ADL5501AKSZ-R21 datasheet(22/28 Pages) AD

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ADL5501

Rev. B | Page 22 of 2

DEVICE CALIBRATION AND ERROR CALCULATION

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

level calibration must be performed to achieve high accuracy.

In general, calibration is performed by applying two input power

levels to the ADL5501 and measuring the corresponding output

voltages. The calibration points are generally chosen to be within

the linear operating range of the device. The best-fit line is char-

acterized by calculating the conversion gain (or slope) and intercept

using the following equations:

Gain = (VRMS2 − VRMS1)/(VIN2 − VIN1)

(2)

Intercept = VRMS1 − (Gain × VIN1)

(3)

where:

VIN is the rms input voltage to RFIN.

VRMS is the voltage output at VRMS.

After gain and intercept are calculated, an equation can be

written that allows calculation of an (unknown) input power

based on the measured output voltage.

VIN = (VRMS − Intercept)/Gain

(4)

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

the measured data can be calculated as

ERROR (dB) =

20 × log [(VRMS, MEASURED − Intercept)/(Gain × VIN, IDEAL)] (5)

Figure 51 includes a plot of the error at 25°C, the temperature

at which the ADL5501 is calibrated. Note that the error is not

zero; this is because the ADL5501 does not perfectly follow the

ideal linear equation, even within its operating region. The

error at the calibration points is, however, equal to zero by

definition.

–3

–2

–1

–25

–20

–15

–10

–5

+85°C

+25°C

–40°C

INPUT (dBm)

Figure 51. Error from Linear Reference vs. Input at −40°C, +25°C, and

+85°C vs. +25°C Linear Reference, Frequency = 1900 MHz, Supply = 5.0 V

Figure 51 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 calibra-

tion in a mass-production environment where calibration at

temperature is not practical.

CALIBRATION FOR IMPROVED ACCURACY

Another way of presenting the error function of the ADL5501

is shown in Figure 52. In this case, the dB error at hot and cold

temperatures is calculated with respect to the transfer function

at ambient. 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. When this alterna-

tive technique is used, the error at ambient becomes equal to

zero by definition (see Figure 52).

–3

–2

–1

–25

–20

–15

–10

–5

+85°C

+25°C

–40°C

INPUT (dBm)

Figure 52. Error from +25°C Output Voltage at −40°C, +25°C, and +85°C

After Ambient Normalization, Frequency = 1900 MHz, Supply = 5.0 V

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 ADL5501 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 wire-

less transmitter, the accuracy of the high power amplifier (HPA)

is most critical at or close to full power. The ADL5501 offers a

tight error distribution in the high input power range, as shown

in Figure 52. The high accuracy range, centered around 9 dBm at

1900 MHz, offers 7 dB of ±0.1 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.

The high accuracy range center varies over frequency. At

1900 MHz, the region is centered at approximately 9 dBm.

At higher frequencies, the high accuracy range is centered

at higher input powers (see Figure 13 through Figure 15 and

Figure 19 through Figure 21).

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