ADF4111BCP datasheet(21/28 Pages) AD

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AD8348

Rev. A | Page 21 of 28

1000pF

MXIP

MXIN

Figure 53. Driving the MX Inputs from a Differential Source

If the MX inputs are to be driven from a single-ended 50 Ω source,

a 4:1 balun can be used to transform the 200 Ω impedance of

the inputs to 50 Ω while performing the required single-ended-

to-differential conversion. The recommended transformer is the

M/A-COM ETK4-2T.

MXIP

1000pF

ETK4-2T

1µF

MXIP

MXIN

Figure 54. Driving the MX Inputs from a Single-Ended 50 Ω Source

BASEBAND OUTPUTS

The baseband amplifier outputs, IOPP, IOPN, QOPP, and QOPN,

should be presented with loads of at least 2 kΩ (single-ended to

ground). They are not designed to drive 50 Ω loads directly. The

typical swing for these outputs is 2 V p-p differential (1 V p-p

single-ended), but larger swings are possible as long as care is taken

to ensure that the signals remain within the lower limit of 0.5 V

and the upper limit of VS − 1 V of the output swing. To achieve

a larger swing, it is necessary to adjust the common-mode bias of

the baseband output signals. Increasing the swing can have the

benefit of improving the signal-to-noise ratio of the baseband

amplifier output.

When connecting the baseband outputs to other devices, care

should be taken to ensure that the outputs are not capacitively

loaded by approximately 20 pF or more. Such loads could

potentially overload the output or induce oscillations. The effect

of capacitive loading on the baseband amplifier outputs can be

mitigated by inserting series resistors of approximately 200 Ω.

OUTPUT DC BIAS LEVEL

The dc bias of the mixer outputs and the baseband amplifier

inputs and outputs is determined by the voltage that is driven

onto the VCMO pin. The range of this voltage is typically

between 500 mV and 4 V when operating with a 5 V supply.

To achieve maximum voltage swing from the baseband amplifiers,

VCMO should be driven at 2.25 V; this allows a swing of up to

7 V p-p differential (3.5 V p-p single-ended).

INTERFACING TO DETECTOR FOR AGC OPERATION

The AD8348 can be interfaced with a detector such as the

AD8362 rms-to-dc converter to provide an automatic signal-

leveling function for the baseband outputs.

1000pF

60.4Ω

1:1

AD8348

100pF

0.1µF

+VS

VREF

+VS

100pF

VREF

1000pF

100pF

1000pF

1µF

100pF

0.1µF

1µF

1000pF

+VS

0.1µF

100pF

+VS

0.1µF

100pF

VCMO

VSET

+VS

AD8362

ACOM

COMM

VTGT

DECL

VPOS

INHI

VOUT

INLO

ACOM

PWDN

CLPF

COMM

VREF

CHPF

VSET

DECL

TO BASEBAND

Q ADC

TO BASEBAND

I ADC

IF INPUT

ZO = 200Ω

1.02kΩ

1.24kΩ

IFIP

MXIP

VPOS2 VGIN

IOFS

QOFS

VREF

ENBL

LOIN

LOIP

VPOS1 COM1

IOPN

QOPN

IOPP

QOPP

VCMO ENVG

IAIN

QAIN

COM3 COM3

IMXO

QMXO

COM2 VPOS3

IFIN

MXIN

Figure 55. AD8362 Configuration for AGC Operation

Assuming the I and Q channels have the same rms power, the

mixer output (or the output of the baseband filter) of one channel

can be used as the input of the AD8362. The AD8362 should be

operated in a region where its linearity error is small. Also, a

voltage divider should be implemented with an external resistor

in series with the 200 Ω input impedance of the AD8362 input.

This attenuates the AD8348 mixer output so that the AD8362

input is not overdriven. The size of the resistor between the

mixer output and the AD8362 input should be chosen so that

the peak signal level at the input of the AD8362 is about 10 dB

less than the approximately 10 dBm maximum of the AD8362

dynamic range.

The other side of the AD8348 baseband output should be

loaded with a resistance equal to the series resistance of the

attenuating resistor in series with the AD8362’s 200 Ω input

impedance. This resistor should be tied to the source driving

VCMO so that there is no dc drawn from the mixer output.

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