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| 型号： | AD7440BRTZ-R2 |
| 厂商： | Analog Devices Inc |
| 文件页数： | 10/29页 |
| 文件大小： | 0K |
| 描述： | IC ADC 10BIT W/DIFF INP SOT-23-8 |
| 标准包装： | 1 |
| 位数： | 10 |
| 采样率（每秒）： | 1M |
| 数据接口： | DSP，MICROWIRE?，QSPI?，串行，SPI? |
| 转换器数目： | 1 |
| 功率耗散（最大）： | 9.25mW |
| 电压电源： | 单电源 |
| 工作温度： | -40°C ~ 85°C |
| 安装类型： | 表面贴装 |
| 封装/外壳： | SOT-23-8 |
| 供应商设备封装： | SOT-23-8 |
| 包装： | 标准包装 |
| 输入数目和类型： | 1 个差分，单极 |
| 其它名称： | AD7440BRTZ-R2DKR |
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AD7440/AD7450A
Rev. C | Page 17 of 28
different values of VREF for VDD = 5 V. It also gives the maximum
and minimum common-mode voltages for each reference value
according to Figure 28.
03051-A
-030
COMMON-MODE (CM)
CMMIN = 1V
CMMAX = 4V
REFERENCE = 2V
VIN–
VIN+
2V p-p
COMMON-MODE (CM)
CMMIN = 1.25V
CMMAX = 3.75V
REFERENCE = 2.5V
VIN–
VIN+
2.5V p-p
Figure 30. Examples of the Analog Inputs to VIN+ and VIN– for
Different Values of VREF for VDD = 5 V
Analog Input Structure
Figure 31 shows the equivalent circuit of the analog input
structure of the AD7440/AD7450A. The four diodes provide
ESD protection for the analog inputs. Care must be taken to
ensure that the analog input signals never exceed the supply
rails by more than 300 mV. This causes these diodes to become
forward biased and start conducting into the substrate. These
diodes can conduct up to 10 mA without causing irreversible
damage to the part. The capacitors, C1 in Figure 31, are
typically 4 pF and can primarily be attributed to pin
capacitance. The resistors are lumped components made up of
the on resistance of the switches. The value of these resistors is
typically about 100 Ω. The capacitors, C2, are the ADC’s
sampling capacitors and have a capacitance of 16 pF typically.
03051-A
-031
C1
C2
R1
D
C1
C2
R1
D
VDD
VIN+
VIN–
Figure 31. Equivalent Analog Input Circuit
Conversion Phase–Switches Open; Track Phase–Switches Closed
For ac applications, removing high frequency components from
the analog input signal through the use of an RC low-pass filter
on the relevant analog input pins is recommended. In applica-
tions where harmonic distortion and signal-to-noise ratio are
critical, the analog input should be driven from a low impe-
dance source. Large source impedances significantly affect the
ac performance of the ADC. This may necessitate the use of an
input buffer amplifier. The choice of op amp is a function of the
particular application.
When no amplifier is used to drive the analog input, the source
impedance should be limited to low values. The maximum
source impedance depends on the amount of total harmonic
distortion (THD) that can be tolerated. The THD increases as
the source impedance increases, and performance degrades.
Figure 32 shows a graph of THD vs. the analog input signal
frequency for different source impedances for VDD = 5 V.
0
–100
–80
–60
–40
–20
10
100
1000
03051-A
-032
INPUT FREQUENCY (kHz)
THD
(dB)
TA = 25°C
VDD = 5V
RIN = 1kΩ
RIN = 510Ω
RIN = 10Ω
RIN = 300Ω
Figure 32. THD vs. Analog Input Frequency for Various Source Impedances
for VDD = 5 V
Figure 33 shows a graph of the THD vs. the analog input
frequency for VDD of 5 V ± 5% and 3 V + 20%/–10%, while
sampling at 1 MSPS with an SCLK of 18 MHz. In this case, the
source impedance is 10 Ω.
–50
–90
–85
–80
–75
–70
–65
–60
–55
10
100
1000
03051-A
-033
INPUT FREQUENCY (kHz)
THD
(dB)
TA = 25°C
VDD = 2.7V
VDD = 3.6V
VDD = 5.25V
VDD = 4.75V
Figure 33. THD vs. Analog Input Frequency for 3 V and 5 V Supply Voltages
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