AD8364ACPZ-R2 datasheet(32/44 Pages) AD

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AD8364

Data Sheet

Rev. C | Page 32 of 44

Once slope and intercept have been calculated, an equation can

be written that will allow calculation of the input power based

on the output voltage of the detector.

PIN (unknown) = (VOUT1(MEASURED)/Slope) + Intercept

The log conformance error of the calculated power is given by

Error (dB) = (VOUT(MEASURED) − VOUT(IDEAL))/Slope

Figure 69 includes a plot of the error at 25°C, the temperature at

which the log amp is calibrated. Note that the error is not zero.

This is because the log amp does not perfectly follow the ideal

VOUT vs. PIN equation, even within the operating region. The

error at the calibration points (−43 dBm and −23 dBm in this

case) will, however, be equal to zero by definition.

Figure 69 also includes error plots for the output voltage at

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

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

in a mass-production environment, where calibration at

temperature is not practical.

SELECTING CALIBRATION POINTS TO IMPROVE

ACCURACY OVER A REDUCED RANGE

In some applications, very high accuracy is required at 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.

Figure 70 shows the same measured data as Figure 69. Notice

that accuracy is very high from −10 dBm to −25 dBm. At

approximately −45 dBm, the error increases to about −0.3 dB

because the calibration points have been changed to −15 dBm

and −25 dBm.

Calibration points must be chosen to suit the application at

hand. In general, though, do not choose calibration points in

the nonlinear portion of the log amp transfer function (above

0 dBm or below −50 dBm in this case).

Figure 71 shows how calibration points can be adjusted to

increase dynamic range, but at the expense of linearity. In this

case, the calibration points for slope and intercept are set at −1 dBm

and −50 dBm. These points are at the end of the device linear

range. At 25°C, there is an error of 0 dB at the calibration points.

Note also that the range over which the AD8364 maintains an error

of <±0.4 dB is extended to 57 dB at 25°C. The disadvantage of this

approach is that linearity suffers, especially at the top end of the

input range.

Another way of presenting the error function of a log amp

detector is shown in Figure 72. In this case, the dB error at hot

and cold temperatures is calculated with respect to the output

voltage at ambient. This is a key difference in comparison to the

previous plots, in which all errors have been calculated with

respect to the ideal transfer function at ambient.

When the alternative technique, the error at ambient becomes

by definition equal to 0 (see Figure 72).

This is valid if the device transfer function perfectly followed

the ideal VOUT = Slope × (PIN − Intercept) equation. However,

since an rms amp, in practice, never perfectly follows this equation

(especially outside of the linear operating range), this plot tends

to artificially improve linearity and extend the dynamic range,

unless enough calibration points were taken to remove the error.

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

particular power level with respect to the (nonideal) output

voltage at ambient.

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