AD831 Analog Devices, AD831 Datasheet - Page 8

• Manufacturer Part Number: AD831
• Description: High-Performance, Low Distortion 500-MHz Mixer
• Manufacturer: Analog Devices

AD831
The mixer has two open-collector outputs (differential cur-
rents) at pins IFN and IFP. These currents may be used to pro-
vide 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 18.
Figure 18. Connections for Transformer Coupling to the IF
Output
Programming the Bias Current
Because the AD831’s RF port is a Class-A circuit, the maxi-
mum RF input is proportional to the bias current. This bias cur-
rent may be reduced by connecting a resistor from the BIAS pin
to the positive supply (Figure 19). 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.
OSCILLATOR
Figure 19. Programming the Quiescent Current
LOCAL
INPUT
4
5
6
7
8
RFP
GND
VN
RFN
VN
INPUT
RF
VP
3
9
BIAS
AN
LON
LOP
RFN
RFP
50
VN
VP
LON
10
11
10
2
6
7
IFN
AMPLIFIER
LIMITING
CURRENT
11
LOP
1
BIAS
VP
Top View
AD831
Q2
Q3
VP
12
20
IFP
50
VPOS
Q4
1k
R4
20
GND
R1
MCLT4-1H
18mA TYP
19
13
36mA TYP
AP
BIAS
COM
VFB
OUT
Q7
VN
IFP
R3
26
IF OUTPUT
1k
R5
20
R2
18
17
16
15
14
Q5
20
1.33k
Q6
2
Q1
NOTE ADDED
RESISTOR
18mA TYP
IFN
VPOS
0.1µF
VP
1
5k
5k
–8–
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 external capacitors; these attenuate the sum component in
a down-conversion application (Figure 20). The corner fre-
quency of this one-pole low-pass filter (f = (2 RC
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
nearest standard value.
Using the Output Amplifier
The AD831’s output amplifier converts the mixer core’s dif-
ferential current output into a single-ended voltage and provides
an output as high as 1 V peak into a 50
For unity gain operation (Figure 21), the inputs AN and AP
connect to the open-collector outputs of the mixer’s core and
OUT connects to VFB.
Figure 21. Output Amplifier Connected for Unity Gain
Operation
Figure 20. Low-Pass Filtering Using External Capacitors
4
5
6
7
8
GND
RFP
RFN
VN
VN
VP
4
5
6
7
8
3
9
AN
GND
RFP
RFN
VN
50
VN
LON
F
10
2
= (2
VP
IFN
3
9
AN
50
LON
LOP
11
C
1
10
VP
F
2
Top View
IFN
AD831
=
C
2
F
VP
12
20
14
1
11
LOP
IFP
1
f R
50
VP
Top View
C
GND
AD831
=
F
19
13
with a tolerance of 20%)
89.7 f
AP
VP
12
BIAS
20
COM
VFB
OUT
1
VN
140 MHz)
IFP
50
GND
18
17
16
15
14
19
13
AP
load (+10 dBm).
BIAS
COM
VFB
OUT
VN
18
17
16
15
14
–1
IF
OUTPUT
F
)
–1
82 pF, the
) should
REV. B