Datasheet

AD9258
Rev. A | Page 28 of 44
The signal characteristics must be considered when selecting
a transformer. Most RF transformers saturate at frequencies
below a few megahertz (MHz). Excessive signal power can also
cause core saturation, which leads to distortion.
network. At higher input frequencies, good performance can be
achieved by using a ferrite bead in series with a resistor and
removing the capacitors. However, these values are dependent
on the input signal and should be used only as a starting guide.
At input frequencies in the second Nyquist zone and above, the
noise performance of most amplifiers is not adequate to achieve
the true SNR performance of the AD9258. For applications in
which SNR is a key parameter, differential double balun coupling
is the recommended input configuration (see Figure 68). In this
configuration, the input is ac-coupled, and the CML is provided
to each input through a 33 Ω resistor. These resistors compensate
for losses in the input baluns to provide a 50 Ω impedance to
the driver.
Table 10. Example RC Network
Frequency
Range
(MHz)
R1 Series
(Ω Each)
C1 Differential
(pF)
R2 Series
(Ω Each)
C2 Shunt
(pF Each)
0 to 100 33 5 15 15
100 to 200 10 5 10 10
100 to 300 10
1
Remove 66 Remove
1
In this configuration, R1 is a ferrite bead with a value of 10 Ω @ 100 MHz.
An alternative to using a transformer-coupled input at
frequencies in the second Nyquist zone is to use the AD8352
differential driver. An example is shown in Figure 69. See the
AD8352 data sheet for more information.
In the double balun and transformer configurations, the value of
the input capacitors and resistors is dependent on the input fre-
quency and source impedance and may need to be reduced or
removed. Table 10 displays recommended values to set the RC
AD9258
R1
0.1µF
0.1µF
2
V p-
p
VIN+
VIN–
VCM
C1
C2
R1
R2
R2
0.1µF
S
0.1µF
C2
33Ω
33Ω
SP
A
P
08124-038
Figure 68. Differential Double Balun Input Configuration
AD8352
0Ω
0Ω
C
D
R
D
R
G
0.1µF
0.1µF
0.1µF
0.1µF
16
1
2
3
4
5
11
0.1µF
0.1µF
10
14
0.1µF
8, 13
V
CC
200Ω
200Ω
ANALOG INPUT
ANALOG INPUT
R
R
C
AD9258
VIN+
VIN–
VCM
08124-039
Figure 69. Differential Input Configuration Using the AD8352