Datasheet

Data Sheet AD7938/AD7939
Rev. C | Page 21 of 36
03715-032
V
REF
p-p
V
IN+
V
IN–
V
REF
p-p
*ADDITIONAL PINS
OMITTED FOR CLARITY.
AD7938/
AD7939*
COMMON-MODE
VOLTAGE
Figure 25. Differential Input Definition
The amplitude of the differential signal is the difference
between the signals applied to the V
IN+
and V
IN−
pins in each
differential pair (that is, V
IN+
− V
IN−
). V
IN+
and V
IN−
should be
simultaneously driven by two signals each of amplitude V
REF
(or
2 × V
REF
depending on the range chosen) that are 180° out of
phase. The amplitude of the differential signal is therefore −V
REF
to +V
REF
peak-to-peak (that is, 2 × V
REF
). This is regardless of
the common mode (CM). The common mode is the average of
the two signals (that is, (V
IN+
+ V
IN−
)/2) and is therefore the
voltage on which the two inputs are centered. This results in the
span of each input being CM ± V
REF
/2. This voltage has to be set
up externally and its range varies with the reference value V
REF
.
As the value of V
REF
increases, the common-mode range
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
REF
for a 5 V power supply using the 0 V
to V
REF
range or 2 × V
REF
range, respectively. The common
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
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 is
used, the input signal amplitude extends from −2 V
REF
to +2 V
REF
after conversion.
V
REF
(V)
COMMON-MODE RANGE (V)
3.5
3.0
2.0
1.5
2.5
1.0
0.5
0
0 0.5 1.51.0 2.0 2.5 3.0
03715-033
T
A
= 25°C
Figure 26. Input Common-Mode Range vs. V
REF
(0 V to V
REF
Range, V
DD
= 5 V)
V
REF
(V)
COMMON-MODE RANGE (V)
4.5
4.0
3.0
1.5
2.0
2.5
3.5
1.0
0.5
0
0.1 0.6 1.61.1 2.1 2.6
03715-034
T
A
= 25°C
Figure 27. 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.
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
REF
level
of the ADC.
The circuit configuration shown in Figure 29 converts a
unipolar, single-ended signal into a differential signal.