CN-0150 Datasheet, PDF (2/5 Pages) Analog Devices – Software-Calibrated, 1 MHz to 8 GHz, 60 dB RF Power Measurement System Using a Logarithmic Detector

CN-0150
CIRCUIT DESCRIPTION
The RF signal being measured is applied to the AD8318. The
device is configured in its so-called measurement mode, with
the VSET and VOUT pins connected together. In this mode,
the output voltage vs. the input signal level is linear-in-dB
(nominally −24 mV/dB) and has a typical output voltage range
of 0.5 V to 2.1 V.
The AD8318 output is connected directly to the AD7887, 12-bit
ADC. The ADC uses its internal reference and is configured for
a 0 V to 2.5 V input, resulting in an LSB size of 610 μV. With the RF
detector providing a nominal −24 mV/dB, the digital resolution
is 39.3 LSBs/dB. With this much resolution, there is little value
in trying to scale the 0.5 V to 2.1 V signal from the RF detector
to exactly fit the 0 V to 2.5 V range of the ADC.
The transfer function of the detector can be approximated by
the equation
VOUT = SLOPE × (PIN − INTERCEPT)
where SLOPE is in mV/dB (−24 mV/dB nominal); INTERCEPT is
the x-axis intercept with a unit of dBm (20 dBm nominal);
and PIN is the input power expressed in dBm. A typical plot
of detector output voltage vs. input power is shown in Figure 2.
2.4
2.0
V_OUT 25°C
2.1
ERROR 25°C
1.5
1.8
1.0
1.5
0.5
1.2
0
0.9
–0.5
RANGE OF
0.6
CALCULATION
–1.0
OF SLOPE AND
INTERCEPT
0.3
–1.5
0
–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5 0
P_IN (dBm)
5 10 15
INTERCEPT
Figure 2. Typical Output Voltage vs. Input Signal Level for the AD8318
At the output of the ADC, the equation can be written as
CODE_OUT = SLOPE_ADC × (PIN − INTERCEPT)
where SLOPE_ADC is in codes/dB and PIN and INTERCEPT
are in dBm. Figure 3 shows a typical detector power sweep in
terms of input power and observed ADC codes.
Because the slope and intercept of the system vary from device
to device, a system level calibration is required. A calibration is
performed by applying two known signal levels close to the
endpoints of the AD8318 linear input range and measuring the
corresponding output codes from the ADC. The calibration
points chosen should be well within the linear operating
range of the device (−10 dBm and −50 dBm in this case).
Circuit Note
Using the two known input power levels, PIN_1 and PIN_2,
and the corresponding observed ADC codes, CODE_1 and
CODE_2, SLOPE_ADC, and INTERCEPT can be calculated
using the following equations:
SLOPE_ADC = (CODE_2 − CODE_1)/(PIN_2 − PIN_1)
INTERCEPT = PIN_2 − (CODE_2/SLOPE_ADC)
Once SLOPE_ADC and INTERCEPT are calculated and stored (in
nonvolatile RAM) during factory calibration, they can be used
to calculate an unknown input power level, PIN, when the
equipment is in operation in the field using the equation
PIN = (CODE_OUT/SLOPE_ADC) + INTERCEPT
Figure 3 through Figure 8 show how the system transfer function
deviates from this straight line equation, particularly at the
endpoints of the transfer function. This deviation is expressed
in dB using the equation
Error (dB) = Measured Input Power − True Input Power =
(CODE_OUT/SLOPE_ADC) + INTERCEPT – PIN_TRUE
where:
CODE_OUT is the ADC output code.
SLOPE_ADC is the stored ADC slope in codes/dB.
INTERCEPT is the stored intercept.
PIN_TRUE is the exact (and unknown) input level.
The plots shown in Figure 3 through Figure 8 show the typical
system performance that can be obtained using the AD8318 and
AD7887BR in an RF power measurement system. The graphs
depict the RF input power in dBm vs. the ADC output code and
output error in dB (scaled on the axes on the right side of the
plots). They were generated from data taken with various input
power levels, frequencies, and temperatures and with both internal
and external ADC voltage references. The charts show improved
system performance and lower temperature drift with the use of
a low drift external ADC voltage reference. (See the Common
Variations section for more details about the use of an external
reference.
A complete design support package for this circuit note can be
found at www.analog.com/CN0150-DesignSupport.
4.0k
3.5k
CODE_2
3.0k
2.5k
4
+25°C CODE
–40°C CODE ₃
+85°C CODE
+25°C ERROR
–40°C ERROR ²
+85°C ERROR
1
2.0k
0
1.5k
–1
CODE_1
1.0k
–2
0.5k
–3
0
–70 –60 –50
–40 –30 –20 –10
INPUT POWER (dBm)
–4
0
10
PIN_2
PIN_1
Figure 3. Input = 900 MHz, ADC Using an Internal 2.5 V Reference
Rev. C | Page 2 of 5