AD8361
–13–
REV. A
The AD8361 can be disabled either by pulling the PWDN (Pin 4)
to VPOS or by simply turning off the power to the device. While
turning off the device obviously eliminates the current consump-
tion, disabling the device reduces the leakage current to less
than 1 µA. Figures 23 and 24 show the response of the output of
the AD8361 to a pulse on the PWDN pin, with no capacitance and
with a filter capacitance of 0.01 µF respectively; the turn-on time is
a function of the filter capacitor. Figure 27 shows a plot of the
output response to the supply being turned on (i.e., PWDN is
grounded and VPOS is pulsed) with a filter capacitor of 0.01 µF
Again, the turn-on time is strongly influenced by the size of the
filter capacitor.
If the input of the AD8361 is driven while the device is disabled
(PWDN = VPOS), the leakage current of less than 1 µA will
increase as a function of input level. When the device is dis-
abled, the output impedance increases to around 16 kΩ.
Volts to dBm Conversion
In many of the plots, the horizontal axis is scaled in both rms
volts and dBm. In all cases, dBm are calculated relative to an
impedance of 50 Ω. To convert between dBm and volts in a
50 Ω. system, the following equations can be used. Figure 40
shows this conversion in graphical form.
Power dBm
V rms
W
V rms
V rms W
dBm
dBm
( ) log
()
.
log ( ( ) )
. log
log /
–
–
=
=
=××
=
(
)
10
50
0 001
10 20
0 001 50
10
10
20
2
2
1
1
Ω
Ω
V rms dBm
+20
+10
0
–10
–20
–30
–40
1
0.1
0.01
0.001
Figure 41. Conversion from dBm to rms Volts
Output Drive Capability and Buffering
The AD8361 is capable of sourcing an output current of approxi-
mately 3 mA. If additional current is required, a simple buffering
circuit can be used as shown in Figure 42c. Similar circuits
can be used to increase or decrease the nominal conversion gain of
7.5 V/V rms (Figure 42a and 42b). In Figure 42b, the AD8031
buffers a resistive divider to give a slope of 3.75 V/V rms. In Figure
42a, the op amp’s gain of two increases the slope to 15 V/V rms.
Using other resistor values, the slope can be changed to an
arbitrary value. The AD8031 rail-to-rail op amp, used in these
examples can swing from 50 mV to 4.95 V on a single 5 V supply
and operate at supply voltages down to 2.7 V. If high output
current is required (>10 mA), the AD8051, which also has rail-
to-rail capability, can be used, down to a supply voltage of 3 V. It
can deliver up to 45 mA of output current.
100pF
0.01F
AD8361
VOUT
VPOS
COMM PWDN
5k⍀
5k⍀
0.01F
5V
15V/V rms
AD8031
a. Slope of 15 V/V rms
AD8361
VOUT
VPOS
COMM PWDN
0.01F
5V
3.75V/V rms
AD8031
10k⍀
5k⍀
5k⍀
100pF
0.01F
b. Slope of 3.75 V/V rms
100pF
AD8361
VOUT
VPOS
COMM PWDN
0.01F
0.01F
5V
7.5V/V rms
AD8031
c. Slope of 7.5 V/V rms
Figure 42. Output Buffering Options
OUTPUT REFERENCE TEMPERATURE DRIFT
COMPENSATION
The error due to low temperature drift of the AD8361 can be
reduced if the temperature is known. Many systems incorporate
a temperature sensor; the output of the sensor is typically digi-
tized, facilitating a software correction. Using this information,
only a two-point calibration at ambient is required.
The output voltage of the AD8361 at ambient (25°C) can be
expressed by the equation:
V GAIN V V
OUT IN OS
=×
()
+
where GAIN is the conversion gain in V/V rms and V
OS
is the
extrapolated output voltage for an input level of 0 V. GAIN and
V
OS
(also referred to as Intercept and Output Reference) can be
calculated at ambient using a simple two-point calibration;
that is, by measuring the output voltages for two specific input
levels. Calibration at roughly 35 mV rms (–16 dBm) and
250 mV rms (+1 dBm) is recommended for maximum linear
dynamic range. However, alternative levels and ranges can be
