Datasheet

AD8366
Rev. A | Page 21 of 28
BASEBAND INTERFACE
In most direct-conversion receiver designs, it is desirable to select a
wanted carrier within a specified band. The desired channel can be
demodulated by tuning the LO to the appropriate carrier frequency.
If the desired RF band contains multiple carriers of interest, the
adjacent carriers would also be down converted to a lower IF
frequency. These adjacent carriers can be a problem if they are
large relative to the desired carrier because they can overdrive
the baseband signal detection circuitry. As a result, it is often
necessary to insert a filter to provide sufficient rejection of the
adjacent carriers.
It is necessary to consider the overall source and load impedance
presented by the AD8366 and the ADC input to design the
filter network. The differential baseband output impedance of
the AD8366 is 25 Ω and is designed to drive a high impedance
ADC input. It may be desirable to terminate the ADC input down
to the lower impedance by using a terminating resistor, such as
500 Ω. The terminating resistor helps to better define the input
impedance at the ADC input at the cost of a slightly reduced gain.
The order and type of filter network depends on the desired high
frequency rejection required, pass-band ripple, and group delay.
Figure 57 shows the schematic for a typical fourth-order, Chebyshev,
low-pass filter. Table 4 shows the typical values of the filter
components for a fourth-order, Chebyshev, low-pass filter with
a differential source impedance of 25  and a differential load
impedance of 200 .
L1
L2
C1
L3
L4
C2
Z
LOAD
Z
SOURCE
07584-051
Figure 57. Schematic of a Fourth-Order, Chebyshev, Low-Pass Filter
Table 4. Typical Values for Fourth-Order, Chebyshev, Low-Pass Filter
3 dB Corner (MHz) Z
SOURCE
(Ω) Z
LOAD
(Ω) L1 (μH) L2 (μH) L3 (μH) L4 (μH) C1 (pF) C2 (pF)
5 25 200 6.6 6.6 6.0 6.0 220 180
10 25 200 3.3 3.3 3 3 110 90
28 25 200 1.2 1.2 1 1 39 33