Data Sheet

ADL5513

APPLICATIONS INFORMATION

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Rev. B | 16 of 25

Intercept=PIN1−VOUTMEASURED

Slope

(6)

Once the slope and intercept are calculated, VOUT(IDEAL) can be

calculated, and the error is determined using the following equation:

Error=VOUTMEASURED−VOUTIDEAL

Slope

(7)

Figure 31 shows a plot of the error at 25°C, the temperature at

which the device is calibrated. Error is not 0 dB over the full

dynamic range. This is because the log amp does not perfectly

follow the ideal VOUT vs. PIN equation, even within its operating

range. The error at the calibrating points of −20 dBm and −40 dBm

is equal to 0 dB by definition.

Figure 31 also shows error plots for output voltages measured

at −40°C and 85°C. These error plots are calculated using slope

and intercept at 25°C, which is consistent in a mass-production

environment, where calibration over temperature is not practical.

This is a measure of the linearity of the device. Error from the

linear response to the CW waveform is not a measure of absolute

accuracy because it is calculated using the slope and intercept of

each device. However, error verifies the linearity of the devices.

Similarly, at temperature extremes, error represents the output

voltage variations from the 25°C ideal line performance. Data pre-

sented in the graphs are the typical error distributions observed

during characterization of the ADL5513. Device performance was

optimized for operation at 85°C; this can be changed by changing

the voltage at TADJ.

ADJUSTING ACCURACY THROUGH CHOICE

OF CALIBRATION POINTS

Choose calibration points to suit the specific application, but usually

they should be in the linear range of the log amp.

In some applications, very high accuracy is required at a reduced

input range; in other applications, good linearity is necessary over

the full power input range. The linearity of the transfer function can

be adjusted by choice of calibration points. Figure 32 and Figure

33 show plots for a typical device at 3600 MHz as an example of

adjusting accuracy through choice of calibration points.

Figure 32. Typical Device at 3600 MHz, Calibration Points at PIN = −20 dBm

and −40 dBm

Figure 33. Typical Device at 3600 MHz, Calibration Points at PIN = −12 dBm

and −40 dBm

In Figure 32, calibration points are chosen so that linearity is

improved over the full dynamic range, but error at the higher power

level at PIN = −10 dBm is 0.5 dB at 25°C. In Figure 33, calibration

points are chosen so that error is smaller at higher power input ,but

with loss of linearity over the full dynamic range.

Figure 34 shows another way of presenting the error of a log amp

detector. The same typical device from Figure 32 and Figure 33

is presented where the error at −40°C, +85°C, and +125°C are

calculated with respect to the output voltage at +25°C. This is the

key difference in presenting the error of a log amp compared with

the plots in Figure 32 and Figure 33 where the error is calculated

with respect to the ideal line at 25°C.