AD641S Просмотр технического описания (PDF) - Analog Devices
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AD641S Datasheet PDF : 16 Pages
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AD641
3
2.5
0dBm
2
1.5
–20dBm
1
–35dBm
0.5
0
–50dBm
4.5
0dBm
4
3.5
–20dBm
3
2.5
2
–50dBm
1.5
1
0.5
–80dBm
–0.5
1
10
100
FREQUENCY – MHz
1000
Figure 31. Single AD641 RSSI vs. Frequency
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.
0
1
10
100
1000
FREQUENCY – MHz
Figure 33. Cascaded AD641s RSSI vs. Frequency
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.
DENOTES A CONNECTION TO THE
GROUND PLANE; OBSERVE COMMON
CONNECTIONS WHERE SHOWN.
ALL UNMARKED CAPACITORS ARE
0.1F CERAMIC. FOR VALUES OF
NUMBERED COMPONENTS SEE TEXT
R5
1.13k⍀ (SEE TEXT)
2
7
U3
AD846 6
3
4
4.7⍀
–6V
4.7⍀
+6V
LOG
OUTPUT
+50mV / d B
(LO)
R3
100⍀
+6V
68⍀
68⍀
R4
100⍀
SIGNAL
INPUT
C1
47pF
20 19 18 17 16 15 14 13 12 11
SIG
+IN
ATN CKT
OUT COM
RG1 RG0 RG2
LOG LOG +VS SIG
OUT COM
+OUT
1k⍀ 1k⍀
R1
U1 AD641
SIG ATN ATN ATN ATN
SIG
–IN LO COM COM IN BL1 –VS ITC BL2 –OUT
1 2 3 4 5 6 7 8 9 10
R2
NC
L1
(SEE
TEXT)
C2
47pF
NC
20 19 18 17 16 15 14 13 12 11
SIG ATN CKT RG1 RG0 RG2 LOG LOG +VS SIG
+IN OUT COM
OUT COM
+OUT
1k⍀ 1k⍀
U2 AD641
SIG ATN ATN ATN ATN
SIG
–IN LO COM COM IN BL1 –VS ITC BL2 –OUT
1 2 3 4 5 6 7 8 9 10
NC
18⍀
18⍀
–6V
Figure 32. Complete 58 dB Dynamic Range Converter for 250 MHz Operation
REV. C
–15–
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