AD8311

Rev. 0 | Page 18 of 24

In a control loop, calibration is performed by applying two

levels to the AD8311’s setpoint voltage and measuring the

corresponding power. The calibration points are generally

chosen to be within the linear-in-dB operating range of the

device (see Figure 37). Calculation of slope and intercept is

done using the equations

)

/(

)

(

1

IN

2

IN

1

SET

2

SET

P

P

V

V

SLOPE

−

−

=

(15)

)

/

(

SLOPE

V

P

INTERCEPT

1

SET

1

=

(16)

Once slope and intercept have been calculated, an equation can

be written which allows calculation of an (unknown) power

based on the setpoint voltage.

INTERCEPT

SLOPE

V

P

SET

IN

+

=

)

/

(

(17)

Using Equation 17 as a reference for the ideal input power, the

log conformance error of the measured data can be calculated:

)

(

)

(

D

IN,MEASURE

IN,IDEAL

P

P

dB

ERROR

−

=

(18)

Figure 37 includes a plot of the error at 25°C, the temperature at

which the AD8311 is calibrated. Note that the error is not zero.

This is because the AD8311 does not perfectly follow the ideal

at the calibration points (0.45 V and 1.15 V in this case) is,

however, equal to zero by definition.

Figure 37 also includes error plots for the output voltage at

−40°C and +85 °C. These error plots are calculated using the

slope and intercept at +25°C. This is consistent with calibration

in a mass-production environment where calibration at

temperature is not practical.

SELECTING CALIBRATION POINTS TO IMPROVE

ACCURACY OVER A REDUCED RANGE

In some applications very high accuracy is required at just one

power level or over a reduced input range. For example, in a

wireless transmitter, the accuracy of the high power amplifier

(HPA) is most critical at or close to full power.

Figure 38 shows the same measured data as Figure 37. Notice

that accuracy is very high from −15 dBm to 0 dBm. Below

−15 dBm the error increases to about −2 dB. This is because the

calibration points have been changed to approximately 0.975 V

and 1.3 V.

10

–60

0

1.6

VSET (V)

0

–10

–20

–30

–40

–50

4

–3

2

1

0

–1

–2

3

0.2

0.4

0.6

0.8

1.0

1.2

1.4

–40°C

+25°C

+85°C

–40°C

+25°C

+85°C

Approximately 0.975 V and 1.3 V

Calibration points should be chosen to suit the application at

hand. In general, though, the calibration points should never be

chosen in the nonlinear portion of the log amp’s transfer

function (above 1.4 V or below 0.35 V in this case).

Figure 39 shows how calibration points can be adjusted to

increase dynamic range, but at the expense of linearity. In this

case the calibration points for slope and intercept are set at

0.37 V and 1.37 V. These points are at the end of the device’s

linear range. Once again at 25°C we see an error of 0 dB at the

calibration points. Note also that the range over which the

AD8311 maintains an error of less than ±0.5 dB is extended to

more than 45 dB at 25°C and more than 40 dB over

temperature. The disadvantage of this approach is that linearity

suffers, especially in the middle of the range.

10

–60

0

1.6

VSET (V)

0

–10

–20

–30

–40

–50

4

–3

2

1

0

–1

–2

3

0.2

0.4

0.6

0.8

1.0

1.2

1.4

–40°C

+25°C

+85°C

–40°C

+25°C

+85°C

Figure 39. Dynamic Range Extension by Choosing Calibration Points that are

Close to the End of the AD8311’s Linear Range