EVAL-AD7934-6CB AD [Analog Devices], EVAL-AD7934-6CB Datasheet - Page 18

• Manufacturer Part Number: EVAL-AD7934-6CB
• Description: 4-Channel, 625 kSPS, 12-Bit Parallel ADC with a Sequencer
• Manufacturer: AD [Analog Devices]

AD7934-6
The amplitude of the differential signal is the difference between
the signals applied to the V
pair (that is, V
driven by two signals, each of amplitude V
depending on the range chosen), which are 180° out of phase.
The amplitude of the differential signal is therefore −V
peak-to-peak (that is, 2 × V
(CM). The common mode is the average of the two signals,
(V
inputs are centered. This results in the span of each input being
CM ± V
range varies with the reference value V
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 25 and Figure 26 show how the common-mode range
typically varies with V
0 to V
mode must be in this range to guarantee the functionality of the
AD7934-6.
When a conversion takes place, the common mode is rejected.
This results in a virtually noise-free signal of amplitude −V
to +V
2 × V
from −2 V
Figure 25. Input Common-Mode Range vs. V
IN+
+ V
REF
REF
REF
COMMON-MODE
3.5
3.0
2.5
2.0
1.5
1.0
0.5
REF
, corresponding to the digital codes 0 to 4096. If the 0 to
range is used, then the input signal amplitude extends
IN−
range or 0 to 2 × V
0
0
/2. This voltage must be set up externally, and its
REF
)/2, and is therefore the voltage on which the two
VOLTAGE
T
A
to +2 V
IN+
= 25 ° C
Figure 24. Differential Input Definition
− V
0.5
*ADDITIONAL PINS OMITTED FOR CLARITY
IN−
REF
REF
). V
.
for a 5 V power supply using the
1.0
IN+
IN+
REF
V
p-p
V
p-p
REF
REF
REF
and V
), regardless of the common mode
and V
range, respectively. The common
V
REF
1.5
IN−
IN−
(V)
REF
pins in each differential
should be simultaneously
REF
V
V
. As the value of V
IN+
IN–
(0 to V
AD7934-6*
2.0
REF
(or 2 × V
REF
Range, V
2.5
REF
REF
DD
to +V
3.0
= 5 V)
REF
REF
Rev. A | Page 18 of 28
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 a dc input provide the
best THD performance over a wide frequency range. Not all
applications have a signal preconditioned for differential
operation, so 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 AD7934-6. The
circuit configurations shown in Figure 27 and Figure 28 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
for use in this configuration to provide differential drive to the
AD7934-6 is the AD8022.
It is advisable to 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 in
Figure 27 converts and level shifts a single-ended, ground-
referenced, bipolar signal to a differential signal centered at
the V
Figure 28 converts a unipolar, single-ended signal into a
differential signal.
Figure 26. Input Common-Mode Range vs. V
REF
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
level of the ADC. The circuit configuration shown in
0
0.1
T
A
= 25 ° C
0.6
1.1
V
REF
REF
, the power supply, and the
(V)
IN+
1.6
REF
and V
(2 × V
REF
IN−
2.1
Range, V
be
DD
2.6
= 5 V)