INL和1 MSPS的差分ADC |
| 文件頁數： | 13/20頁 |
| 文件大小： | 322K |
| 代理商： | AD7677 |
REV. 0
AD7677
–13–
FREQUENCY
–
Hz
90
C
–
45
75
10k
10M
1k
1M
80
65
100k
55
85
70
60
50
Figure 7. Analog Input CMRR vs. Frequency
During the acquisition phase, for ac signals, the AD7677 behaves
like a one-pole RC filter consisting of the equivalent resis-
tance R+ , R–, and C
S
. The resistors R+ and R– are typically
168 V and are lumped components made up of some serial
resistors and the on resistance of the switches. The capacitor C
S
is
typically 60 pF and is mainly the ADC sampling capacitor. This
one-pole filter with a typical –3 dB cutoff frequency of 15.8 MHz
reduces undesirable aliasing effect and limits the noise com-
ing from the inputs.
Because the input impedance of the AD7677 is very high, the
AD7677 can be driven directly by a low impedance source
without gain error. That allows the user to input, as shown in
Figure 5, an external one-pole RC filter between the output of
the amplifier output and the ADC analog inputs to even further
improve the noise filtering done by the AD7677 analog input
circuit. However, the source impedance has to be kept low
because it affects the ac performances, especially the total har-
monic distortion. The maximum source impedance depends
on the amount of total harmonic distortion (THD) that can
be tolerated. The THD degrades proportionally to the source
impedance.
Single to Differential Driver
For applications using unipolar analog signals, a single-ended-
to-differential driver will allow for a differential input into the
part. The schematic is shown in Figure 8.
U2
590
590
2.5V REF
C
C
AD8021
590
AD7677
IN+
IN
–
REF
2.5V REF
U1
ANALOG INPUT
(UNIPOLAR)
C
C
AD8021
590
Figure 8. Single-Ended-to-Differential Driver Circuit
This configuration, when provided an input signal of 0 to V
REF
,
will produce a differential
±
2.5 V with midscale at 1.25 V.
If the application can tolerate more noise, the AD8138 can
be used.
Driver Amplifier Choice
Although the AD7677 is easy to drive, the driver amplifier needs
to meet at least the following requirements:
The driver amplifier and the AD7677 analog input circuit
have to be able together to settle for a full-scale step of the
capacitor array at a 16-bit level (0.0015%). In the amplifier’s
data sheet, the settling at 0.1% or 0.01% is more commonly
specified. It could significantly differ from the settling time at
a 16-bit level and, therefore, it should be verified prior to the
driver selection. The tiny op-amp, AD8021, which combines
ultralow noise and a high gain bandwidth, meets this settling
time requirement even when used with a high gain up to 13.
The noise generated by the driver amplifier needs to be kept
as low as possible in order to preserve the SNR and transi-
tion noise performance of the AD7677. The noise coming
from the driver is filtered by the AD7677 analog input circuit
one-pole, low-pass filter made by R+,
R–, and C
S
. The SNR
degradation due to the amplifier is:
SNR
LOG
f
N e
LOSS
dB
N
=
+
(
)
20
28
784
4
3
2
π
–
where
f
–3
dB
is the –3
dB
input bandwidth in MHz of the AD7677
(15.8 MHz) or the cutoff frequency of the input filter if
any used.
N
is the noise factor of the amplifiers (1 if in buffer con-
figuration).
e
N
is the equivalent input noise voltage of each opamp in
nV/(Hz)
1/2
.
For instance, a driver with an equivalent input noise of 2 nV/
√
Hz
(like the AD8021) and configured as a buffer, thus with a noise
gain of +1, the SNR degrades by only 0.07 dB with the filter in
Figure 5, and 0.27 dB without.
The driver needs to have a THD performance suitable to
that of the AD7677.
The AD8021 meets these requirements and is usually appropri-
ate for almost all applications. The AD8021 needs an external
compensation capacitor of 10 pF. This capacitor should have
good linearity as an NPO ceramic or mica type.
The AD8022 could also be used where a dual version is needed
and gain of 1 is used.
The AD8132 or the AD8138 could also be used to generate a
differential signal from a single-ended signal.
The AD829 is another alternative where high-frequency (above
1 MHz) performance is not required. In gain of 1, it requires an
82 pF compensation capacitor.
The AD8610 is also another option where low bias current is
needed in low-frequency applications.
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相關代理商/技術參數 |
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|---|---|
| AD7677AAST | 制造商:Analog Devices 功能描述: |
| AD7677ACP | 制造商:Analog Devices 功能描述:ADC Single SAR 1Msps 16-bit Parallel/Serial 48-Pin LFCSP EP 制造商:Rochester Electronics LLC 功能描述:- Bulk |
| AD7677ACPRL | 制造商:Analog Devices 功能描述:ADC Single SAR 1Msps 16-bit Parallel/Serial 48-Pin LFCSP EP T/R |
| AD7677ACPZ | 功能描述:IC ADC 16BIT DIFF INP 48-LFCSP RoHS:是 類別:集成電路 (IC) >> 數據采集 - 模數轉換器 系列:PulSAR® 標準包裝:1 系列:- 位數:14 采樣率（每秒）:83k 數據接口:串行，并聯 轉換器數目:1 功率耗散（最大）:95mW 電壓電源:雙 ± 工作溫度:0°C ~ 70°C 安裝類型:通孔 封裝/外殼:28-DIP（0.600"，15.24mm） 供應商設備封裝:28-PDIP 包裝:管件 輸入數目和類型:1 個單端，雙極 |
| AD7677ACPZRL | 制造商:Analog Devices 功能描述:ADC Single SAR 1Msps 16-bit Parallel/Serial 48-Pin LFCSP EP T/R 制造商:Analog Devices 功能描述:ADC SGL SAR 1MSPS 16BIT PARALLEL/SERL 48LFCSP EP - Tape and Reel |
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