AD8366 datasheet(21/28 Pages) AD

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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 Ω and a differential load

impedance of 200 Ω.

ZLOAD

ZSOURCE

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)

ZSOURCE (Ω)

ZLOAD (Ω)

L1 (μH)

L2 (μH)

L3 (μH)

L4 (μH)

C1 (pF)

C2 (pF)

200

6.6

6.0

220

180

200

3.3

110

200

1.2

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