AD9432
Rev. F | Page 15 of 16
APPLICATIONS INFORMATION
USING THE AD8138 TO DRIVE THE AD9432
The AD8138 differential output op amp can be used to drive the
AD9432 in dc-coupled applications. The AD8138 was specifically
designed for ADC driver applications. Superior SNR performance
is maintained up to analog frequencies of 30 MHz. The AD8138
op amp provides single-ended-to-differential conversion, which
allows for a low cost alternative to transformer coupling for ac
applications, as well.
The circuit in Figure 29 was breadboarded, and the measured
performance is shown in Figure 30 and Figure 31. These figures
are for ±5 V supplies at the AD8138; with a single 5 V supply at
the AD8138, performance dropped by about 1 dB to 2 dB.
Figure 30 shows SNR and SINAD for a −1 dBFS analog input
frequency varied from 2 MHz to 40 MHz with an encode rate
of 105 MSPS. The measurements are for nominal conditions
at room temperature. Figure 31 shows the second-order and
third-order harmonic distortion performance under the same
conditions.
The dc common-mode voltage for the AD8138 outputs can be
adjusted via the V
OCM
input to provide the 3 V common-mode
voltage that the AD9432 inputs require.
00587-028
AD8138
AD9432
10pF
0.1µF
25Ω 500Ω
500Ω
50Ω
50Ω
50Ω
2kΩ
3kΩ
5V
500Ω
VIN
AIN
AIN
V
OCM
10pF
500Ω
22pF
Figure 29. AD8138/AD9432 Schematic
SNR/SINAD (dB)
AIN (MHz)
02040
00587-029
62
63
64
65
66
61
60
60
SNR
SINAD
Figure 30. Measured SNR and SINAD (Encode = 105 MSPS)
HARMONICS (dB)
AIN (MHz)
02040
00587-030
–90
–80
60
70
–100
H2
H3
Figure 31. Measured Second-Order and Third-Order Harmonic Distortion
(Encode = 105 MSPS)
