AD8138
+3V
3 V OPERATION
+3V
The circuit in Figure 41 shows a simplified front end connection
for an AD8138 driving an AD9203, a 10-bit, 40 MSPS A/D
converter that is specified to work on a single +3 V supply. The
A/D works best when driven differentially to make the best use
of the signal swing available within the 3 V supply. The appro-
priate outputs of the AD8138 are connected to the appropriate
differential inputs of the AD9203 via a low-pass filter.
0.1F
0.1F
499⍀
0.1F
10k⍀
499⍀
49.9⍀
AVDD DRVDD
AINN
+
20pF
DIGITAL
OUTPUTS
49.9⍀
AD9203
AD8138
AINP
AVSS DRVSS
523⍀
49.9⍀
20pF
0.1F
10k⍀
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.
499⍀
Figure 41. AD8138 Driving an AD9203, a 10-Bit, 40 MSPS
A/D Converter
The signal generator has ground-referenced, bipolar output, i.e.,
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.
–40
–45
–50
The output common-mode is raised up to midsupply by the
voltage divider that biases VOCM. In this way, the AD8138 pro-
vides dc-coupling and level-shifting of a bipolar signal, without
inverting the input signal.
–55
AD8138-2V
–60
The low-pass filter between the AD8138 and the AD9203 pro-
vides filtering that helps to improve the signal-to-noise ratio.
Lower noise can be realized by lowering the pole frequency, but
the bandwidth of the circuit will be lowered.
–65
AD8138-1V
–70
–75
–80
The circuit was tested with a –0.5 dBFS signal at various fre-
quencies. Figure 42 shows a plot of the total harmonic distor-
tion (THD) vs. frequency at signal amplitudes of 1 V and 2 V
differential drive levels.
0
5
10
15
20
25
FREQUENCY – MHz
Figure 43 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 perfor-
mance degrades when trying to swing too close to the supply
rails.
Figure 42. AD9203 THD @ –0.5 dBFS AD8138
65
63
61
59
57
AD8138-1V
55
AD8138-2V
53
51
49
47
45
0
5
10
15
20
25
FREQUENCY – MHz
Figure 43. AD9203 SINAD @ –0.5 dBFS AD8138
REV. A
–13–
ADI [ ADI ]
