AD8364ACPZ-R2 Analog Devices, AD8364ACPZ-R2 Datasheet - Page 32

• Manufacturer Part Number: AD8364ACPZ-R2
• Description: LF to 2.7GHz, Dual 60dB TruPwr Detector
• Manufacturer: Analog Devices

AD8364
Calculation of the slope and intercept is done using the
equations:
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.
The log conformance error of the calculated power is given by
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
V
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.
OUT
Slope = (V
Intercept = P
P
Error (dB) = (V
vs. P
IN
(unknown) = (V
IN
equation, even within its operating region. The
OUT1
IN1
− V
− (V
OUT(MEASURED)
OUT2
OUT1(measured)
OUT1
)/(P
/Slope)
IN1
− V
− P
/Slope) + Intercept
OUT(IDEAL)
IN2
)
)/Slope
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Calibration points should be chosen to suit the application at
hand. In general, though, do not choose calibration points in
the nonlinear portion of the log amp’s 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’s 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 would be valid if the device transfer function perfectly
followed the ideal V
However, since an rms amp, in practice, never perfectly follows
this equation (especially outside of its 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 temper-
ature drift at a particular power level with respect to the (nonideal)
output voltage at ambient.
OUT
= Slope × (P
IN
− Intercept) equation.