CN-0178 Datasheet, PDF (2/5 Pages) Analog Devices – Software-Calibrated, 50 MHz to 9 GHz, RF Power Measurement System

CN-0178
Circuit Note
Data is shown for the two devices operating over a −40°C to
+85°C temperature range.
CIRCUIT DESCRIPTION
The RF signal being measured is applied to the input of the
ADL5902, a linear-in-dB rms-responding rms detector. The
external 60.4 Ω resistor, R3, combined with the relatively high
input impedance of the ADL5902 ensures a broadband 50 Ω
match to the RF input. The ADL5902 is configured in its
so-called “measurement mode,” with the VSET and VOUT
pins connected together. In this mode the output voltage is
proportional to the logarithm of the rms value of the input. In
other words, the reading is presented directly in decibels and is
scaled to 1.06 V per decade, or 53 mV/dB.
The power supply voltage and reference voltage for the AD7466
12-bit ADC are provided by the ADL5902 internal 2.3 V
reference. Because the AD7466 consumes so little current
(16 µA when sampling at 10 kSPS), the ADL5902’s reference
voltage output can supply the ADC, as well as the temperature
compensating and rms accuracy-scaling network consisting of
R9, R10, R11, and R12.
The ADC full-scale voltage is equal to 2.3 V. The maximum
detector output voltage (when operating in its linear input
range) is approximately 3.5 V (see ADL5902 data sheet figures
6, 7, 8, 12, 13, and 14) and must, therefore, be scaled down by a
factor of 0.657 before driving the AD7466. This scaling is
implemented using a simple resistor divider R10 and R11
(1.21 kΩ and 2.0 kΩ). These values provide an actual scaling
factor of 0.623, which ensures that the ADL5902 RF detector
does not overdrive the ADC by building in some room for
resistor tolerance.
A typical plot of detector output voltage vs. input power is
shown in Figure 2 (without output scaling).
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–70 –60 –50 –40 –30 –20 –10
0
10
INTERCEPT
PIN (dBm)
Figure 2. ADL5902 RMS Detector, Output Voltage vs. Input Power @ 900 MHz
The transfer function of the detector can be approximated by
the equation
VOUT = SLOPE_DETECTOR × (PIN − INTERCEPT)
where SLOPE_DETECTOR is in mV/dB; INTERCEPT is the
x-axis intercept with a unit of dBm; PIN is the input power
in dBm.
At the output of the ADC, VOUT is replaced by the ADC’s
output code, and the equation can be rewritten as
CODE = SLOPE × (PIN − INTERCEPT)
where SLOPE is the combined slope of the detector, the scaling
resistors, and the ADC, and has the unit of counts/dB; PIN and
INTERCEPT still have the unit of dBm.
Figure 3 shows a typical detector power sweep in terms of input
power and observed ADC output codes for a 700 MHz input
signal.
4096
3755
3413
3072
2731
+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
2389
2048
1707
1365
1024
683
341
0
–70 –60 –50 –40 –30 –20 –10 0
PIN (dBm)
6
5
4
3
2
1
0
–1
–2
–3
–4
–5
–6
10
Figure 3. ADC Output Code and Error vs. RF Input Power @ 700 MHz
Overall SLOPE and INTERCEPT will vary from system to
system. This variation is caused by part to part variations in the
transfer function of the RF detector, the scaling resistors, and
the ADC. As a result, a system level calibration is required to
determine the complete system SLOPE and INTERCEPT. In this
application, a 4-point calibration is used to correct for some
nonlinearity in the RF detector’s transfer function, particularly
at the low end. This 4-point calibration scheme yields three
SLOPE and three INTERCEPT calibration coefficients, which
should be stored in nonvolatile RAM (NVM) after calibration.
Rev. A| Page 2 of 5