ADL5902-EVALZ AD [Analog Devices], ADL5902-EVALZ Datasheet - Page 23

• Manufacturer Part Number: ADL5902-EVALZ
• Description: 50 MHz to 9 GHz 65 dB TruPwr Detector
• Manufacturer: AD [Analog Devices]

SYSTEM CALIBRATION AND ERROR CALCULATION
The measured transfer function of the ADL5902 at 2.14 GHz is
shown in Figure 50, which contains plots of both output voltage
vs. input amplitude (power) and calculated error vs. input level. As
the input level varies from −62 dBm to +3 dBm, the output
voltage varies from ~0.25 V to ~3.5 V.
Because slope and intercept vary from device to device, board-
level calibration must be performed to achieve high accuracy.
The equation for the idealized output voltage can be written as
where:
Slope is the change in output voltage divided by the change in
input power (dB).
Intercept is the calculated input power level at which the output
voltage is 0 V (note that Intercept is an extrapolated theoretical
value not a measured value).
In general, calibration is performed during equipment manu-
facture by applying two or more known signal levels to the
input of the ADL5902 and measuring the corresponding output
voltages. The calibration points are generally within the linear-
in-dB operating range of the device.
With a two-point calibration, the slope and intercept are
calculated as follows:
After the slope and intercept are calculated and stored in non-
volatile memory during equipment calibration, an equation can
be used to calculate an unknown input power based on the
output voltage of the detector.
The log conformance error is the difference between this
straight line and the actual performance of the detector.
Figure 50 includes a plot of this error when using a two-point
calibration (calibration points are 0 dBm and −40 dBm). The
Figure 50. 2.14 GHz Transfer Function, Using Various Calibration Techniques
V
Slope = (V
Intercept = P
P
Error (dB) = (V
6
5
4
3
2
1
0
IN
–70
OUT(IDEAL)
(Unknown) = (V
–60
= Slope × (P
OUT1
IN1
–50
− V
OUT(MEASURED)
− (V
V
ERROR 2-POINT CAL AT 0dBm, AND 40dBm
ERROR 3-POINT CAL AT 0 dBm,
ERROR 4-POINT CAL AT 0dBm, –20dBm,
OUT
–45dBm, AND 60dBm
–45dBm, AND –60dBm
OUT2
–40
OUT1
OUT1(MEASURED)
)/(P
IN
P
/Slope)
IN
− Intercept)
–30
(dBm)
IN1
− V
− P
OUT(IDEAL)
–20
IN2
/Slope) + Intercept
)
–10
)/Slope
0
10
6
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6
(18)
(19)
(20)
(21)
(22)
Rev. 0 | Page 23 of 28
error at the calibration points (in this case, −40 dBm and 0 dBm)
is equal to 0 by definition.
The residual nonlinearity of the transfer function that is
apparent in the two-point calibration error plot can be reduced
by increasing the number of calibration points. Figure 50 shows
the postcalibration error plots for three-point and four-point
calibrations. With a multipoint calibration, the transfer function
is segmented, with each segment having its own slope and
intercept. Multiple known power levels are applied, and
multiple voltages are measured. When the equipment is in
operation, the measured voltage from the detector is first used
to determine which of the stored slope and intercept calibration
coefficients are to be used. Then the unknown power level is
calculated by inserting the appropriate slope and intercept into
Equation 21.
Figure 51 shows the output voltage and error at 25°C and over
temperature when a four-point calibration is used (calibration
points are 0 dBm, −20 dBm, −45 dBm, and −60 dBm). When
choosing calibration points, there is no requirement for, or
value, in equal spacing between the points. There is also no
limit to the number of calibration points used. However, using
more calibration points increases calibration time.
The −40°C and +85°C error plots in Figure 51 are generated
using the 25°C calibration coefficients. This is consistent with
equipment calibration in a mass production environment where
calibration at just a single temperature is practical.
HIGH FREQUENCY PERFORMANCE
The ADL5902 is specified to 6 GHz; however, operation is
possible to as high as 9 GHz with sufficient dynamic range for
many purposes. Figure 52 shows the typical V
conformance error at 7 GHz, 8 GHz, and 9 GHz.
Figure 51. 2.14 GHz Transfer Function and Error at +25°C, −40°C, and +85°C
Using a Four-Point Calibration (0 dBm, −20 dBm, −45 dBm, −60 dBm)
6
5
4
3
2
1
0
–70
+85°C V
+25°C V
–40°C V
+85°C ERROR 4-POINT CAL
+25°C ERROR 4-POINT CAL AT 0dBm,
–40°C ERROR 4-POINT CAL
–20dBm, –45dBm, AND –60dBm
–60
OUT
OUT
OUT
–50
–40
P
IN
–30
(dBm)
–20
–10
OUT
0
response and
ADL5902
10
6
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6