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| 型號： | AD7938BCP |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | ADC |
| 英文描述： | 8-Channel, 1.5 MSPS, 12-Bit and 10-Bit Parallel ADCs with a Sequencer |
| 中文描述： | 8-CH 12-BIT SUCCESSIVE APPROXIMATION ADC, PARALLEL ACCESS, QCC32 |
| 封裝： | MO-220-VHHD-2, LFCSP-32 |
| 文件頁數(shù)： | 21/32頁 |
| 文件大?。?/td> | 1332K |
| 代理商： | AD7938BCP |
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Preliminary Technical Data
AD7938/AD7939
When a conversion takes place, the common mode is rejected
resulting in a virtually noise free signal of amplitude V
REF
to
+V
REF
corresponding to the digital codes of 0 to 4096 for the
AD7938 and 0 to 1024 for the AD7939. If the 2 × V
REF
range was
used then the input signal amplitude would extend from 2V
REF
to +2V
REF
after conversion.
Using an Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to one of the analog input pairs of the AD7938/AD7939.
The circuit configurations shown in
show how a dual op amp can be used to convert a single-ended
signal into a differential signal for both a bipolar and unipolar
input signal, respectively.
and
Figure 30
Figure 30
Figure 30
Figure 30. Dual Op Amp Circuit to Convert a Single-Ended
Unipolar Signal into a Differential Signal
Figure 31
Figure 31
Figure 31
Figure 31. Dual Op Amp Circuit to Convert a Single-Ended
Bipolar Signal into a Differential Unipolar Signal
V
REF
(V)
C
3.5
3.0
2.0
1.5
2.5
1.0
0.5
0
0
0.5
1.5
1.0
2.0
2.5
3.0
0
The voltage applied to Point A sets up the common-mode
voltage. In both diagrams, it is connected in some way to the
reference, but any value in the common-mode range can be
input here to set up the common mode. A suitable dual op amp
that could be used in this configuration to provide differential
drive to the AD7938/AD7939 is the AD8022.
Take care when choosing the op amp; the selection depends on
the required power supply and system performance objectives.
The driver circuits in
and
dc coupling applications requiring best distortion performance.
are optimized for
The circuit configuration shown in
unipolar, single-ended signal into a differential signal.
converts a
Figure 28. Input Common-Mode Range vs. V
REF
(0 to V
REF
Range, V
DD
= 5 V)
The differential op amp driver circuit in
to convert and level shift a single-ended, ground-referenced
(bipolar) signal to a differential signal centered at the V
REF
level
of the ADC.
is configured
V
REF
(V)
C
4.5
4.0
3.0
1.5
2.0
2.5
3.5
1.0
0.5
0
0.1
0.6
1.6
1.1
2.1
2.6
0
2
×
V
REF
p-p
GND
390
220
220
220
20k
220
10k
27
27
V+
V–
V+
V–
A
V
IN+
V
IN–
V
REF
AD7938/
AD7939
0.47
μ
F
0
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
Figure 29. Input Common-Mode Range vs. V
REF
(2 × V
REF
Range, V
DD
= 5 V)
Driving Differential Inputs
Differential operation requires that V
IN+
and V
IN
be
simultaneously driven with two equal signals that are 180° out
of phase. The common mode must be set up externally and has
a range that is determined by V
REF
, the power supply, and the
particular amplifier used to drive the analog inputs. Differential
modes of operation 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.
2
×
V
REF
p-p
V
REF
GND
390
220
220
20k
220
10k
27
27
V+
V–
V+
V–
A
V
IN+
V
IN–
V
REF
AD7938/
AD7939
0.47
μ
F
0
3.75V
2.5V
1.25V
3.75V
2.5V
1.25V
Rev. PrN | Page 21 of 32
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