AD831
–8–
Low-Pass Filtering
A simple low-pass lter may be added between the mixer and
the output amplier 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 lter (f = (2 RC
F
)
–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 C
F
= (2 14 140 MHz)
–1
82 pF, the nearest
standard value.
Figure 4. Low-Pass Filtering Using External Capacitors
Using the Output Amplier
The AD831’s output amplier 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 Amplier 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 for Transformer 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.
Ω
Figure 3. Programming the Quiescent Current
REV. C
