ADL5502ACBZ-P2 datasheet(20/28 Pages) AD

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ADL5502

Rev. A | Page 20 of 28

For an ideal (known) input power, the law conformance error of

the measured data can be calculated as

ERROR

(dB) =

20 × log [(VVRMS, MEASURED − Intercept)/(Gain × VIN, IDEAL)] (6)

Figure 55 includes a plot of the error at 25°C, the temperature

at which the ADL5502 is calibrated. Note that the error is not

zero; this is because the ADL5502 does not perfectly follow the

ideal linear equation, even within its operating region. The error

at the calibration points is, however, equal to 0 by definition.

+85ºC +25ºC

–40ºC

–3

–2

–1

INPUT (dBm)

–25

–20

–15

–10

–5

Figure 55. VRMS Error from Linear Reference vs. Input at −40°C, +25°C, and

+85°C vs. +25°C Linear Reference, Frequency 1900 MHz, Supply 3.0 V

Figure 55 also includes error plots for the output voltage at

−40°C and +85°C. These error plots are calculated using the

gain and intercept at 25°C. This is consistent with calibration in

a mass production environment where calibration at temperature is

not practical.

The same procedure should be followed to calculate the linearity

error for the PEAK output. In this case, replace VVRMS with

VPEAK in the preceding equations.

CALIBRATION FOR IMPROVED ACCURACY

Another way of presenting the error function of the ADL5502 is

shown in Figure 56. In this case, the dB error at hot and cold

temperatures is calculated with respect to the transfer function

at ambient temperature. This is a key difference in comparison to

the previous plots. Up until now, the errors were calculated with

respect to the ideal linear transfer function at ambient temperature.

When this alternative technique is used, the error at ambient

temperature becomes equal to zero by definition (see Figure 56).

This plot is a useful tool for estimating temperature drift at a

particular power level with respect to the (nonideal) response at

ambient. The linearity and dynamic range tend to be improved

artificially with this type of plot because the ADL5502 does not

perfectly follow the ideal linear equation (especially outside of

its linear operating range). Achieving this level of accuracy in

an end application requires calibration at multiple points in the

operating range of the device.

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) is most critical at or close to full power. The ADL5502

offers a tight error distribution in the high input power range,

as shown in Figure 56. The high accuracy range, beginning

around 4 dBm at 1900 MHz, offers 8 dB of ±0.15 dB detection

error over temperature. Multiple point calibration at ambient

temperature in the reduced range offers precise power

measurement with near 0 dB error from −40°C to +85°C.

+85ºC

+25ºC

–40ºC

–3

–2

–1

INPUT (dBm)

–25

–20

–15

–10

–5

Figure 56. VRMS Error from +25°C Output Voltage at −40°C, +25°C, and

+85°C After Ambient Normalization, Frequency 1900 MHz, Supply 3.0 V

Note that the high accuracy range center varies over frequency

(see Figure 13, Figure 14, Figure 15, Figure 19, Figure 20, and

Figure 21).

CALCULATION OF CREST FACTOR (CF)

The ADL5502 is a true rms power detector in combination with

an envelope detector that accurately determines the crest factor

of a modulated signal. The device has two outputs, VRMS and

PEAK, which respectively provide the rms and envelope peak of

the RF waveform present at RFIN. Therefore, these two outputs can

be used to accurately calculate the crest factor of the waveform.

Before CF can be measured and calculated, both of the ADL5502

outputs must be calibrated (see the Device Calibration and Error

Calculation section for the calibration procedure for VRMS and

PEAK). It is suggested that the calibration step be completed by

applying at least two input power levels with a CW signal. The

CW signal (with a CF of 0 dB) serves as the reference for the CF

calculation. When the characteristics (slope and intercept) of

the VRMS and PEAK outputs are known, the calibration for the

CF calculation is complete.

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