AD8363ACPZ-R2 datasheet(19/29 Pages) AD

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Data Sheet

AD8363

Rev. B | Page 19 of 29

SYSTEM CALIBRATION AND ERROR CALCULATION

The measured transfer function of the AD8363 at 1.9 GHz is

shown in Figure 44, which contains plots of both output voltage

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

the input level varies from −55 dBm to +0 dBm, the output

voltage varies from ~0 V to ~3.1 V.

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)

(12)

where:

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

input power (dB).

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

–1

–2

–3

–4

–60

–50

–40

–30

–20

–10

PIN (dBm)

Figure 44. 1.9 GHz Transfer Function and Linearity Error using a Two-Point

Calibration (Calibration Points −20 dBm and −40 dBm)

Intercept is the calculated input power level at which the output

voltage would equal 0 V (note that Intercept is an extrapolated

theoretical value not a measured value).

In general, calibration, which establishes the Slope and Intercept,

is performed during equipment manufacture by applying two

or more known signal levels to the input of the AD8363 and

measuring the corresponding output voltages. The calibration

points are generally chosen 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)

(13)

Intercept = PIN1 − (VOUT1/Slope)

(14)

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

volatile 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) = (VOUT1(MEASURED)/Slope) + Intercept

(15)

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

(16)

Figure 44 includes a plot of this error when using a two-point

calibration (calibration points are −20 dBm and −40 dBm). The

error at the calibration points 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 45 shows

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

a multipoint calibration, the transfer function is segmented,

with each segment having its own slope and intercept. During

calibration, multiple known power levels 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 into

Equation 15.

Figure 45 shows the output voltage and error at 25°C and over

temperature when a three-point calibration is used (calibration

points are 0 dBm, −10 dBm and −40 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.

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0.5

–1

–2

–3

–4

–60

–50

–40

–30

–20

–10

PIN (dBm)

Figure 45. 1.9 GHz Transfer Function and Error at +25°C, −40°C, and +85°C

Using a Three-Point Calibration (0 dBm, −10 dBm and −40 dBm)

The −40°C and +85°C error plots in Figure 44 and Figure 45

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.

OPERATION TO 125°C

The AD8363 operates up to 125°C with slightly degraded

performance. Figure 46 shows the typical operation (Errors are

plotted using two-point calibration) at 125°C as compared to

other temperatures using the TCM1 and TCM2 values in Table 4.

Temperature compensation can be optimized for operation

above 85°C by modifying the voltages on the TCM1 and TCM2

pins from those shown in Table 4.

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