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參數(shù)資料
| 型號： | AD7720BRUZ-REEL7 |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 3/16頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC MODULATOR SIGMA-DELTA 28TSSOP |
| 標準包裝： | 1,000 |
| 位數(shù)： | 16 |
| 采樣率（每秒）： | 12.5M |
| 數(shù)據(jù)接口： | 并聯(lián) |
| 轉(zhuǎn)換器數(shù)目： | 1 |
| 功率耗散（最大）： | 215mW |
| 電壓電源： | 模擬和數(shù)字 |
| 工作溫度： | -40°C ~ 85°C |
| 安裝類型： | 表面貼裝 |
| 封裝/外殼： | 28-TSSOP（0.173"，4.40mm 寬） |
| 供應商設備封裝： | 28-TSSOP |
| 包裝： | 帶卷 (TR) |
| 輸入數(shù)目和類型： | 1 個差分，單極；1 個差分，雙極 |
AD7720
–11–
REV. 0
R
100
C
2.7nF
VIN(+)
VIN(–)
C
2.7nF
C
2.7nF
R
100
Figure 24. Differential Input with Antialiasing
A capacitor between the two input pins sources or sinks charge
to allow most of the charge that is needed by one input to be
effectively supplied by the other input. This minimizes undesir-
able 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 (f3 dB) of the
antialias filter is given by Equation 1, and the attenuation of the
filter is given by Equation 2.
f3dB = 1/(2
π R
EXT CEXT)
(1)
Attenuation = 20 log 1/ 1+ f / f 3dB
()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
digital 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, Polystyrene or Polycarbonate are suitable.
If ceramic capacitors are used, they must have NPO dielectric.
Applying the Reference
The reference circuitry used in the AD7720 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 reference, connect 100 nF between REF1 and AGND.
If the internal reference is required to bias external circuits, use
an external precision op amp to buffer REF1.
3k
REFERENCE
BUFFER
1V
2.5V
REFERENCE
SWITCHED-CAP
DAC REF
REF1
REF2
COMPARATOR
100nF
Figure 25. Reference Circuit Block Diagram
The AD7720 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. How-
ever, there will be an error 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 connected to REF2.
In all cases, since the REF2 voltage connects to the analog
modulator, a 220 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).
A
B
4pF
A
B
A
B
MCLK
REF2
A
4pF
SWITCHED-CAP
DAC REF
220nF
Figure 26. REF2 Equivalent Circuit
The AD780 is ideal to use as an external reference with the
AD7720. Figure 27 shows a suggested connection diagram.
AD780
1
2
3
4
8
7
6
5
NC
+VIN
TEMP
GND
O/P
SELECT
NC
VOUT
TRIM
22nF
1 F
REF2
22 F
220nF
REF1
+5V
Figure 27. External Reference Circuit Connection
Input Circuits
Figures 28 and 29 show two simple circuits for bipolar mode
operation. Both circuits accept a single-ended bipolar signal
source and create the necessary differential signals at the input
to the ADC.
The circuit in Figure 28 creates a 0 V to 2.5 V signal at the
VIN(+) pin to form a differential signal around an initial bias
voltage of 1.25 V. For single-ended applications, best THD
performance is obtained with VIN(–) set to 1.25 V rather than
2.5 V. The input to the AD7720 can also be driven differen-
tially with a complementary input as shown in Figure 29.
In this case, the input common-mode voltage is set to 2.5 V.
The 2.5 V p-p full-scale differential input is obtained with a
1.25 V p-p signal at each input in antiphase. This configuration
minimizes the required output swing from the amplifier circuit
and is useful for single supply applications.
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| AD7721AR | 功能描述:IC ADC 16BIT SIGMA-DELTA 28-SOIC RoHS:否 類別:集成電路 (IC) >> 數(shù)據(jù)采集 - 模數(shù)轉(zhuǎn)換器 系列:- 標準包裝:1,000 系列:- 位數(shù):12 采樣率（每秒）:300k 數(shù)據(jù)接口:并聯(lián) 轉(zhuǎn)換器數(shù)目:1 功率耗散（最大）:75mW 電壓電源:單電源 工作溫度:0°C ~ 70°C 安裝類型:表面貼裝 封裝/外殼:24-SOIC（0.295"，7.50mm 寬） 供應商設備封裝:24-SOIC 包裝:帶卷 (TR) 輸入數(shù)目和類型:1 個單端，單極；1 個單端，雙極 |
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