AD7357_08 AD [Analog Devices], AD7357_08 Datasheet - Page 14

• Manufacturer Part Number: AD7357_08
• Description: Differential Input, Dual, Simultaneous Sampling, 4.2 MSPS, 14-Bit, SAR ADC
• Manufacturer: AD [Analog Devices]

AD7357
If the analog inputs source being used has zero impedance, all
four resistors (R
source has a 50 Ω impedance and a 50 Ω termination, for
example, the value of R
balance this parallel impedance on the input and thus ensure
that both the positive and negative analog inputs have the same
gain. The outputs of the amplifier are perfectly matched
balanced differential outputs of identical amplitude and are
exactly 180° out of phase.
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 AD7357. The
circuit configurations shown in Figure 21 and Figure 22 show
how an op amp pair can be used to convert a single-ended
signal into a differential signal for 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. The
amp that could be used in this configuration to provide
differential drive to the AD7357.
V
GND
*ADDITIONAL PINS OMITTED FOR CLARITY.
*MOUNT AS CLOSE TO THE AD7357 AS POSSIBLE
AND ENSURE HIGH PRECISION R
R
R
REF
Figure 21. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
S
G
Figure 20. Using the AD8138 as a Single-Ended-to-Differential Amplifier
2 = 523Ω.
– 33Ω; R
+2.048V
GND
–2.048V
2 × V
G
1 = R
REF
F
1 = R
p-p
440Ω
G
1, R
51Ω
A
F
2 = 499Ω; C
G
2, R
V
R
220Ω
220Ω
220Ω
R
G
OCM
G
G
2 should be increased by 25 Ω to
1
2
S
V+
V–
V+
V–
F
RESISTORS ARE USED.
1, and R
F
AD8138
1 = C
C
R
C
R
10kΩ
F
F
F
F
1
1
2
2
27Ω
27Ω
AD8022
10kΩ
F
2 = 39pF;
F
2) should be the same. If the
R
R
10kΩ
S
S
*
*
is a suitable dual op
2.048V
1.024V
0V
2.048V
1.024V
0V
2.048V
1.024V
0V
2.048V
0V
1.024V
V
V
V
V
IN–
IN+
IN+
IN–
AD7357*
AD7357
REF
REF
A
A
/REF
/REF
10µF
10µF
Rev. PrF | Page 14 of 20
B
B
V
ADC TRANSFER FUNCTION
The output coding for the AD7357 is straight binary. The
designed code transitions occur at successive LSB values (such
as, 1 LSB, 2 LSBs). The LSB size is (2 × V
transfer characteristic of the AD7357 is shown in Figure 23.
GND
*ADDITIONAL PINS OMITTED FOR CLARITY.
Figure 22. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
REF
111 ... 111
111 ... 110
111 ... 101
000 ... 010
000 ... 001
000 ... 000
2 × V
–V
REF
Figure 23. AD7357 Deal Transfer Characteristic
REF
p-p
440Ω
20kΩ
+ 0.5 LSB
A
–V
a Differential Unipolar Signal
Preliminary Technical Data
REF
into a Differential Signal
220Ω
220Ω
220Ω
+ 1 LSB
V+
V–
V+
V–
10kΩ
ANALOG INPUT
27Ω
27Ω
+V
2.048V
1.024V
0V
REF
REF
2.048V
1.024V
0V
)/16384. The ideal
– 1.5 LSB
+V
V
V
IN+
IN–
REF
AD7357*
REF
– 1 LSB
A
/REF
10µF
B