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AD8311

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tolerance of the external resistor. This method of matching is

most useful in wideband applications or in multiband systems

where there is more than one operating frequency.

A reactive match can also be implemented as shown in

Figure 34. This is not recommended at low frequencies because

device tolerances dramatically vary the quality of the match due

to the large input resistance. For low frequencies, Figure 33 or

Figure 35 is recommended.

In Figure 34, the matching components are drawn as generic

reactances. Depending on the frequency, the input impedance,

and the availability of standard value components either a

capacitor or an inductor is used. As in the previous case, the

input impedance at a particular frequency is plotted on a Smith

Chart and matching components are chosen (shunt or series L,

shunt or series C) to move the impedance to the center of the

chart.

RFIN

AD8311

RIN

CIN

RSHUNT

52.3

Figure 33. Broadband Resistive Input Coupling Option

RFIN

AD8311

RIN

CIN

Figure 34. Narrow Band Reactive Input Coupling Option

RFIN

AD8311

RIN

CIN

RATTN

ANTENNA

STRIPLINE

Figure 35. Series Attention Input Coupling Option

Figure 35 shows a third method for coupling the input signal

into the AD8311. A series resistor connected to the RF source

combines with the input impedance of the AD8311 to

resistively divide the input signal being applied to the input.

This has the advantage of very little power being tapped off in

RF power transmission applications.

TEMPERATURE DRIFT

Figure 36 shows the log slope and error over temperature for a

0.9 GHz input signal. Error due to drift over temperature

consistently remains within ±1 dB and only begins to exceed

this limit when the ambient temperature goes above +65 °C and

below −20 °C. For all frequencies using a reduced temperature

range, higher measurement accuracy is achievable.

–60

0.2

1.6

VSET (V)

–10

–20

–30

–40

–50

0.4

0.6

0.8

1.0

1.2

1.4

–3

–1

–2

–40°C

–20°C

0°C

+25°C

+45°C

+65°C

+85°C

Figure 36. Typical Drift at 900 GHz for Various Temperatures

DEVICE CALIBRATION AND ERROR CALCULATION

The measured transfer function of the AD8311 at 0.9 GHz is

shown in Figure 37. The figure shows plots of both input power

and calculated error vs. setpoint voltage.

The vertical axis represents the input power required at the RFIN

pin to null the control loop when a VSET voltage is applied. As

the setpoint voltage varies from about 0.2 V to 1.5 V, the

corresponding input power varies from −60 dBm to +10 dBm.

–60

1.6

VSET (V)

–10

–20

–30

–40

–50

–3

–1

–2

0.2

0.4

0.6

0.8

1.0

1.2

1.4

PIN2

PIN1

VSET1

INTERCEPT

VSET2

PINIDEAL = (VSET1/ SLOPE) + INTERCEPT

ERROR (dB) = (PINIDEAL – PIN)

SLOPE = (VSET2 – VSET1)/(PIN1 – PIN2)

INTERCEPT = PIN1 – (VSET1 / SLOPE)

+85°C

–40°C

+25°C

–40°C

+25°C

+85°C

Figure 37. Transfer Function of AD8311 at 0.9 GHz

Because slope and intercept vary from device to device, board-

level calibration must be performed to achieve high accuracy.

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