AD640 Analog Devices, AD640 Datasheet - Page 12

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

AD640
can be adjusted by adding or subtracting a small current to the
output. Since the slope current is 1 mA/decade, a 50 A incre-
ment will move the intercept by 1 dB. Note that any error in
this current will invalidate the calibration of the AD640. For
example, if one of the 5 V supplies were used with a resistor to
generate the current to reposition the intercept by 20 dB, a
absolute calibration. Of course, slope calibration is unaffected.
Source Resistance and Input Offset
The bias currents at the signal inputs (Pins 1 and 20) are typi-
cally 7 A. These flow in the source resistances and generate
input offset voltages which may limit the dynamic range because
the AD640 is direct coupled and an offset is indistinguishable
from a signal. It is good practice to keep the source resistances
as low as possible and to equalize the resistance seen at each
input. For example, if the source resistance to Pin 20 is 100 , a
compensating resistor of 100
Pin l. The residual offset is then due to the bias current offset,
which is typically under 1 A, causing an extra offset uncertainty
of 100 V in this example. For a single AD640 this will rarely be
troublesome, but in some applications it may need to be nulled
out, along with the internal voltage offset component. This may
be achieved by adding an adjustable voltage of up to 250 V at
the unused input. (Pins l and 20 may be interchanged with no
change in function.)
In most applications there will be no need to use any offset
adjustment. However, a general offset trimming circuit is shown
in Figure 25. R
rf sources may include a blocking capacitor and have no dc path to
ground, or may be transformer coupled and have a near zero resis-
tance to ground. Determine whether the source resistance is zero,
25
the correct value of bias compensating resistor, R
should optimally be equal to R
use R
provide a 250 V trim range. To null the offset, set the source
voltage to zero and use a DVM to observe the logarithmic out-
put voltage. Recall that the LOG OUT current of the AD640
exhibits an absolute value response to the input voltage, so the offset
potentiometer is adjusted to the point where the logarithmic output
“turns around” (reaches a local maximum or minimum).
Figure 25. Optional Input Offset Voltage Nulling Circuit;
See Text for Component Values
At high frequencies it may be desirable to insert a coupling
capacitor and use a choke between Pin 20 and ground, when
Pin 1 should be taken directly to ground. Alternatively, trans-
former coupling may be used. In these cases, there is no added
offset due to bias currents. When using two dc coupled AD640s
(overall gain 100,000), it is impractical to maintain a sufficiently
low offset voltage using a manual nulling scheme. The section
10% variation in this supply will cause a 2 dB error in the
or 50
B
= 5 . The value of R
20k
(with the generator terminated in 50 ) to find
+5V
–5V
S
is the source resistance of the signal. Note: 50
(SOURCE RESISTANCE
OF TERMINATED
GENERATOR)
OS
R
R
R
S
OS
, unless R
S
B
should be placed in series with
should be set to 20,000 R
S
20
1
AD640
= 0, in which case
19
2
B
, which
B
to
–12–
CASCADED OPERATION explains how the offset can be
automatically nulled to submicrovolt levels by the use of a nega-
tive feedback network.
Using Higher Supply Voltages
The AD640 is calibrated using 5 V supplies. Scaling is very
insensitive to the supply voltages (see dc SPECIFICATIONS)
and higher supply voltages will not directly cause significant
errors. However, the AD640 power dissipation must be kept
below 500 mW in the interest of reliability and long-term stabil-
ity. When using well regulated supply voltages above 6 V, the
decoupling resistors shown in the application schematics can be
increased to maintain 5 V at the IC. The resistor values are
calculated using the specified maximum of 15 mA current into
the +V
–V
resistor of (9 V–5 V)/15 mA, about 261 , should be included in
the +V
in each –V
with in a similar way.
Using the Attenuator
In applications where the signal amplitude is sufficient, the on-
chip attenuator should be used because it provides a tempera-
ture independent dynamic range (compare Figures 18 and 19).
Figure 26 shows this attenuator in more detail. R1 is a thin-film
resistor of nominally 270
(TC). It is trimmed to calibrate the intercept to 10 mV dc (or
–24 dBm for sinusoidal inputs), that is, to an attenuation of
nominally 20 dBs at 27 C. R2 has a nominal value of 30
has a high positive TC, such that the overall attenuation factor
is 0.33%/ C at 27 C. This results in a transmission factor that is
proportional to absolute temperature, or PTAT. (See Intercept
Stabilization for further explanation.) To improve the accuracy
of the attenuator, the ATN COM nodes are bonded to both
Pin 3 and Pin 4. These should be connected directly to the “SIG-
NAL LOW” of the source (for example, to the grounded side of
the signal connector, as shown in Figure 32) not to an arbitrary
point on the ground plane.
R4 is identical to R2, and in shunt with R3 (270
forms a 27
of the attenuator. By connecting Pin 1 to ATN LOW (Pin 2)
this resistance minimizes the offset caused by bias currents. The
offset nulling scheme shown in Figure 25 may still be used, with
the external resistor R
bility is improved because the compensating voltage introduced
at Pin 20 is now PTAT. Drifts of under 1 V/ C (referred to
Pins 1 and 20) can be maintained using the attenuator.
S
terminal (Pin 7). For example, when using 9 V supplies, a
S
S
lead to each AD640, and (9 V–5 V)/60 mA, about 64.9 ,
terminal (Pin 12) and a maximum of 60 mA into the
Figure 26. Details of the Input Attenuator
S
lead. Of course, asymmetric supplies may be dealt
resistor with the same TC as the output resistance
SIG
+IN
SIG
–IN
20
1
B
ATN
ATN
OUT
LO
omitted and R
19
2
R2
R4
R1
R3
COM
and low temperature coefficient
ATN
18
3
COM
ATN
17
4
INPUT
OS
ATN
AD640
16
IN
5
AMPLIFIER
= 500 k . Offset sta-
FIRST
thin film)
REV. C
and