AD641 AD [Analog Devices], AD641 Datasheet - Page 15

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

Figure 31 shows a plot of RSSI vs. frequency for various input
signal amplitudes. It can be seen that at higher frequencies the
output drops off as explained in the section “Effect of Fre-
quency on Calibration.’’ If the RSSI circuit is to be operated at
a known frequency with limited bandwidth, the compensation
techniques described in that section can be used to enhance
accuracy.
250 MHz RSSI Converter with 58 dB Dynamic Range
For a larger dynamic range two AD641s can be cascaded, as
shown in Figure 32. The low end usefulness of the circuit will
be set by the noise floor of the overall environment that the circuit
sees. This includes all sources of both radiated and conducted
noise. Proper layout to avoid conducted noise and good shield-
ing to minimize radiated noise are essential for good low signal
operation.
REV. C
–0.5
SIGNAL
INPUT
2.5
1.5
0.5
Figure 31. Single AD641 RSSI vs. Frequency
3
2
1
0
1
R1
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
R2
SIG
SIG
–IN
+IN
20
1
ATN
OUT
ATN
LO
19
2
Figure 32. Complete 58 dB Dynamic Range Converter for 250 MHz Operation
10
COM
CKT
COM
ATN
FREQUENCY – MHz
18
3
0dBm
–20dBm
–35dBm
–50dBm
COM
ATN
RG1
17
4
1k
U1 AD641
RG0
ATN
16
IN
5
1k
RG2
BL1
100
15
100
6
R 3
–V
LOG
OUT
14
7
S
47pF
C1
COM
LOG
ITC
NC
13
8
BL2
+V
12
9
1000
S
+OUT
–OUT
SIG
SIG
11
10
68
18
–15–
L1
(SEE
TEXT)
–6V
+6V
Filtering between the devices and input offset nulling techniques
described elsewhere are also useful for extending the dynamic
range of two cascaded devices.
Figure 33 shows a plot of this circuit vs. frequency for various
input amplitudes. The drop off at high frequency can be seen to
be greater than for the single device case due to the compound-
ing effects of the bandwidth limiting of the extra stages.
18
68
SIG
SIG
+IN
–IN
20
1
Figure 33. Cascaded AD641s RSSI vs. Frequency
4.5
3.5
2.5
1.5
0.5
ATN
OUT
ATN
LO
4
3
19
2
1
0
2
1
COM
COM
CKT
ATN
18
3
COM
RG1
ATN
17
4
1k
2
3
U2 AD641
1.13k
AD846
ATN
RG0
R 5
16
IN
5
U3
10
1k
7
4
FREQUENCY – MHz
RG2
BL1
(SEE TEXT)
–80dBm
15
6
4.7
–V
6
LOG
–50dBm
OUT
14
7
47pF
S
R 4
100
–20dBm
C 2
–6V
COM
LOG
ITC
13
8
0dBm
100
BL2
+V
12
9
4.7
S
–OUT
+OUT
SIG
NC
SIG
NC
11
10
+6V
LOG
OUTPUT
+50mV/dB
(LO)
AD641
1000