AD8138AR Analog Devices Inc, AD8138AR Datasheet - Page 22

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AD8138AR

Manufacturer Part Number
AD8138AR
Description
IC,Differential Amplifier,BIPOLAR,SOP,8PIN,PLASTIC
Manufacturer
Analog Devices Inc
Type
ADC Driverr
Datasheet

Specifications of AD8138AR

Rohs Status
RoHS non-compliant
Design Resources
DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7352 (CN0040) DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7356 (CN0041) DC-Coupled, Single-Ended-to-Differential Conversion Using AD8138 and AD7357 (CN0061)
Applications
Data Acquisition
Mounting Type
Surface Mount
Package / Case
8-SOIC (3.9mm Width)
Lead Free Status / RoHS Status

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AD8138
3 V OPERATION
The circuit in Figure 47 shows a simplified front-end
connection for an AD8138 driving an AD9203, a 10-bit,
40 MSPS ADC that is specified to work on a single 3 V supply.
The ADC works best when driven differentially to make the
best use of the signal swing available within the 3 V supply.
The appropriate outputs of the AD8138 are connected to the
appropriate differential inputs of the
The AD8138 is configured for unity gain for a single-ended
input to differential output. The additional 23 Ω at the input to
−IN is to balance the impedance of the 50 Ω source and its 50 Ω
termination that drives the noninverting input.
The signal generator has ground-referenced, bipolar output,
that is, it can drive symmetrically above and below ground.
Even though the AD8138 has ground as its negative supply, it
can still function as a level-shifter with such an input signal.
The output common mode is raised up to midsupply by the
voltage divider that biases V
provides dc coupling and level-shifting of a bipolar signal,
without inverting the input signal.
The low-pass filter between the AD8138 and the
provides filtering that helps to improve the signal-to-noise ratio
(SNR). Lower noise can be realized by lowering the pole
frequency, but the bandwidth of the circuit is lowered.
Figure 47. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS A/D Converter
0.1µF
49.9Ω
10kΩ
10kΩ
0.1µF
499Ω
523Ω
8
2
1
+
AD8138
3V
6
3
499Ω
499Ω
5
4
OCM
49.9Ω
49.9Ω
20pF
20pF
. In this way, the AD8138
AD9203
0.1µF
25
26
AINN
AINP
AVDD
AVSS DRVSS
via a low-pass filter.
28
27
AD9203
3V
DRVDD
AD9203
2
1
0.1µF
DIGITAL
OUTPUTS
Rev. F | Page 22 of 24
The circuit was tested with a −0.5 dBFS signal at various
frequencies. Figure 48 shows a plot of the total harmonic
distortion (THD) vs. frequency at signal amplitudes of 1 V and
2 V differential drive levels.
Figure 49 shows the signal-to-noise-plus distortion (SINAD)
under the same conditions as above. For the smaller signal
swing, the AD8138 performance is quite good, but its
performance degrades when trying to swing too close to the
supply rails.
–40
–45
–50
–55
–60
–65
–70
–75
–80
65
63
61
59
57
55
53
51
49
47
45
0
0
Figure 49.
Figure 48.
5
5
AD8138–2V
AD8138–1V
AD9203
AD9203
FREQUENCY (MHz)
FREQUENCY (MHz)
10
10
SINAD @ −0.5 dBFS AD8138
THD @ −0.5 dBFS AD8138
AD8138–1V
AD8138–2V
15
15
20
20
25
25

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