ADL5906SCPZN-R7 datasheet(24/30 Pages) AD

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Data Sheet

ADL5906

THEORY OF OPERATION

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Rev. B | 24 of 30

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.

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 nonvola-

tile 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. System Calibration and Error

Calculation 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.

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.

USING VTEMP TO IMPROVE INTERCEPT

TEMPERATURE DRIFT

In applications where VTEMP and VRMS are both being digitized by

an ADC, the VTEMP voltage can be used to further improve the

temperature drift of the ADL5906.

ADL5906SCPZN-R7 数据表(PDF) 24 Page - Analog Devices

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