ADL5902

Data Sheet

Rev. B | Page 24 of 28

SYSTEM CALIBRATION AND ERROR CALCULATION

The measured transfer function of the ADL5902 at 2.14 GHz is

shown in Figure 50, which contains plots of both output voltage

vs. input amplitude (power) and calculated error vs. input level. As

the input level varies from −62 dBm to +3 dBm, the output

voltage varies from ~0.25 V to ~3.5 V.

0

1

2

3

4

5

6

–70

–60

–50

–40

–30

–20

–10

10

0

VOUT

ERROR 2-POINT CAL AT 0dBm, AND 40dBm

ERROR 3-POINT CAL AT 0 dBm,

–45dBm, AND 60dBm

ERROR 4-POINT CAL AT 0dBm, –20dBm,

–45dBm, AND –60dBm

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

PIN (dBm)

Figure 50. 2.14 GHz Transfer Function, Using Various Calibration Techniques

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

level calibration must be performed to achieve high accuracy.

The equation for the idealized output voltage can be written as

VOUT(IDEAL) = Slope × (PIN − Intercept)

(18)

where:

Slope is the change in output voltage divided by the change in

input power (dB).

Intercept is the calculated input power level at which the output

voltage is 0 V (note that Intercept is an extrapolated theoretical

value not a measured value).

In general, calibration is performed during equipment manu-

facture by applying two or more known signal levels to the

input of the ADL5902 and measuring the corresponding output

voltages. The calibration points are generally within the linear-

in-dB operating range of the device.

With a two-point calibration, the slope and intercept are

calculated as follows:

Slope = (VOUT1 − VOUT2)/(PIN1 − PIN2)

(19)

Intercept = PIN1 − (VOUT1/Slope)

(20)

After the slope and intercept are calculated and stored in non-

volatile memory during equipment calibration, an equation can

calculate an unknown input power based on the output voltage

of the detector.

PIN (Unknown) = (VOUT1(MEASURED)/Slope) + Intercept

(21)

The log conformance error is the difference between this

straight line and the actual performance of the detector.

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

(22)

Figure 50 includes a plot of this error when using a two-point

calibration (calibration points are 0 dBm and −40 dBm). The

error at the calibration points (in this case, −40 dBm and 0 dBm)

is equal to 0 by definition.

The residual nonlinearity of the transfer function that is

apparent in the two-point calibration error plot can be reduced

by increasing the number of calibration points. Figure 50 shows

the postcalibration error plots for three-point and four-point

calibrations. With a multipoint calibration, the transfer function

is segmented, with each segment having a slope and intercept.

Multiple known power levels are applied, and multiple voltages are

measured. When the equipment is in operation, the measured

voltage from the detector first determines which of the stored

slope and intercept calibration coefficients are to be used. Then

the unknown power level is calculated by inserting the

appropriate slope and intercept into Equation 21.

Figure 51 shows the output voltage and error at 25°C and over

temperature when a four-point calibration is used (calibration

points are 0 dBm, −20 dBm, −45 dBm, and −60 dBm). When

choosing calibration points, there is no requirement for, or

value, in equal spacing between the points. There is also no

limit to the number of calibration points used. However, using

more calibration points increases calibration time.

0

1

2

3

4

5

6

–6

–5

–4

–3

–2

–1

0

1

2

3

4

5

6

–70

–60

–50

–40

–30

–20

–10

10

0

PIN (dBm)

+85°C VOUT

+25°C VOUT

–40°C VOUT

+85°C ERROR 4-POINT CAL

+25°C ERROR 4-POINT CAL AT 0dBm,

–20dBm, –45dBm, AND –60dBm

–40°C ERROR 4-POINT CAL

Figure 51. 2.14 GHz Transfer Function and Error at +25°C, −40°C, and +85°C

Using a Four-Point Calibration (0 dBm, −20 dBm, −45 dBm, −60 dBm)

The −40°C and +85°C error plots in Figure 51 are generated

using the 25°C calibration coefficients. This is consistent with

equipment calibration in a mass production environment where

calibration at just a single temperature is practical.