LTC5596 datasheet(20/22 Pages) LINER

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LTC5596

5596f

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applicaTions inForMaTion

High Accuracy Power Measurement

Thepowermeasurementaccuracyachievedusingapower

detector is not only determined by the performance of the

power detector device itself, but also by the approach/

methods used to interpret the DC power detector output

signal. This can be understood by considering Figure 8.

response using linear regression over a suitable power

range (where the detector response is close to linear).

Better accuracy/smaller errors are obtained if SLOPE and

PINTERCEPT are determined for:

• Each detector device individually

• Each operating temperature

• Each operating frequency

To achieve the best accuracy, it is recommended to de-

termine SLOPE and PINTERCEPT for each individual unit,

requiring a 2-point factory calibration. When temperature

drifteffectsaretobeincluded,SLOPEandPINTERCEPTneed

to be determined at different operating temperatures and

the system needs to incorporate a temperature sensor to

determine which parameter values to use for the current

operating temperature.

The LOG-linearity error curves in the Typical Performance

Characteristics section were obtained using linear regres-

sion, applied to the response of the individual detector

devices at T = 25°C. For frequencies up to 28GHz, the

input power range from –37dBm to –5dBm was used. The

resulting LOG-linearity error tends to have larger negative

values than positive values. To center the error curves

within the ±1dB range, an additional 0.5dB was added

to the PINTERCEPT parameter. This slightly increases the

measurement error at T = 25°C, but results in a smaller

error over the full temperature range. The calculated

LOG-slope and LOG-intercept numbers are displayed in

the tables on page 3 and 4.

A better measurement accuracy is achieved if the inter-

preter uses the actual detector response at T = 25°C as

model for the detector, instead of the perfect linear-in-dB

response described above. The resulting measurement

error, the temperature drift error, equals:

TemperatureDriftError=[VOUT(T)–VOUT(25°C)]/SLOPE

A system that achieves this measurement error should

store the full output voltage vs input power response of

the detector with suitable resolution. The error curves

displayed on page 10 and 11 represent the achieved power

measurement accuracy using this configuration.

5596 F08

PMEAS

VOUT

PACT

RF INPUT

SIGNAL

POWER

DETECTOR

INTERPRETER

(ESTIMATOR)

LF DETECTOR

OUTPUT VOLTAGE

MEASURED

INPUT POWER

Systems for accurate power level measurements on RF

signals can conceptually be thought to consist of two

elements:

• A high accuracy power detector (like the LTC5596),

converting the power level of an RF signal into a DC

voltage or current;

• An interpreter (also called an estimator), translating

the DC output voltage or current of the power detector

back to a power level.

In Figure 8, PMEAS represents the power level measured

by the system, i.e. the power level the system thinks is

present at its input, while PACTrepresentstheactualpower

levelpresentatthedetectorinput.Thepowermeasurement

error thus equals the difference: PERR = PMEAS – PACT.

The more the interpreter knows about the operating

conditions and transfer of the detector, the smaller the

measurement error that can be achieved. For example,

the interpreter may assume that the detector response is

perfectly linear in dB, such that the relationship between

input power and output voltage is a straight line:

VOUT = SLOPE • (PMEAS - PINTERCEPT)

This results in a power measurement error equal to:

LOG-Linearity Error = VOUT/SLOPE +PINTERCEPT – PACT

TheparametersSLOPEandPINTERCEPT,theLOG-slopeand

LOG-intercept, are best obtained from the actual detector

Figure 8. Power Measurement Concept

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