AD7450BRZ Analog Devices Inc, AD7450BRZ Datasheet - Page 13

• Manufacturer Part Number: AD7450BRZ
• Description: 3V/5V DIFFERENTIAL INPUT 12 BIT SAR IC
• Manufacturer: Analog Devices Inc

Specifications of AD7450BRZ

• Number Of Bits: 12
• Sampling Rate (per Second): 1M
• Data Interface: DSP, MICROWIRE™, QSPI™, Serial, SPI™
• Number Of Converters: 1
• Power Dissipation (max): 9.25mW
• Voltage Supply Source: Single Supply
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 8-SOIC (0.154", 3.90mm Width)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• For Use With: EVAL-AD7450CBZ - BOARD EVALUATION FOR AD7450
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

Differential Amplifier
An ideal method of applying differential drive to the AD7450 is to
use a differential amplifier, such as the AD8138. This part can be
used as a single-ended-to-differential amplifier or as a differential-
to-differential amplifier. In both cases, the analog input needs to
be bipolar. It also provides common-mode level shifting and buffer-
ing of the bipolar input signal. Figure 14 shows how the AD8138
can be used as a single-ended-to-differential amplifier. The positive
and negative outputs of the AD8138 are connected to the respective
inputs on the ADC via a pair of series resistors to minimize the
effects of switched capacitance on the front end of the ADC.
The RC low-pass filter on each analog input is recommended in
ac applications to remove the high-frequency components of the
analog input. The architecture of the AD8138 results in outputs
that are highly balanced over a wide frequency range without
requiring tightly matched external components.
If the analog input source being used has zero impedance then all
four resistors (Rg1, Rg2, Rf1, and Rf2) should be the same. If the
source has a 50 Ω impedance and a 50 Ω termination, for example,
the value of Rg2 should be increased by 25 Ω to balance this paral-
lel impedance on the input and thus ensure that both the positive
and negative analog inputs have the same gain (see Figure 14).
The outputs of the amplifier are perfectly matched, balanced
differential outputs of identical amplitude and exactly 180
of phase.
The AD8138 is specified with 3 V, 5 V, and ± 5 V power supplies,
but the best results are obtained when it is supplied by ± 5 V.
A lower cost device that could also be used in this configuration
with slight differences in characteristics to the AD8138, but with
similar performance and operation, is the AD8132.
Op Amp Pair
An op amp pair can be used to directly couple a differential
signal to the AD7450. The circuit configurations shown in
Figures 15a and 15b show how a dual op amp can be used to
convert a single-ended signal into a differential signal for both a
bipolar and a unipolar input signal, respectively.
REV. 0
Figure 14. Using the AD8138 as a Single-Ended-to-Differential Amplifier
*MOUNT AS CLOSE TO THE AD7450
AS POSSIBLE AND ENSURE HIGH
PRECISION Rs AND Cs ARE USED
Rs – 50R; C – 1nF;
Rg1 = Rf1 = Rf2 = 499R; Rg2 = 523R
+2.5V
GND
–2.5V
51R
o
out
Rg1
Rg2
V
OCM
–13–
AD8138
Rf2
Rf1
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. Examples of suitable dual op amps
that could be used in this configuration to provide differential
drive to the AD7450 are the AD8042, AD8056, and AD8022.
Care must be taken when choosing the op amp, since the selec-
tion will depend on the required power supply and the system
performance objectives. The driver circuits in Figure 15a and
Figure 15b are optimized for dc coupling applications requiring
optimum distortion performance.
The differential op amp driver circuit in Figure 15a 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.
GND
Figure 15a. Dual Op Amp Circuit to Convert a
Single-Ended Bipolar Input into a Differential Input
Rs*
Rs*
2
C*
C*
V
REF
p-p
V
V
IN–
IN+
AD7450
3.75V
2.5V
1.25V
390
3.75V
2.5V
1.25V
220
20k
V REF (2.5V)
EXTERNAL
A
V
220
220
220
REF
V+
V–
V+
V–
10k
27
27
AD7450
V
V
IN–
IN+
EXTERNAL
AD7450
V
REF
V
V
DD
REF
REF
level
0.1 F