Datasheet

Data Sheet AD9255
Rev. C | Page 27 of 44
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.
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
network. However, these values are dependent on the input
signal and should be used only as a starting guide.
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
15
18
15
Open
100 to 300 10 10 10 10
An alternative to using a transformer-coupled input at frequencies
in the second Nyquist zone and higher is to use the ADL5562
differential driver. The ADL5562 provides three selectable gain
options up to 15.5 dB. An example circuit is shown in Figure 69;
additional filtering between the ADL5562 output and the AD9255
input may be required to reduce out-of-band noise. See the
ADL5562 data sheet for more information.
ADC
R1
0.1µF
0.1µF
2V p-p
VIN+
VIN–
VCM
C1
C2
R1
R2
R2
0.1µF
S
0.1µF
C2
33Ω
33Ω
SP
A
P
08505-041
Figure 68. Differential Double Balun Input Configuration
ADL5562
0Ω
0Ω
0.1µF
0.1µF
0.1µF
0.1µF
2
1
3
11
0.1µF
0.1µF
10
9
0.1µF
5, 6, 7, 8
V
CC
100Ω
100Ω
ANALOG INPUT
ANALOG INPUT
5pF
15Ω
AD9255
VIN+
VIN–
VCM
08505-042
15Ω
15Ω
20Ω
20Ω
15Ω
10pF
10pF
4
Figure 69. Differential Input Configuration Using the ADL5562