AD831 Analog Devices, AD831 Datasheet - Page 9

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

REV. B
For gains other than unity, the amplifier’s output at OUT is
connected via an attenuator network to VFB; this determines
the overall gain. Using resistors R1 and R2 (Figure 22), the gain
setting expression is
Driving Filters
The output amplifier can be used for driving reverse-terminated
loads. When driving an IF bandpass filter (BPF), for example,
proper attention must be paid to providing the optimal source
and load terminations so as to achieve the specified filter re-
sponse. The AD831’s wideband highly linear output amplifier
affords an opportunity to increase the RF-to-IF gain to compen-
sate for a filter’s insertion and termination losses.
Figure 23 indicates how the output amplifier’s low impedance
(voltage source) output can drive a doubly-terminated bandpass
filter. The typical 10 dB of loss (4 dB of insertion loss and 6 dB
due to the reverse-termination) be made up by the inclusion of a
feedback network that increases the gain of the amplifier by
10 dB ( 3.162). When constructing a feedback circuit, the sig-
nal path between OUT and VFB should be as short as possible.
Figure 22. Output Amplifier Feedback Connections for
Increasing Gain
4
5
6
7
8
RFP
GND
VN
RFN
VN
VP
3
9
AN
G
50
dB
LON
10
2
IFN
20 log
LOP
11
1
VP
Top View
AD831
10
VP
12
20
IFP
50
R1 R2
GND
19
13
R2
AP
BIAS
COM
VFB
OUT
VN
18
17
16
15
14
R1
R2
IF
OUTPUT
Equation 2
–9–
Figure 23. Connections for Driving a Doubly-Terminated
Bandpass Filter
Higher gains can be achieved, using different resistor ratios, but
with concomitant reduction in the bandwidth of this amplifier
(Figure 24). Note also that the Johnson noise of these gain-set-
ting resistors, as well as that of the BPF terminating resistors, is
ultimately reflected back to the mixer’s input; thus they should
be as small as possible, consistent with the permissible loading
on the amplifier’s output.
Figure 24. Output Amplifier 1 dB Compression Point for
Gains of 1, 2, and 4 (Gains of 0 dB, 6 dB, and 12 dB,
Respectively)
4
5
6
7
8
RFP
GND
VN
RFN
VN
VP
12
10
3
9
8
6
4
2
0
AN
10
50
LON
G = 1
G = 2
G = 4
10
2
IFN
11
LOP
1
VP
Top View
AD831
20
VP
12
IFP
50
FREQUENCY – MHz
GND
19
13
AP
BIAS
COM
VFB
OUT
VN
100
18
17
16
15
14
51.1
110
R2
R1
R
T
BPF
AD831
R
1000
T
IF
OUTPUT