CN-0178 Datasheet, PDF (3/5 Pages) Analog Devices – Software-Calibrated, 50 MHz to 9 GHz, RF Power Measurement System

Circuit Note
CN-0178
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
6
+85°C CODE
3755
–40°C CODE
5
+25°C CODE
3413
+25°C ERROR 4-POINT CAL @ 0dBm,
4
–20dBm, –45dBm, AND, –58dBm
3072
+85°C ERROR 4-POINT CAL
–40°C ERROR 4-POINT CAL
3
2731
2
2389
1
2048
0
1707
–1
1365
–2
1024
–3
683
–4
341
–5
0
–70 –60 –50 –40 –30 –20 –10 0
PIN (dBm)
–6
10
Figure 4. ADC Output Code and Error vs. RF Input Power @ 1 GHz
4096
6
+85°C CODE
3755
–40°C CODE
5
+25°C CODE
3413
+25°C ERROR 4-POINT CAL @ 0dBm,
–20dBm, –45dBm, AND, –58dBm
4
3072
+85°C ERROR 4-POINT CAL
–40°C ERROR 4-POINT CAL
3
2731
2
2389
1
2048
0
1707
–1
1365
–2
1024
–3
683
–4
341
–5
0
–70 –60 –50 –40 –30 –20 –10
0
PIN (dBm)
–6
10
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.
Rev. A| Page 3 of 5