AD641 AD [Analog Devices], AD641 Datasheet - Page 10

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

AD641
SIGNAL MAGNITUDE
The AD641 is a calibrated device. It is, therefore, important to
be clear in specifying the signal magnitude under all waveform
conditions. For dc or square wave inputs there is, of course, no
ambiguity. Bounded periodic signals, such as sinusoids and
triwaves, can be specified in terms of their simple amplitude
(peak value) or alternatively by their rms value (which is a mea-
sure of power when the impedance is specified). It is generally bet-
ter to define this type of signal in terms of its amplitude because
the AD641 response is a consequence of the input voltage, not
power. However, provided that the appropriate value of inter-
cept for a specific waveform is observed, rms measures may be
used. Random waveforms can only be specified in terms of rms
value because their peak value may be unbounded, as is the case
for Gaussian noise. These must be treated on a case-by-case
basis. The effective intercept given in Table I should be used for
Gaussian noise inputs.
On the other hand, for bounded signals the amplitude can be
expressed either in volts or dBV (decibels relative to 1 V). For
example, a sine wave or triwave of 1 mV amplitude can also be
defined as an input of –60 dBV, one of 100 mV amplitude as
–20 dBV, and so on. RMS value is usually expressed in dBm
(decibels above 1 mW) for a specified impedance level. Through-
out this data sheet we assume a 50
impedance level for high speed systems, when referring to signal pow-
ers in dBm. Bearing in mind the above discussion of the effect of
waveform on the intercept calibration of the AD641, it will be
apparent that a sine wave at a power of, say, –10 dBm will not
produce the same output as a triwave or square wave of the
same power. Thus, a sine wave at a power level of –10 dBm has
an rms value of 70.7 mV or an amplitude of 100 mV (that is, 2
times as large, the ratio of amplitude to rms value for a sine
wave), while a triwave of the same power has an amplitude
which is 3 or 1.73 times its rms value, or 122.5 mV.
“Intercept” and “Logarithmic Offset”
If the signals are expressed in dBV, we can write the output
current in a simpler form, as:
where Input
and X
form) in dBV. This form shows more clearly why the intercept is
I
OUT
dBV
= 50 A (Input
is the appropriate value of the intercept (for a given wave-
TERMINATION
dBV
OPTIONAL
RESISTOR
SIGNAL
OFFSET BALANCE
INPUT
is the input voltage amplitude (not rms) in dBV
ALL UNMARKED CAPACITORS ARE
0.1 F CERAMIC (SEE TEXT).
OPTIONAL
DENOTES A SHORT, DIRECT CONNECTION
TO THE GROUND PLANE.
RESISTOR
dBV
Figure 24. Connections for a Single AD641 to Verify Basic Performance
– X
dBV
SIG
SIG
–IN
+IN
20
1
)
environment, the customary
ATN
ATN
OUT
LO
19
2
COM
COM
ATN
CKT
18
3
COM
ATN
RG1
17
4
1k
ATN
RG0
16
IN
Equation (4)
AD641
5
1k
RG2
BL1
15
6
–V
LOG
OUT
14
7
S
–10–
COM
LOG
ITC
NC
13
8
often referred to as the logarithmic offset. For dc or square wave
inputs, V
Equation (4) becomes
Alternatively, for a sinusoidal input measured in dBm (power in
dB above 1 mW in a 50
because the intercept for a sine wave expressed in volts rms is at
1.414 mV (from Table I) or –44 dBm.
OPERATION OF A SINGLE AD641
Figure 24 shows the basic connections for a single device, using
100
slope of –100 mV per decade; output B is positive going with a
slope of +100 mV per decade. For applications where absolute
calibration of the intercept is essential, the main output (from
LOG OUT, Pin 14) should be used; the LOG COM output can
then be grounded. To evaluate the demodulation response, a
simple low pass output filter having a time constant of roughly
500 s (3 dB corner of 320 Hz) is provided by a 4.7 F (–20%
+80%) ceramic capacitor (Erie type RPE117-Z5U-475-K50V)
placed across the load. A DVM may be used to measure the
averaged output in verification tests. The voltage compliance at
Pins 13 and 14 extends from 0.3 V below ground up to 1 V
below +V
zero signal to +2.3 mA when fully limited (dc input of >300 mV)
the output never drops below –230 mV. On the other hand, the
current out of Pin 13 ranges from –0.2 mA to +2.3 mA, and if
desired, a load resistor of up to 2 k can be used on this output;
the slope would then be 2 V per decade. Use of the LOG COM
output in this way provides a numerically correct decibel read-
ing on a DVM (+100 mV = +1.00 dB).
Board layout is very important. The AD641 has both high gain
and wide bandwidth; therefore every signal path must be very
carefully considered. A high quality ground plane is essential,
but it should not be assumed that it behaves as an equipotential
plane. Even though the application may only call for modest
bandwidth, each of the three differential signal interface pairs
(SIG IN, Pins l and 20, SIG OUT, Pins 10 and 11, and LOG,
Pins 13 and 14) must have their own “starred” ground points to
avoid oscillation at low signal levels (where the gain is highest).
BL2
+V
12
9
S
I
I
–OUT
+OUT
OUT
OUT
SIG
NC
SIG
11
10
NC
load resistors. Output A is a negative going voltage with a
10
X
= 50 A (Input
= 50 A (Input
S
4.7 F
. Since the current into Pin 14 is from –0.2 mA at
is 1 mV so the numerical value of X
4.7
100
0.1%
R
dBm
dBV
LA
4.7 F
system) the output can be written
+ 44)
+ 60)
100
0.1%
R
LB
–5V
+5V
OUTPUT A
OUTPUT B
dBV
is –60, and
Equation (5)
Equation (6)
REV. C