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AD7357YRUZ Datasheet(PDF) 14 Page - Analog Devices

Part No. AD7357YRUZ
Description  Differential Input, Dual, Simultaneous Sampling, 4.2 MSPS, 14-Bit, SAR ADC
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7357YRUZ Datasheet(HTML) 14 Page - Analog Devices

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AD7357
Rev. 0 | Page 14 of 20
GND
2 × VREF p-p
27
27
V+
V–
V+
V–
VREF
1.024V
2.048V
0V
1.024V
2.048V
0V
REFA/REFB
VIN+
AD7357*
VIN–
220
10µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
220
220
10k
A
440
Differential Amplifier
An ideal method of applying differential drive to the AD7357
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. The AD8138 also provides
common-mode level shifting. Figure 20 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 architecture of the AD8138 results in
outputs that are very highly balanced over a wide frequency
range without requiring tightly matched external components.
Figure 21. Dual Op Amp Circuit to Convert a Single-Ended Unipolar Signal
into a Differential Signal
If the analog inputs source being used has zero impedance, 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 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.
GND
2 × VREF p-p
27
27
V+
V–
V+
V–
VREF
1.024V
2.048V
0V
1.024V
2.048V
0V
REFA/REFB
VIN+
AD7357*
VIN–
220
10µF
*ADDITIONAL PINS OMITTED FOR CLARITY.
220
220
10k
A
440
20k
+2.048V
GND
–2.048V
AD8138
RG1
RS*
RS*
RG2
RF2
VOCM
RF1
2.048V
VIN+
VIN–
1.024V
0V
2.048V
1.024V
0V
AD7357
CF2
CF1
10k
10k
10µF
REFA/REFB
*MOUNT AS CLOSE TO THE AD7357 AS POSSIBLE
AND ENSURE THAT HIGH PRECISION RS RESISTORS ARE USED.
RS – 33Ω; RG1 = RF1 = RF2 = 499Ω; CF1 = CF2 = 39pF;
RG2 = 523Ω
51
Figure 22. Dual Op Amp Circuit to Convert a Single-Ended Bipolar Signal into
a Differential Unipolar Signal
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 × VREF)/16,384. The ideal
transfer characteristic of the AD7357 is shown in Figure 23.
000 ... 000
000 ... 001
000 ... 010
111 ... 101
111 ... 110
111 ... 111
ANALOG INPUT
–VREF + 0.5 LSB
–VREF + 1 LSB
+VREF – 1.5 LSB
+VREF – 1 LSB
Figure 20. Using the AD8138 as a Single-Ended-to-Differential Amplifier
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, Point A is
connected in some way to the reference. The AD8022 is a
suitable dual op amp that can be used in this configuration
to provide differential drive to the AD7357.
Figure 23. Deal Transfer Characteristic


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