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ADL5501 Datasheet(Fiches technique) 21 Page  Analog Devices 

21 page ADL5501 Rev. 0  Page 21 of 28 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 bestfit line is char acterized by calculating the conversion gain (or slope) and intercept using the following equations: Gain = (VRMS2 − VRMS1)/(VIN2 − VIN1) (3) Intercept = VRMS1 − (Gain × VIN1) (4) where: VIN is the rms input voltage to RFIN. VRMS is the voltage output at VRMS. Once 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 (5) 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)] (6) 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 0 by definition. 3 –3 –2 –1 0 1 2 –25 –20 –15 –10 –5 0 5 +85°C +25°C –40°C 10 15 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 calibration in a massproduction 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 to now, the errors were calculated with respect to the ideal linear transfer function at ambient. When this alternative technique is used, the error at ambient becomes equal to 0 by definition (see Figure 52). 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 wireless 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. 3 –3 –2 –1 0 1 2 –25 –20 –15 –10 –5 0 5 +85°C +25°C –40°C 10 15 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 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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