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 k

Ω.

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

0.01 F

AD8361

VOUT

VPOS

COMM PWDN

5k

5k

0.01 F

5V

15V/V rms

AD8031

a. Slope of 15 V/V rms

AD8361

VOUT

VPOS

COMM PWDN

0.01 F

5V

3.75V/V rms

AD8031

10k

5k

5k

100pF

0.01 F

b. Slope of 3.75 V/V rms

100pF

AD8361

VOUT

VPOS

COMM PWDN

0.01 F

0.01 F

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

=×

extrapolated output voltage for an input level of 0 V. GAIN and

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