AD640 Analog Devices, AD640 Datasheet - Page 15

• Manufacturer Part Number: AD640
• Description: DC-Coupled Demodulating 120 MHz Logarithmic Amplifier
• Manufacturer: Analog Devices

REV. C
diminishes the risk of instability due to poor grounding. Never-
theless, it must be remembered that at high frequencies even
very small lengths of wire, including the leads to capacitors,
have significant impedance. The ground plane itself can also
generate small but troublesome voltages due to circulating cur-
rents in a poor layout. A printed circuit evaluation board is
available from Analog Devices (Part Number ADEB640) to
facilitate the prototyping of an application using one or two
AD640s, plus various external 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 AD640s 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.
50 MHz–150 MHz Converter with 70 dB Dynamic Range
Figure 30 shows a logarithmic converter using two AD640s
which can provide at least 70 dB of dynamic range, limited
mostly by first stage noise. In this application, an rf choke (L1)
prevents the transmission of dc offset from the first to the sec-
ond AD640. One or two turns in a ferrite core will generally
suffice for operation at frequencies above 30 MHz. For ex-
ample, one complete loop of 20 gauge wire through the two
holes in a Fair-Rite type 2873002302 core provides an inductance
of 5 H, which presents an impedance of 1.57 k at 50 MHz.
The shunting effect across the 150
the signal interface is thus fairly slight.
The signal source is optionally terminated by R1. To minimize
the input offset voltage R2 should be chosen to match the dc
resistance of the terminated source. (However, the offset voltage
is not a critical consideration in this ac-coupled application.)
Note that all unused inputs are grounded; this improves the
isolation from the outputs back to the inputs.
FEEDBACK
A
NULLING
B
+15V
+15V
–15V
–15V
OFFSET
NC = NO CONNECT
5k
5k
0.1 F
0.1 F
SIG
SIG
+IN
–IN
DENOTES A CONNECTION TO THE GROUND PLANE;
OBSERVE COMMON CONNECTIONS WHERE SHOWN.
ALL UNMARKED CAPACITORS ARE 0.1 F CERAMIC.
TO U3
AND U4
20
1
OUT
ATN
ATN
LO
19
2
9.1V
9.1V
COM
COM
ATN
CKT
TO U3
AND U4
18
3
COM
TO U1
AND U2
RG1 RG0 RG2 LOG
ATN
SIGNAL INPUT
17
4
+6V
1k
–6V
U1 AD640
ATN
16
IN
5
1k
BL1
15
6
OUT
Figure 32. Complete 95 dB Dynamic Range Converter
differential impedance at
–V
14
7
S
COM
LOG
ITC
13
8
BL2
+V
12
9
S
–OUT
+OUT
SIG
SIG
11
10
68
18
+6V
–6V
C1
(SEE
TEXT)
C2
(SEE
TEXT)
68
18
100 F
–15–
SIG
SIG
+IN
–IN
C3
20
1
ATN
OUT
ATN
A transimpedance op amp (U3, AD844) converts the summed
logarithmic output currents of U1 and U2 to a ground referenced
voltage scaled 1 V per decade. The resistor R5 is nominally 1 k
but is increased slightly to compensate for the slope deficit at the
operating frequency, which can be determined from Figure 12.
The inverting input of U3 forms a virtual ground, so that each
logarithmic output of U1 and U2 is loaded by 100
R4). These resistors in conjunction with capacitors C1 and C2
form independent low-pass filters with a time constant of about
Figure 31. Logarithmic Output and Nonlinearity for Circuit
of Figure 30, for a Sine Wave Input at f = 80 MHz
5 ns. These capacitors should be connected directly across Pins
13 and 14, as shown, to prevent high frequency output currents
from circulating in the ground plane. A second 5 ns time con-
stant is formed by feedback resistor R5 in conjunction with the
transcapacitance of U3.
This filtering is adequate for input frequencies of 50 MHz or
above; more elaborate filtering can be devised for pulse
applications requiring a faster rise time. In applications where
only a long term measure of the input is needed, C1 and C2 can
LO
19
2
49.9
R1
COM
COM
ATN
CKT
18
3
1
COM
4
3
2
0
–70
RG1 RG0 RG2 LOG
ATN
NC
17
4
1k
U2 AD640
ATN
16
IN
2
3
5
4.7 F
1k
C4
–60
U3a
BL1
15
6
AD712
1/2
OUT
–V
14
–50
7
1
S
INPUT LEVEL – dBm IN 50
LOG
COM
ITC
13
8
50k
R2
–40
BL2
+V
12
9
0.1 F
S
50k
+OUT
–OUT
R3
C6
SIG
SIG
10
11
3.3M
3.3M
–30
R6
R7
0.1 F
C5
5
6
R5
200k
U3b
C7
4.7 F
C8
4.7 F
–20
AD712
200k
R4
1/2
3
2
5
6
7
–10
U4a
U4b
AD712
AD640
AD712
+100mV/dB
1/2
OUTPUT
1/2
FEEDBACK
LOG
(R3 or
1
NULLING
7
OFFSET
0
+1
–1
0
B
A