AD8138
3 V OPERATION
The circuit in Figure 47 shows a simplified front-end
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.
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 AD9203 via a low-pass filter.
–40
–45
–50
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.
–55
AD8138–2V
–60
–65
AD8138–1V
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.
–70
–75
–80
0
5
10
15
20
25
The output common mode is raised up to midsupply by the
voltage divider that biases VOCM. In this way, the AD8138
provides dc coupling and level-shifting of a bipolar signal,
without inverting the input signal.
FREQUENCY (MHz)
Figure 48. AD9203 THD @ −0.5 dBFS AD8138
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.
The low-pass filter between the AD8138 and the AD9203
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.
65
63
61
3V
3V
0.1µF
0.1µF
499Ω
0.1µF
10kΩ
28
2
3
59
49.9Ω
499Ω
AVDD DRVDD
+
5
8
2
25
AINN
57
20pF
DIGITAL
OUTPUTS
AD8138–1V
49.9Ω
AD9203
AD8138
AINP
523Ω
49.9Ω
55
26
4
1
AVSS DRVSS
27
6
AD8138–2V
20pF
1
53
0.1µF
10kΩ
499Ω
51
49
47
45
Figure 47. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS A/D Converter
0
5
10
15
20
25
FREQUENCY (MHz)
Figure 49. AD9203 SINAD @ −0.5 dBFS AD8138
Rev. F | Page 22 of 24
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