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AD8318

Rev. A | Page 16 of 24

Using the equation for the ideal output voltage (7) as a reference, the

log conformance error of the measured data can be calculated

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

(12)

Figure 31 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 its operating region. The error at the calibra-

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

to zero by definition.

Figure 31 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 environ-

ment 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 32 shows the same measured data as Figure 31. 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.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.6

0.4

0.2

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–65 –60 –55 –50 –45 –40 –35 –30

–20

–10

–5

PIN (dBm)

PIN1

PIN2

ERROR +25

°C

ERROR –40

°C

ERROR +85

°C

VOUT +25°C

VOUT –40°C

VOUT +85°C

VOUT2

VOUT1

Figure 32. 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 33 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.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

PIN (dBm)

2.5

–2.5

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

VOUT +25°C

VOUT –40°C

VOUT +85°C

ERROR +25

°C

ERROR –40

°C

ERROR +85

°C

58dB DYNAMIC RANGE (

±1dB ERROR)

Figure 33. 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 34. 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 32 and Figure 33. Previously, all errors were

calculated with respect to the ideal transfer function at ambient.

When this alternative technique is used, the error at ambient

becomes, by definition, equal to 0 (see Figure 34)

. 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.

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