AD7938-6 Analog Devices, AD7938-6 Datasheet - Page 19

• Manufacturer Part Number: AD7938-6
• Description: 8-Channel, 625 kSPS, 12-Bit Parallel ADCs with a Sequencer
• Manufacturer: Analog Devices

Specifications of AD7938-6

• Resolution (bits): 12bit
• # Chan: 8
• Sample Rate: 625kSPS
• Interface: Par
• Analog Input Type: Diff-Uni,SE-Uni
• Ain Range: Uni (Vref),Uni (Vref) x 2
• 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
pair (that is, V
driven by two signals each of amplitude V
depending on the range chosen) that are 180° out of phase. The
amplitude of the differential signal is therefore −V
peak-to-peak (that is, 2 × V
common mode (CM). The common mode is the average of the
two signals (that is, (V
on which the two inputs are centered. This results in the span of
each input being CM ± V
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 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
AD7938-6.
When a conversion takes place, the common mode is rejected,
resulting in a virtually noise-free signal of amplitude −V
+V
2 × V
−2 V
Figure 25. Input Common-Mode Range vs. V
REF
REF
REF
REF
corresponding to the digital codes of 0 to 4096. If the
COMMON-MODE
range or 2 × V
to +2 V
3.5
3.0
2.5
2.0
1.5
1.0
0.5
range is used, the input signal amplitude extends from
0
0
T
VOLTAGE
A
IN+
= 25°C
REF
Figure 24. Differential Input Definition
− V
REF
0.5
after conversion.
IN−
increases, the common-mode range
*ADDITIONAL PINS OMITTED FOR CLARITY.
REF
). V
REF
IN+
range, respectively. The common
for a 5 V power supply using the 0 V
+ V
REF
IN+
1.0
IN+
REF
/2. This voltage has to be set up
V
p-p
V
p-p
and V
IN−
REF
REF
and V
). This is regardless of the
)/2) and is therefore the voltage
V
REF
1.5
IN−
(V)
IN−
should be simultaneously
REF
pins in each differential
V
V
(0 V to V
IN+
IN–
AD7938-6*
2.0
REF
(or 2 × V
REF
Range, V
2.5
REF
to +V
REF
REF
REF
DD
,
3.0
= 5 V)
REF
.
to
Rev. C | Page 19 of 32
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 AD7938-6. 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
that can be used in this configuration to provide differential
drive to the AD7938-6 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 differential op amp driver circuit 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 configuration shown in Figure 28 converts a
unipolar, single-ended signal into a differential signal.
Figure 26. 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+
1.6
REF
and V
(2 × V
REF
IN−
Range, V
2.1
AD7938-6
be
REF
DD
= 5 V)
2.6
level