Data Sheet

AD8318

USING THE AD8318

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Rev. E | 16 of 24

vs. PIN equation, even within its operating region. The error at the

calibration points (−12 dBm and −52 dBm, in this case) is, however,

equal to 0 by definition.

Figure 32 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 method is consistent with 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 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.

Figure 33 shows the same measured data as Figure 32. Note that

accuracy is very high from −10 dBm to −30 dBm. Below −30 dBm,

the error increases to about −1 dB. This is because the calibration

points have changed to −14 dBm and −26 dBm.

Figure 33. Output Voltage and Error vs. PIN with 2-Point Calibration at −10

dBm and −30 dBm

Calibration points are chosen to suit the application at hand. In

general, the calibration points are never chosen in the nonlinear

portion of the transfer function of the log amp (above −5 dBm or

below −60 dBm, in this case).

Figure 34 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 −4 dBm and −60

dBm. These points are at the end of the linear range of the device.

=

Once again, at 25°C, an error of 0 dB is seen at the calibration

points. Note also that the range over which the AD8318 maintains

an error of < ±1 dB is extended to 60 dB at 25°C and 58 dB over

temperature. The disadvantage of this approach is that linearity

suffers, especially at the top end of the input range.

Figure 34. Dynamic Range Extension by Choosing Calibration Points Close

to the End of the Linear Range

Another way of presenting the error function of a log amp detector

is shown in Figure 35. 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 plots in

Figure 33 and Figure 34. Previously, all errors were calculated with

respect to the ideal transfer function at ambient.

When this alternative technique is used, the error at ambient be-

comes, by definition, equal to 0 (see Figure 35). This is valid if the

device transfer function perfectly follows the ideal VOUT = Slope ×

(PIN − Intercept) equation. However, because a log 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. This plot is a useful tool for estimating

temperature drift at a particular power level with respect to the

(nonideal) output voltage at ambient. However, to achieve this level

of accuracy in an end application requires calibration at multiple

points in the operating range of the device.

Figure 35. Error vs. Temperature with Respect to Output Voltage at 25°C

(Does Not Take Transfer Function Nonlinearities at 25°C into Account)