ADL5906ACPZN-R2 datasheet(24/32 Pages) AD

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ADL5906

Data Sheet

Rev. 0 | Page 24 of 32

SYSTEM CALIBRATION AND ERROR CALCULATION

The measured transfer function of the ADL5906 at 2.14 GHz is

shown in Figure 53, which contains plots of both output voltage

vs. input level and linearity error vs. input level. As the input level

varies from −65 dBm to +5 dBm, the output voltage varies from

~0.25 V to ~3.9 V.

–6

–5

–4

–3

–2

–1

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

–65

–55

–45

–35

–25

–15

–5

PIN (dBm)

OUTPUT VOLTAGE –40°C

OUTPUT VOLTAGE +25°C

OUTPUT VOLTAGE +85°C

ERROR –40°C

ERROR +25°C

ERROR +85°C

Figure 53. 2.14 GHz VRMS and Log Conformance Error at +25°C, −40°C, and

+85°C Using Two-Point Calibration at 0 dBm and −40 dBm

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

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

(13)

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 equal to 0 V (note that Intercept is an extrapolated

theoretical value and not a measured value).

In general, calibration is performed during equipment manufacture

by applying two or more known signal levels to the input of the

ADL5906 and measuring the corresponding output voltages.

The calibration points must be within the linear operating range

of the device.

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

as follows:

Slope = (VRMS1 − VRMS2)/(PIN1 − PIN2)

(14)

Intercept = PIN1 − (VRMS1/Slope)

(15)

After the slope and intercept are calculated and stored in nonvolatile

memory during equipment calibration, an equation can be used

to calculate an unknown input power based on the output

voltage of the detector.

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

(16)

The log conformance error is the difference between this straight

line and the actual performance of the detector.

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

(17)

Figure 53 includes a plot of this error at +25°C, −40°C, and +85°C

when using a two-point calibration (calibration points are 0 dBm

and −40 dBm). The error at the calibration points at 25°C (in

this case, −40 dBm and 0 dBm) is equal to 0 dB 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 54 shows

the post-calibration error plots for a three-point calibration. With

a multipoint calibration, the transfer function is segmented, with

each segment having its own slope and intercept. Multiple known

power levels (three levels in this case) are applied, and multiple

voltages are measured. When the equipment is in operation, the

measured voltage from the detector is first used to determine

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 values into

Equation 16.

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, when

more calibration points are used, calibration time increases.

PIN (dBm)

–6

–5

–4

–3

–2

–1

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

–65

–55

–45

–35

–25

–15

–5

OUTPUT VOLTAGE –40°C

OUTPUT VOLTAGE +25°C

OUTPUT VOLTAGE +85°C

ERROR –40°C

ERROR +25°C

ERROR +85°C

VTADJ = 0.35V

Figure 54. 2.14 GHz VRMS and Log Conformance Error at +25°C, −40°C, and

+85°C Using Three-Point Calibration at 0 dBm, −40 dBm, and −55 dBm

The −40°C and +85°C error plots in Figure 54 are generated using

the +25°C slope and intercept values. This is consistent with

equipment calibration in a mass production environment where

calibration of multiple temperatures is not practical.

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