AD641 AD [Analog Devices], AD641 Datasheet - Page 14

• Manufacturer Part Number: AD641
• Description: 250 MHz Demodulating Logarithmic Amplifier
• Manufacturer: AD [Analog Devices]

AD641
PRACTICAL APPLICATIONS
We show here two applications, using AD641s to achieve a wide
dynamic range. As already mentioned, the use of a differential
signal path and differential logarithmic outputs diminishes the
risk of instability due to poor grounding. Nevertheless, it must
be remembered that at high frequencies even very small lengths
of wire, including the leads to capacitors, have significant im-
pedance. The ground plane itself can also generate small but
troublesome voltages due to circulating currents in a poor lay-
out. A printed circuit evaluation board is available from Analog
Devices (Part Number AD641-EB) to facilitate the prototyping
of an application using one or two AD641s, plus various exter-
nal components.
At very low signal levels various effects can cause significant
deviation from the ideal response, apart from the inherent non-
linearities of the transfer function already discussed. Note that
any spurious signal presented to the AD641s is demodulated and
added to the output. Thus, in the absence of thorough shielding,
emissions from any radio transmitters or RFI from equipment
operating in the locality will cause the output to appear too
high. The only cure for this type of error is the use of very care-
ful grounding and shielding techniques.
SIGNAL
INPUT
R1
R2
SIG
SIG
+IN
–IN
20
1
ATN
OUT
ATN
LO
19
2
COM
COM
ATN
CKT
18
3
COM
RG1
ATN
17
4
1k
U1 AD641
ATN
RG0
16
IN
5
1k
RG2
BL1
15
6
Figure 30. RSSI Using Single AD641
–V
LOG
OUT
14
7
S
47pF
C1
COM
LOG
ITC
NC
13
8
100
R3
+V
BL2
12
9
S
+OUT
–OUT
SIG
SIG
11
10
68
18
–14–
–6V
+6V
RSSI APPLICATIONS
The AD641 can be used to perform an RSSI (Received Signal
Strength Indicator) function. This is a commonly used function
in radio receivers, but can be used in other instrumentation such
as photomultiplier tubes. The signal strength indicator on FM
radios is one example of an RSSI application. It is this signal
that is monitored to determine where to stop during seek or
scan operations.
The AD641 is used to measure the strength of the incoming RF
signal and outputs a current that is proportional to the loga-
rithm of its ac amplitude. In this manner signal amplitudes with
a wide dynamic range and wide bandwidth can be measured.
250 MHz RSSI Converter with 44 dB Dynamic Range
Figure 30 shows the schematic for an RSSI circuit that uses a
single AD641. The dynamic range for this circuit using a single
AD641 is 44 dB. The AD641 amplifies and full wave rectifies
(detects) the input and outputs a current. The AD846 is used to
convert the current to a ground referenced voltage. With a 1 k
feedback resistor, the output varies by 1 V/decade or 50 mV/dB.
DENOTES A CONNECTION TO THE
GROUND PLANE; OBSERVE COMMON
CONNECTIONS WHERE SHOWN.
ALL UNMARKED CAPACITORS ARE
0.1 F CERAMIC. FOR VALUES OF
NUMBERED COMPONENTS SEE TEXT
2
3
AD846
1.0k
U3
7
4
4.7
6
–6V
4.7
+6V
(LO)
RSSI
OUTPUT
+50mV/dB
REV. C