Datasheet

AD8138
Rev. F | Page 22 of 24
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
AD9203 via a low-pass filter.
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
OCM
. In this way, the AD8138
provides dc coupling and level-shifting of a bipolar signal,
without inverting the input signal.
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.
49.9Ω
0.1µF
10kΩ
523Ω
499Ω
10kΩ
20pF
49.9Ω
20pF
499Ω
499Ω
0.1µF
3V
DRVDDAVDD
AINN
AINP
0.1µF 0.1µF
3
V
49.9Ω
AD8138
+
AD9203
8
2
1
3
5
4
6
28 2
27
25
26
1
AVSS DRVSS
DIGITAL
OUTPUTS
01073-046
Figure 47. AD8138 Driving an
AD9203, a 10-Bit, 40 MSPS A/D Converter
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.
FREQUENCY (MHz)
–
40
THD (dBc)
–45
–50
–55
–60
–65
–70
–75
–80
AD8138–2V
AD8138–1V
0 5 10 15 20 25
01073-047
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.
FREQUENCY (MHz)
65
SINAD (dBc)
63
61
59
57
55
53
51
45
49
47
AD8138–1V
AD8138–2V
0 5 10 15 20 25
01073-048
Figure 49.
AD9203 SINAD @ −0.5 dBFS AD8138