Circuit Note

CN-0178

Rev. A| Page 3 of 5

The calibration is performed by applying four known signal

levels to the ADL5902 and measuring the corresponding output

codes from the ADC. The calibration points chosen should be

within the linear operating range of the device. In this example,

calibration points at 0 dBm, −20 dBm, −45 dBm, and −58 dBm

were used.

The SLOPE and INTERCEPT calibration coefficients are

calculated using the equations

SLOPE1 = ( CODE _1 – CODE_2)/(PIN_1 − PIN_2)

INTERCEPT1= CODE_1/(SLOPE_ADC × PIN_1)

This calculation is then repeated using CODE_2/CODE_3

and CODE_3/CODE_4 to calculate SLOPE2/INTERCEPT2

and SLOPE3/INTERCEPT3, respectively. The six calibration

coefficients should then be stored in NVM along with CODE_1,

CODE_2, CODE_3, and CODE_4.

When the circuit is in operation in the field, these calibration

coefficients are used to calculate an unknown input power level,

PIN, using the equation

PIN = (CODE/SLOPE) + INTERCEPT

In order to retrieve the appropriate SLOPE and INTERCEPT

calibration coefficients during circuit operation, the observed

CODE from the ADC must be compared to CODE_1, CODE_2,

CODE_3, and CODE_4. For example if the CODE from the

ADC is between CODE_1 and CODE_2, then the SLOPE1 and

INTERCEPT1 should be used. This step can also be used to

provide an underrange or overrange warning. For example, if

the CODE from the ADC is greater than CODE_1 or less than

CODE_4, it indicates that the measured power is outside of the

calibration range

Figure 3 also shows the transfer function variation of the circuit

vs. the above straight line equations. This error function is

caused by bending at the edges of the transfer function, small

ripple in the linear operating range, and drift over temperature.

The error is expressed in dB using the equation

Error (dB) = Calculated RF Power − True Input Power

= (CODE/SLOPE) + INTERCEPT – PIN_TRUE

Figure 3 also includes plots of error vs. temperature. In this case

the measured ADC codes at +85°C and −40°C are compared to

the straight line equations at ambient. This is consistent with a

real world system where system calibration is generally only

practical at ambient temperature.

Figure 4 and Figure 5 show the performance of the circuit at

1 GHz and 2.2 GHz, respectively.

4096

3755

3413

3072

2731

2389

2048

1707

1365

1024

683

341

0

6

5

4

3

2

1

0

–1

–2

–3

–4

–5

–6

–70

–60

–50

–40

–30

–20

–10

0

10

PIN (dBm)

+85°C CODE

–40°C CODE

+25°C CODE

+25°C ERROR 4-POINT CAL @ 0dBm,

–20dBm, –45dBm, AND, –58dBm

+85°C ERROR 4-POINT CAL

–40°C ERROR 4-POINT CAL

Figure 4. ADC Output Code and Error vs. RF Input Power @ 1 GHz

4096

3755

3413

3072

2731

2389

2048

1707

1365

1024

683

341

0

6

5

4

3

2

1

0

–1

–2

–3

–4

–5

–6

–70

–60

–50

–40

–30

–20

–10

0

10

PIN (dBm)

+85°C CODE

–40°C CODE

+25°C CODE

+25°C ERROR 4-POINT CAL @ 0dBm,

–20dBm, –45dBm, AND, –58dBm

+85°C ERROR 4-POINT CAL

–40°C ERROR 4-POINT CAL

Figure 5. ADC Output Code and Error vs. RF Input Power @ 2.2 GHz

The performance of this or any high speed circuit is highly

dependent on proper PCB layout. This includes, but is not

limited to, power supply bypassing, controlled impedance lines

(where required), component placement, signal routing, and

power and ground planes. (See MT-031 Tutorial, MT-101 Tutorial,

and article, A Practical Guide to High-Speed Printed-Circuit-

Board Layout, for more detailed information regarding PCB

layout.)

A complete design support package for this circuit note can be

found at www.analog.com/CN0178-DesignSupport.