ADL5904ACPZN-R7 datasheet(20/27 Pages) AD

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ADL5904

Data Sheet

Rev. B | Page 20 of 27

VRMS CALIBRATION AND ERROR CALCULATION

The measured transfer function of the ADL5904 at 900 MHz is

shown in Figure 47, which contains plots of both output voltage

and log conformance error vs. input level for one device. As the

input level varies from −30 dBm to +15 dBm, the output voltage

varies from 200 mV to approximately 1.7 V.

–6

–5

–4

–3

–2

–1

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

–40

–30

–20

–10

PIN (dBm)

VOUT +25°C

VOUT –40°C

VOUT +85°C

ERROR +25°C

ERROR –40°C

ERROR +85°C

Figure 47. VRMS and Log Conformance Error at 900 MHz, −40°C, +25°C, and

+85°C with Log Conformance Error Calculated Based on Two-Point

Calibration at −20 dBm and +10 dBm

Calibration must be performed to achieve high accuracy

because the output voltage for a particular input level varies

from device to device. For a two-point calibration, the equation

for the idealized output voltage is

VRMS (IDEAL) = Slope × (PIN − Intercept)

(1)

where:

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

input level (unit is mV/dB).

PIN is the input level (unit is dBm).

Intercept is the calculated input level at which the output voltage

is equal to 0 V (note that Intercept is an extrapolated theoretical

value and not a measured value). Intercept has a unit of dBm.

In general, calibration is performed during equipment

manufacture by applying two or more known signal levels to the

input of the ADL5904 and measuring the corresponding output

voltages. The calibration points must be within the linear

operating range of the device.

With a two-point calibration, calculate the slope and intercept

as follows:

Slope = (VRMS1 − VRMS2)/(PIN1 − PIN2)

(2)

Intercept = PIN1 − (VRMS1/Slope)

(3)

After the slope and intercept are calculated (and stored in some

form), use the following equation to calculate an unknown

input level based on the output voltage of the detector:

PIN (Unknown) = (VRMS (MEASURED)/Slope) + Intercept

(4)

The log conformance error is the difference between this

straight line and the actual performance of the detector.

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

(5)

Use multipoint calibration to extend the measurement dynamic

range further. In this case, the transfer function is segmented,

with each segment having its own slope and intercept. Figure 48

shows the error plot of the same device with calibration points

at −20 dBm, 0 dBm, and +10 dBm. The three-point calibration

results in tighter log conformance and a slight extension of the

linear operating range of the device.

–6

–5

–4

–3

–2

–1

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

–40

–30

–20

–10

PIN (dBm)

VOUT +25°C

VOUT –40°C

VOUT +85°C

ERROR +25°C

ERROR –40°C

ERROR +85°C

Figure 48. VRMS and Log Conformance Error at 900 MHz, −40°C, +25°C, and

+85°C with Log Conformance Error Calculated Based on Three-Point

Calibration at −20 dBm, 0 dBm, and +10 dBm

Where three-point calibration is used, two values of slope and

two values of intercept must be calculated and stored during

calibration. In addition, the transition point between the two

calibration regions must be recorded so that the system knows

which slope/intercept pair to use. In a typical system, the output

of the ADL5904 is sampled by a precision ADC. For the example in

Figure 48 (calibration points at −20 dBm, 0 dBm, and +10 dBm),

the ADC output code for an input power of 0 dBm is stored

with the calculated slopes and intercept. When the system is in

operation in the field, the code from the ADC is compared to

this stored code to determine whether to use the upper or lower

slope/intercept pair.

The calibration scheme for ADL5904 can be extended beyond

three points. This technique can be used, for example, to

linearize the response for input powers below −30 dBm. This

effort, however, is less beneficial if the device is to be used over

a wide temperature range. The multidevice plots (see Figure 15,

Figure 19 to Figure 21, Figure 25 to Figure 27, and Figure 31)

show how temperature stability becomes less predictable at low

input power level.

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