AD7450 查看數據表（PDF） - Analog Devices
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AD7450 Datasheet PDF : 24 Pages
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AD7450
PRELIMINARY TECHNICAL DATA
0
VDD
VIN+
D
R1
C2
C1
D
0
VIN-
C1
VDD
D
D
R1
C2
0
0
0
0
0
0
0
0
T ITLE
Figure 11. Equivalent Analog Input Circuit.
Conversion Phase - Switches Open
Track Phase - Switches Closed
When no amplifier is used to drive the analog input, the
source impedance should be limited to low values. The
maximum source impedance will depend on the amount of
Total Harmonic Distortion (THD) that can be tolerated.
The THD will increase as the source impedance increases
and performance will degrade. Figure 12 shows a graph
of the THD versus analog input signal frequency for dif-
ferent source impedances.
0
Figure 13.THD vs Analog Input Frequency for 3V and 5V
Supply Voltages TBD
DRIVING DIFFERENTIAL INPUTS
Differential operation requires that VIN+ and VIN- be si-
multaneously driven with two equal signals that are 180o
out of phase. The common mode must be set up exter-
nally and has a range which is determined by VREF, the
power supply and the particular amplifier used to drive the
analog inputs (see figure 8). Differential modes of opera-
tion with either an ac or dc input, provide the best THD
performance over a wide frequency range. Since not all
applications have a signal preconditioned for differential
operation, there is often a need to perform single ended to
differential conversion.
0
0
0
0
0
0
0
0
0
TITLE
Figure 12.THD vs Analog Input Frequency for Various
Source Impedances TBD
Figure 13 shows a graph of THD versus analog input
frequency for VDD of 5V and 3V, while sampling at
1MHz and 833kHz with a SCLK of 18 MHz and 15MHz
respectively.
Differential Amplifier
An ideal method of applying dc differential drive to the
AD7450 is to use a differential amplifier such as the AD8138.
This part can be used as a single ended to differential
amplifier or as a differential to differential amplifier. In both
cases the analog input needs to be bipolar. It also provides
common mode level shifting and buffering of the bipolar
input signal. Figure 14 shows how the AD8138 can be used
as a single ended to differential amplifier. The positive and
negative outputs of the AD8138 are connected to the respec-
tive inputs on the ADC via a pair of series resistors to
minimize the effects of switched capacitance on the front end
of the ADC. The RC low pass filter on each analog input is
recommended in ac applications to remove high frequency
components of the analog input. The architecture of the
AD8138 results in outputs that are very highly balanced over
a wide frequency range without requiring tightly matched
external components.
If the analog input source being used has no impedance then
all four resistors (Rg1, Rg2, Rf1, Rf2) should be the same. If
the source has a 50 ⍀ impedance and a 50 ⍀ termination for
example, the value of Rg2 should be increased by 25 ⍀ to
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the
same gain (see figure 14). The outputs of the amplifier are
perfectly matched, balanced differential outputs of identical
amplitude and are exactly 180o out of phase.
–14–
REV. PrJ
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