Datasheet

AD7933/AD7934
Rev. B | Page 20 of 32
V
REF
p-p
V
IN+
V
IN–
V
REF
p-p
*ADDITIONAL PINS OMITTED FOR CLARITY.
AD7933/
AD7934*
COMMON-MODE
VOLTAGE
03713-032
Figure 24. 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 output voltage swing of the amplifier.
Figure 25 and Figure 26 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
AD7933/AD7934.
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 1024 for the
AD7933, and 0 to 4096 for the AD7934. If the 2 × V
REF
range is
used, the input signal amplitude extends from −2 V
REF
to
+2 V
REF
.
V
REF
(V)
COMMON-MODE RANGE (V)
3.5
3.0
2.0
1.5
2.5
1.0
0.5
0
00.5 1.51.0 2.0 2.5 3.0
T
A
= 25°C
03713-033
Figure 25. 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
T
A
= 25°C
03713-034
Figure 26. 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 simultane-
ously 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 applica-
tions 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 AD7933/AD7934.
The circuit configurations shown in
Figure 27 and Figure 28
show how a dual op amp converts 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. The
AD8022 is a
suitable dual op amp that can be used in this configuration to
provide differential drive to the AD7933/AD7934.
Take care when choosing the op amp; the selection depends on
the required power supply and system performance objectives.
The driver circuits in
Figure 27 and Figure 28 are optimized for
dc coupling applications requiring best distortion performance.
The circuit configuration shown in
Figure 27 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 in
Figure 28 converts a unipolar, single-ended signal
into a differential signal.