RSOURCE () T HD ( d B) –40 –45 –" />
參數(shù)資料
型號(hào)： EVAL-AD7938CBZ
廠商： Analog Devices Inc
文件頁(yè)數(shù)： 13/36頁(yè)
文件大小： 0K
描述： EVAL BOARD FOR AD7938
標(biāo)準(zhǔn)包裝： 1
ADC 的數(shù)量： 1
位數(shù)： 12
采樣率（每秒）： 1.5M
數(shù)據(jù)接口： 并聯(lián)
輸入范圍： ±VREF
在以下條件下的電源（標(biāo)準(zhǔn)）： 13.5mW @ 5V
工作溫度： -40°C ~ 85°C
已用 IC / 零件： AD7938
已供物品： 板，CD
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其它名稱： Q3627458
AD7938/AD7939
Data Sheet
Rev. C | Page 20 of 36
RSOURCE ()
T
HD
(
d
B)
–40
–45
–50
–55
–80
–75
–70
–65
–60
–90
–85
10
100
1k
03715-
018
FIN = 50kHz
VDD = 5V
VDD = 3V
Figure 21. THD vs. Source Impedance in Single-Ended Mode
RSOURCE ()
T
HD
(
d
B)
–60
–80
–75
–70
–65
–100
–85
–90
–95
10
100
1k
03715-
019
FIN = 50kHz
VDD = 5V
VDD = 3V
Figure 22. THD vs. Source Impedance in Fully Differential Mode
Figure 23 shows a graph of the THD vs. the analog input
frequency for various supplies while sampling at 1.5 MHz
with an SCLK of 25.5 MHz. In this case, the source impedance
is 10 .
INPUT FREQUENCY (kHz)
T
HD
(
d
B)
–50
–60
–70
–80
–110
–100
–90
–120
0
100
400
200
300
600
500
700
03715-
020
FSAMPLE = 1.5MSPS
RANGE = 0 TO VREF
VDD = 3V
SINGLE-ENDED MODE
VDD = 5V/3V
DIFFERENTIAL MODE
VDD = 5V
SINGLE-ENDED MODE
Figure 23. THD vs. Analog Input Frequency for Various Supply Voltages
ANALOG INPUTS
The AD7938/AD7939 have software-selectable analog input
configurations. The user can choose eight single-ended inputs,
four fully differential pairs, four pseudo differential pairs, or
seven pseudo differential inputs. The analog input
configuration is chosen by setting the MODE0/MODE1 bits in
the internal control register (see Table 10).
Single-Ended Mode
The AD7938/AD7939 can have eight single-ended analog input
channels by setting the MODE0 and MODE1 bits in the control
register to 0. In applications where the signal source has a high
impedance, it is recommended to buffer the analog input before
applying it to the ADC. An op amp suitable for this function is
the AD8021. The analog input range of the AD7938/AD7939
can be programmed to be either 0 V to VREF or 0 V to 2 × VREF.
If the analog input signal to be sampled is bipolar, the internal
reference of the ADC can be used to externally bias up this
signal to make it the correct format for the ADC.
Figure 24 shows a typical connection diagram when operating
the ADC in single-ended mode. This diagram shows a bipolar
signal of amplitude ±1.25 V being preconditioned before it is
applied to the AD7938/AD7939. In cases where the analog
input amplitude is ±2.5 V, the 3R resistor can be replaced with a
resistor of value R. The resultant voltage on the analog input of
the AD7938/AD7939 is a signal ranging from 0 V to 5 V. In this
case, the 2 × VREF mode can be used.
0.47F
+1.25V
VIN
R
3R
0V
–1.25V
+2.5V
0V
VIN0
VIN7
VREFOUT
AD7938/
AD7939*
R
*ADDITIONAL PINS OMITTED FOR CLARITY.
03715-
031
Figure 24. Single-Ended Mode Connection Diagram
Differential Mode
The AD7938/AD7939 can have four differential analog input
pairs by setting the MODE0 and MODE1 bits in the control
register to 0 and 1, respectively.
Differential signals have some benefits over single-ended
signals, including noise immunity based on the device’s
common-mode rejection and improvements in distortion
performance. Figure 25 defines the fully differential analog
input of the AD7938/AD7939.
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