EVAL-AD7934CB Analog Devices Inc, EVAL-AD7934CB Datasheet - Page 20

• Manufacturer Part Number: EVAL-AD7934CB
• Manufacturer: Analog Devices Inc

Specifications of EVAL-AD7934CB

• Lead Free Status / Rohs Status: Not Compliant

AD7933/AD7934
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
180° out of phase. The amplitude of the differential signal is,
therefore, −V
regardless of the common mode (CM). The common mode is
the average of the two signals (that is (V
therefore, the voltage on which the two inputs are centered.
This results in the span of each input being CM ± V
voltage has to be set up externally and its range varies with the
reference value, V
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
to V
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
+V
AD7933, and 0 to 4096 for the AD7934. If the 2 × V
used, the input signal amplitude extends from −2 V
+2 V
REF
REF
REF
(or 2 × V
REF
corresponding to the digital codes of 0 to 1024 for the
COMMON-MODE
range or 2 × V
.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
T
VOLTAGE
Figure 25. Input Common-Mode Range vs. V
A
REF
REF
= 25°C
Figure 24. Differential Input Definition
to +V
depending on the range chosen) that are
0.5
REF
(0 V to V
. As the value of V
*ADDITIONAL PINS OMITTED FOR CLARITY.
REF
REF
REF
range, respectively. The common
peak-to-peak (that is, 2 × V
for a 5 V power supply using the 0 V
1.0
IN+
REF
V
p-p
V
p-p
− V
REF
REF
Range, V
V
IN−
REF
1.5
). V
(V)
IN+
DD
REF
IN+
= 5 V)
and V
V
V
IN+
IN+
IN–
2.0
increases, the
and V
AD7934*
AD7933/
+ V
IN−
IN−
IN−
pins in each
)/2) and is,
2.5
REF
should be
REF
REF
REF
REF
/2. This
to
range is
). This is
REF
3.0
to
Rev. B | Page 20 of 32
Driving Differential Inputs
Differential operation requires that V
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
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
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
of the ADC.
The circuit in Figure 28 converts a unipolar, single-ended signal
into a differential signal.
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.1
T
Figure 26. Input Common-Mode Range vs. V
A
= 25°C
0.6
(2 × V
REF
REF
, the power supply, and the particular
Range, V
1.1
V
REF
DD
(V)
IN+
= 5 V)
1.6
and V
AD8022
IN−
2.1
REF
be simultane-
is a
REF
2.6
level