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參數資料
| 型號： | AD7724 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Dual CMOS Modulators |
| 中文描述： | 調制器雙通道CMOS |
| 文件頁數： | 11/14頁 |
| 文件大小： | 552K |
| 代理商： | AD7724 |
REV. A
AD7724
–11–
An alternative circuit configuration for driving the differential
inputs to the AD7724 is shown in Figure 24.
R
100
V
C
2.7nF
VIN(+)
VIN(–)
C
2.7nF
C
2.7nF
R
100
V
Figure 24. Differential Input with Antialiasing
A capacitor between the two input pins sources or sinks charge
to allow most of the charge needed by one input to be effectively
supplied by the other input. This minimizes undesirable charge
transfer from the analog inputs to and from ground. The series
resistor isolates the operational amplifier from the current spikes
created during the sampling process and provides a pole for
antialiasing. The 3 dB cutoff frequency of the antialias filter is
given by Equation 1, and the attenuation of the filter is given by
Equation 2.
f
3dB
= 1/(2
π
R
EXT
C
EXT
)
(1)
Attenuation
= 20 log
1/ 1
+
f
/
f
3
dB
(
)
2
(2)
The choice of the filter cutoff frequency will depend on the
amount of roll-off that is acceptable in the passband of the digi-
tal filter and the required attenuation at the first image frequency.
The capacitors used for the input antialiasing circuit must have
low dielectric absorption to avoid distortion. Film capacitors
such as polypropylene or polycarbonate are suitable. If ceramic
capacitors are used, they must have NPO dielectric.
Applying the Reference
The reference circuitry used in the AD7724 includes an on-chip
2.5 V bandgap reference and a reference buffer circuit. The
block diagram of the reference circuit is shown in Figure 25.
The internal reference voltage is connected to REF1 via a 3 k
resistor and is internally buffered to drive the analog modulator’s
switched capacitor DAC (REF2). When using the internal refer-
ence, connect 110 nF between REF1 and AGND. If the internal
reference is required to bias external circuits, use an external
precision op amp to buffer REF1.
4k
V
REFERENCE
BUFFER
1V
2.5V
REFERENCE
SWITCHED-CAP
DAC REF
REF1
REF2
COMPARATOR
110nF
Figure 25. Reference Circuit Block Diagram
The AD7724 can operate with its internal reference, or an
external reference can be applied in two ways. An external
reference can be connected to REF1, overdriving the internal
reference. However, an error will be introduced due to the
offset of the internal buffer amplifier. For lowest system gain
errors when using an external reference, REF1 is grounded
(disabling the internal buffer) and the external reference is con-
nected to REF2.
In all cases, since the REF2 voltage connects to the analog modu-
lator, a 110 nF capacitor must connect directly from REF2 to
AGND. The external capacitor provides the charge required for
the dynamic load presented at the REF2 pin (Figure 26).
f
A
f
B
f
B
4pF
f
A
f
B
f
A
f
B
MCLK
REF2
f
A
4pF
SWITCHED-CAP
DAC REF
110nF
Figure 26. REF2 Equivalent Circuit
The AD780 is ideal to use as an external reference with the
AD7724. Figure 27 shows a suggested connection diagram.
AD780
1
2
3
4
8
7
6
5
NC
+V
IN
TEMP
GND
O/P
SELECT
NC
V
OUT
TRIM
22nF
1
m
F
REF2
22
m
F
110nF
REF1
5V
NC = NO CONNECT
Figure 27. External Reference Circuit Connection
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|---|---|
| AD7724ACPZ | 功能描述:IC MOD SIGMA-DELTA DUAL 制造商:analog devices inc. 系列:* 零件狀態(tài):上次購買時間 標準包裝:1 |
| AD7724ACPZ-REEL | 功能描述:IC MOD SIGMA-DELTA DUAL 制造商:analog devices inc. 系列:* 零件狀態(tài):上次購買時間 標準包裝:1 |
| AD7724AST | 制造商:Analog Devices 功能描述:ADC Dual Delta-Sigma 30Msps 15-bit Serial 48-Pin LQFP |
| AD7724AST-REEL | 制造商:Analog Devices 功能描述:ADC Dual Delta-Sigma 30Msps 15-bit Serial 48-Pin LQFP T/R |
| AD7724ASTZ | 功能描述:IC MOD SIGMA-DELTA DUAL 48LQFP RoHS:是 類別:集成電路 (IC) >> 數據采集 - ADCs/DAC - 專用型 系列:- 產品培訓模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標準包裝:50 系列:- 類型:數據采集系統(tǒng)（DAS） 分辨率（位）:16 b 采樣率（每秒）:21.94k 數據接口:MICROWIRE?，QSPI?，串行，SPI? 電壓電源:模擬和數字 電源電壓:1.8 V ~ 3.6 V 工作溫度:-40°C ~ 85°C 安裝類型:表面貼裝 封裝/外殼:40-WFQFN 裸露焊盤 供應商設備封裝:40-TQFN-EP（6x6） 包裝:托盤 |
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