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AD8318

Rev. 0 | Page 16 of 24

In general, the calibration is performed by applying two known

signal levels to the AD8318’s input and measuring the

corresponding output voltages. The calibration points are

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

the device (see Figure 30). Calculation of slope and intercept is

done using the equations

Slope = (VOUT1 − VOUT2)/(PIN1 – PIN2)

(9)

Intercept = PIN1 − VOUT1/Slope

(10)

Once Slope and Intercept have been calculated, an equation can

be written which will allow calculation of an (unknown) input

power based on the output voltage of the detector.

PIN(unknown) = VOUT(measured)/Slope + Intercept

(11)

Using the equation for the ideal output voltage (7) as a reference,

the log conformance error of the measured data can be

calculated:

Error(dB) = (VOUT(MEASURED) − VOUT(IDEAL))/Slope

(12)

Figure 30 includes a plot of the error at 25°C, the temperature at

which the log amp is calibrated. Note that the error is not zero.

This is because the log amp does not perfectly follow the ideal

VOUT versus PIN equation, even within its operating region. The

error at the calibration points (−12 dBm and −52 dBm in this

case) will, however, be equal to zero by definition.

Figure 30 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) will be most critical at or close to full power.

Figure 31 shows the same measured data as Figure 30. Notice

that accuracy is very high from −10 dBm to −30 dBm. Below

−30 dBm the error increases to about −1 dB. This is because the

calibration points have been changed to −14 dBm and −26 dBm.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.6

0.4

0.2

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–65 –60 –55

–45 –40 –35 –30

–20

–10

–5

PIN (dBm)

PIN1

PIN2

ERROR +25

°C

ERROR –40

°C

ERROR +85

°C

VOUT +25°C

VOUT –40°C

VOUT +85°C

VOUT2

VOUT1

Figure 31. Output Voltage and Error vs. PIN with 2-Point Calibration at

–10 dBm and –30 dBm

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 −5 dBm or below −60 dBm in this

case).

Figure 32 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 −4 dBm and −60 dBm. 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 AD8318 maintains an error of < ±1 dB is

extended to 60 dB at 25°C and 58 dB over temperature. The

disadvantage of this approach is that linearity suffers,

especially at the top end of the input range.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

PIN (dBm)

2.5

–2.5

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

VOUT +25°C

VOUT –40°C

VOUT +85°C

ERROR +25

°C

ERROR –40

°C

ERROR +85

°C

58dB DYNAMIC RANGE (

±1dB ERROR)

Figure 32. Dynamic Range Extension by Choosing Calibration Points

that are Close to the End of the Linear Range

Another way of presenting the error function of a log amp

detector is shown in Figure 33. In this case, the dB error at

hot and cold temperatures is calculated with respect to the

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