AD640
REV. D –15–
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 µH, which presents an impedance of 1.57 kΩ at 50 MHz.
The shunting effect across the 150 Ω differential impedance at
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.
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 kΩ
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 Ω (R3 or
R4). These resistors in conjunction with capacitors C1 and C2
form independent low-pass filters with a time constant of about
INPUT LEVEL – dBm IN 50V
0–60 –50 –40 –30 –20 –10
0
+1
–1
ERROR – dB
4
1
0
–70
LOW-PASS FILTERED OUTPUT – V
2
3
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
5kV
0.1mF
0.1mF
+15V
–15V
TO U3
AND U4
15 13141619 18 17 1112
20
6
8
753
4 91
2
10
SIG
+IN
ATN
OUT
CKT
COM
RG1 RG0 RG2 LOG
OUT
LOG
COM
+V
S
SIG
+OUT
SIG
–IN
ATN
LO
ATN
COM BL1 BL2ITC–V
S
SIG
–OUT
1kV 1kV
ATN
COM
ATN
IN
U2 AD640
C6
0.1mF
68V
+6V
68V
R7
3.3MV
1/2
AD712
DENOTES A CONNECTION TO THE GROUND PLANE;
OBSERVE COMMON CONNECTIONS WHERE SHOWN.
ALL UNMARKED CAPACITORS ARE 0.1mF CERAMIC.
OFFSET
NULLING
FEEDBACK
U4a
5kV
SIGNAL INPUT
15
131416
19
18
17 11
12
20
SIG
+IN
ATN
OUT
CKT
COM
RG1 RG0 RG2 LOG
OUT
LOG
COM
+V
S
SIG
+OUT
SIG
–IN
ATN
LO
ATN
COM BL1 BL2ITC–V
S
SIG
–OUT
1kV 1kV
ATN
COM
ATN
IN
U1 AD640
6
8
753
4912 10
C1
(SEE
TEXT)
C2
(SEE
TEXT)
C7
4.7mF
C8
4.7mF
1
2
3
B
OFFSET
NULLING
FEEDBACK
U4b
7
6
5
A
1/2
AD712
1/2
AD712
U3a
1
2
3
NC
R2
50kV
R3
50kV
U3b
1/2
AD712
7
6
5
R4
200kV
R5
200kV
LOG
OUTPUT
+100mV/dB
C4
4.7mF
R1
49.9V
C3
100mF
C5
0.1mF
A
B
+15V
–15V
TO U1
AND U2
TO U3
AND U4
9.1V
9.1V
+6V
–6V
18V
–6V
18V
NC = NO CONNECT
R6
3.3MV
Figure 32. Complete 95 dB Dynamic Range Converter
