ADL5513ACPZ-R2 datasheet(16/25 Pages) AD

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ADL5513

Data Sheet

Rev. A | Page 16 of 25

ERROR CALCULATIONS

The measured transfer function of the ADL5513 at 100 MHz is

shown in Figure 31. The figure shows plots of measured output

voltage, calculated error, and an ideal line. The input power and

output voltage are used to calculate the slope and intercept values.

The slope and intercept are calculated using linear regression

over the input range from −40 dBm to −20 dBm. The slope and

intercept terms are used to generate an ideal line. The error is

the difference in measured output voltage compared to the ideal

output line.

3.0

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–3.0

PIN (dBm)

IDEAL LINE

VOUT AND ERROR @ +25°C

VOUT AND ERROR @ –40°C

VOUT AND ERROR @ +85°C

PIN2

PIN1

VOUT2

VOUT1

2.4

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

Figure 31. Typical Output Voltage vs. Input Signal

The equation for output voltage can be written as

VOUT = Slope × (PIN − Intercept)

where:

Slope is the change in output voltage divided by the change in

input power, PIN. Slope is expressed in volts per decibel (V/dB).

Intercept is the calculated power in decibels (dB) at which the

output voltage is 0 V. Note that VOUT = 0 V can never be achieved.

Calibration is performed by applying two known signal levels to

the ADL 5513 and measuring the corresponding voltage outputs.

The calibration points are in general chosen to be within the

linear-in-dB range of the device.

Calculation of the slope and intercept are accomplished by

using the following equations:

IN2

IN1

MEASURED

OUT

MEASURED

OUT

Slope

−

)

(

)

(

Slope

Intercept

MEASURED

OUT

IN1

)

(

−

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

calculated, and the error is determined using the following

equation:

Slope

Error

IDEAL

OUT

MEASURED

OUT

)

(

)

(

)

(

−

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

presented 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 exam-

ple of adjusting accuracy through choice of calibration points.

2.50

2.25

2.00

1.75

1.50

1.25

1.00

0.75

0.50

0.25

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–2.5

–70 –65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5

PIN (dBm)

+25°C

–40°C

+85°C

+125°C

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

and −40 dBm

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