AD8362
OPERATION IN MEASUREMENT MODES
Figure 50 shows the general connections for operating the
AD8362 as an RF power detector, more correctly viewed
as an accurate measurement system. The full performance
potential of this part, particularly at very high frequencies
(above 500 MHz), is realized only when the input is presented
to the AD8362 in differential (balanced) form. In this example, a
flux-coupled transformer is used at the input. Having a 1:4
impedance ratio (1:2 turns ratio), the 200 Ω differential input
resistance of the AD8362 becomes 50 Ω at the input to the
transformer, whose outputs can be connected directly to INHI
and INLO. If a center-tapped transformer is used, connect the
tap to the DECL pins, which are biased to the same potential as
the inputs (~3.6 V). Over the 0.9 GHz to 2.2 GHz range, a
transmission line transformer (balun) may be used, as explained
later. (The evaluation board is supplied with a M/A-COM
ETC1.6-4-2-3, 0.5 GHz to 2.5 GHz, 4:1 balun.)
LAW CONFORMANCE ERROR
In practice, the response deviates slightly from the ideal straight
line suggested by Equation 11. This deviation is called the law
conformance error. In defining the performance of high
accuracy measurement devices, it is customary to provide plots
of this error. In general terms, it is computed by extracting the
best straight line to the measured data using linear regression
over a substantial region of the dynamic range and under
clearly specified conditions.
3.0
3.8
3.5
2.5
2.0
1.5
1.0
0.5
–40°C
3.2
2.9
2.6
2.3
0
2.0
1.7
1.4
1.1
0.8
0.5
0.2
V
S
–0.5
–1.0
–1.5
–2.0
–2.5
–3.0
+25°C
+5V nom, @ 24mA
+85°C
AD8362
–40°C
1:4 Z-RATIO
(1:2 TURNS RATIO)
3.3Ω
1
2
3
4
5
6
7
8
COMM ACOM 16
+25°C
+85°C
NC
CHPF
DECL
INHI
VREF 15
VTGT 14
VPOS 13
0.1µF
SIGNAL INPUT
Z = 50Ω
1nF
1nF
–60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5
INPUT AMPLITUDE (dBm)
0
5
10 15
200Ω
12
INLO
DECL
VOUT
V
RAIL-TO-RAIL
OUT
CONTROL OUTPUT
VSET 11
Figure 51. Output Voltage and Law Conformance Error,
at TA = −40°C, +25°C, and +85°C
1nF
PWDN ACOM 10
COMM CLPF
9
Figure 51 shows the output of the circuit of Figure 50 over the
full input range. The agreement with the ideal function (law
conformance) is also shown. This was determined by linear
regression on the data points over the central portion of the
transfer function (35 mV to 250 mV rms) for the 25°C data.
The error at +25°C, −40°C, and +85°C was then calculated by
subtracting the ideal output voltage at each input signal level
from the actual output and dividing this quantity by the mean
slope of the regression equation to provide a measurement of
the error in decibels (scaled on the right-hand axis of Figure 51).
300pF
Figure 50. Connections for RF Power Measurement
The output in this mode of use is a continuous, decibel-scaled
voltage ranging from about 0.5 V to 3.5 V.
VOUT =
PIN PZ
× 50 mV dB
(11)
The equivalent input power, PIN, is expressed in dBm (decibels
above 1 mW) in a particular system impedance, which in this
case is 50 Ω. The intercept, PZ, is that input power for which
the back-extrapolated output crosses zero. Expressed as a
voltage, it is 0.447 mV rms (−67 dBV, laser-calibrated at
100 MHz), corresponding to a PZ of −60 dBm in 200 Ω.
However, the 1:2 turns ratio of the transformer halves the
required input voltage, which moves the intercept down by
6 dB to 0.224 mV rms (−73 dBV) at the transformer’s input.
The error curves generated in this way reveal not only the
deviations from the ideal transfer function at a nominal
temperature but also all of the additional errors caused by
temperature changes. Notice there is a small temperature
dependence in the intercept (the vertical position of the error
plots); this variation is within 0.5 dB at high powers.
Figure 51 further reveals that there is a periodic ripple in the
conformance curves. This is due to the interpolation technique
used to select the signals from the attenuator, not only at
discrete tap points, but anywhere in between, thus providing
continuous attenuation values. The selected signal is then
applied to the 3.5 GHz, 40 dB fixed gain amplifier in the
remaining stages of the AD8362’s VGA.
Impedance mismatches and attenuation in the coupling
elements significantly affect the intercept position. This error
is stable over temperature and time, and thus can be removed
during calibration in a specific system. The logarithmic slope of
50 mV/dB varies only slightly with frequency; corrected values
for several common frequencies are provided in the
Specifications section.
Rev. B | Page 22 of 36
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