参数资料
型号: AD7688BCPZRL7
厂商: Analog Devices Inc
文件页数: 6/28页
文件大小: 0K
描述: IC ADC 16BIT SAR 500KSPS 10LFCSP
标准包装: 1
系列: PulSAR®
位数: 16
采样率(每秒): 500k
数据接口: DSP,MICROWIRE?,QSPI?,串行,SPI?
转换器数目: 1
功率耗散(最大): 21.5mW
电压电源: 模拟和数字
工作温度: -40°C ~ 85°C
安装类型: 表面贴装
封装/外壳: 10-WFDFN 裸露焊盘,CSP
供应商设备封装: 10-LFCSP-WD(3x3)
包装: 标准包装
输入数目和类型: 2 个单端,单极;2 个单端,双极;1 个差分,单极;1 个差分,双极
其它名称: AD7688BCPZRL7DKR
AD7688
Rev. A | Page 14 of 28
ANALOG INPUT
Figure 27 shows an equivalent circuit of the input structure of
the AD7688.
The two diodes, D1 and D2, provide ESD protection for the
analog inputs IN+ and IN. Care must be taken to ensure that
the analog input signal never exceeds the supply rails by more
than 0.3 V because this causes these diodes to begin to forward-
bias and start conducting current. These diodes can handle a
forward-biased current of 130 mA maximum. For instance,
these conditions could eventually occur when the input buffer’s
(U1) supplies are different from VDD. In such a case, an input
buffer with a short-circuit current limitation can be used to
protect the part.
CIN
RIN
D1
D2
CPIN
IN+
OR IN–
GND
VDD
02973-026
Figure 27. Equivalent Analog Input Circuit
The analog input structure allows the sampling of the true
differential signal between IN+ and IN. By using these
differential inputs, signals common to both inputs are rejected,
as shown in Figure 28, which represents the typical CMRR over
frequency.
02973-
027
FREQUENCY (kHz)
10000
1
10
100
1000
CMRR
(
d
B)
80
70
60
VDD = 5V
Figure 28. Analog Input CMRR vs. Frequency
During the acquisition phase, the impedance of the analog
inputs (IN+ or IN) can be modeled as a parallel combination
of capacitor, CPIN, and the network formed by the series
connection of RIN and CIN. CPIN is primarily the pin capacitance.
RIN is typically 600 Ω and is a lumped component made up of
some serial resistors and the on resistance of the switches. CIN is
typically 30 pF and is mainly the ADC sampling capacitor.
During the conversion phase, where the switches are opened,
the input impedance is limited to CPIN. RIN and CIN make a
1-pole, low-pass filter that reduces undesirable aliasing effects
and limits the noise.
When the source impedance of the driving circuit is low, the
AD7688 can be driven directly. Large source impedances
significantly affect the ac performance, especially total
harmonic distortion (THD). The dc performances are less
sensitive to the input impedance. The maximum source
impedance depends on the amount of THD that can be
tolerated. The THD degrades as a function of the source
impedance and the maximum input frequency, as shown in
FREQUENCY (kHz)
THD
(dB)
–60
–70
–80
–90
–100
–110
–120
0
25
50
75
100
02973-
028
RS = 250Ω
RS = 100Ω
RS = 50Ω
RS = 33Ω
Figure 29. THD vs. Analog Input Frequency and Source Resistance
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