AD7938 Analog Devices, AD7938 Datasheet - Page 21

• Manufacturer Part Number: AD7938
• Description: 8-Channel, 1.5 MSPS, 12-Bit Parallel ADCs with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7938

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 1.5MSPS
• Interface: Byte,Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: (2Vref) p-p,5V p-p,Uni (Vref),Uni (Vref) x 2,Uni 2.5V,Uni 5.0V
• Adc Architecture: SAR
• Pkg Type: CSP,QFP

Data Sheet
The amplitude of the differential signal is the difference
between the signals applied to the V
differential pair (that is, V
simultaneously driven by two signals each of amplitude V
2 × V
phase. The amplitude of the differential signal is therefore −V
to +V
the common mode (CM). The common mode is the average of
the two signals (that is, (V
voltage on which the two inputs are centered. This results in the
span of each input being CM ± V
up externally and its range varies with the reference value V
As the value of V
decreases. When driving the inputs with an amplifier, the actual
common-mode range is determined by the amplifier’s output
voltage swing.
Figure 26 and Figure 27 show how the common-mode range
typically varies with V
to V
mode must be in this range to guarantee the functionality of
the AD7938/AD7939.
When a conversion takes place, the common mode is rejected,
resulting in a virtually noise-free signal of amplitude −V
+V
AD7938 and 0 to 1024 for the AD7939. If the 2 × V
used, the input signal amplitude extends from −2 V
after conversion.
Figure 26. Input Common-Mode Range vs. V
REF
REF
REF
, corresponding to the digital codes of 0 to 4096 for the
REF
COMMON-MODE
range or 2 × V
3.5
3.0
2.5
2.0
1.5
1.0
0.5
depending on the range chosen) that are 180° out of
peak-to-peak (that is, 2 × V
0
0
VOLTAGE
T
A
= 25°C
Figure 25. Differential Input Definition
REF
0.5
increases, the common-mode range
*ADDITIONAL PINS OMITTED FOR CLARITY.
REF
REF
range, respectively. The common
for a 5 V power supply using the 0 V
1.0
IN+
IN+
V
p-p
V
p-p
− V
+ V
REF
REF
V
IN−
REF
IN−
REF
1.5
). V
)/2) and is therefore the
/2. This voltage has to be set
(V)
IN+
REF
REF
and V
IN+
). This is regardless of
V
V
(0 V to V
IN+
IN–
2.0
and V
AD7939*
AD7938/
IN−
REF
pins in each
IN−
REF
Range, V
2.5
should be
REF
to +2 V
range is
REF
DD
REF
3.0
= 5 V)
REF
to
REF
Rev. C | Page 21 of 36
(or
REF
.
Driving Differential Inputs
Differential operation requires that V
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
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.
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 Figure 28 and Figure 29
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.
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 can 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 Figure 28 and Figure 29 are optimized for
dc coupling applications requiring best distortion performance.
The differential op amp driver circuit in Figure 28 is configured
to convert and level shift a single-ended, ground-referenced
(bipolar) signal to a differential signal centered at the V
of the ADC.
The circuit configuration shown in Figure 29 converts a
unipolar, single-ended signal into a differential signal.
Figure 27. Input Common-Mode Range vs. V
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.1
T
A
= 25°C
0.6
1.1
REF
V
REF
, the power supply, and the
(V)
IN+
AD7938/AD7939
1.6
REF
and V
(2 × V
REF
IN−
2.1
Range, V
be
REF
DD
2.6
= 5 V)
level