AD831APZ datasheet(8/16 Pages) AD

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AD831

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AD831

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Low-Pass Filtering

A simple low-pass filter may be added between the mixer and

the output amplifier by shunting the internal resistive loads

(an equivalent resistance of about 14  with a tolerance of 20%)

with external capacitors; these attenuate the sum component in

a downconversion application (Figure 4). The corner frequency

of this one-pole low-pass filter (f = (2  RCF)–1) should be placed

about an octave above the difference frequency IF. Thus, for a

70 MHz IF, a –3 dB frequency of 140 MHz might be chosen,

using CF = (2   14   140 MHz)–1  82 pF, the nearest

standard value.

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Figure 4. Low-Pass Filtering Using External Capacitors

Using the Output Amplifier

The AD831’s output amplifier converts the mixer core’s differential

current output into a single-ended voltage and provides an output

as high as ±1 V peak into a 50 V load (+10 dBm). For unity gain

operation (Figure 5), the inputs AN and AP connect to the open-

collector outputs of the mixer’s core and OUT connects to VFB.

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Figure 5. Output Amplifier Connected for Unity

Gain Operation

The mixer has two open-collector outputs (differential currents) at

pins IFN and IFP.These currents may be used to provide nominal

unity RF to IF gain by connecting a center-tapped transformer

(1:1 turns ratio) to pins IFN and IFP as shown in Figure 2.

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Figure 2. Connections forTransformer Coupling to

the IF Output

Programming the Bias Current

Because the AD831’s RF port is a Class-A circuit, the maximum

RF input is proportional to the bias current.This bias current

may be reduced by connecting a resistor from the BIAS pin to the

positive supply (Figure 3). For normal operation, the BIAS pin is

left unconnected. For lowest power consumption, the BIAS pin is

connected directly to the positive supply.The range of adjustment

is 100 mA for normal operation to 45 mA total current at minimum

power consumption.

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Figure 3. Programming the Quiescent Current

REV. C

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