AD8362
TEMPERATURE COMPENSATION AT VARIOUS WiMAX FREQUENCIES UP TO 3.8 GHz
The AD8362 is ideally suited for measuring WiMAX type
signals because crest factor changes in the modulation scheme
have very little affect on the accuracy of the measurement.
However, at higher frequencies, the AD8362 drifts more over
temperature often making temperature compensation necessary.
Temperature compensation is possible because the part-to-part
variation over temperature is small, and temperature change
only causes a shift in the AD8362’s intercept. Typically, users
choose to compensate for temperature changes digitally. How-
ever, temperature compensation is possible using an analog
temperature sensor. Because the drift of the output voltage is
due mainly to intercept shift, the whole transfer function tends
to drop with increasing temperature, while the slope remains
quite stable. This makes the temperature drift independent of
input level. Compensating the drift based on a particular
input level (for example, −15 dBm), holds up well over the
dynamic range.
Table 5 shows the resultant values for R2 and R1 for frequen-
cies ranging from 2350 MHz to 3650 MHz. Figure 59 through
Figure 63 show the performance over temperature for the
AD8362 with temperature compensation at frequencies across
the WiMAX band. The compensation factor chosen optimizes
temperature drift in the 25°C to 85°C range. This can be altered
depending on the temperature requirements for the application.
Table 5. Recommended Resistor Values for Temperature
Compensation at Various Frequencies
Average
Drift @
−15 dBm
Average
Drift @
−15 dBm
Freq.
Slope
R1
R2
(MHz)
(dB/°C)
(mV/dB)
(mV/°C)
(kΩ)
(kΩ)
2350
2600
2800
3450
3650
−0.0345
−0.0440
−0.0486
−0.0531
−0.0571
51
−1.7600
−2.2639
−2.5102
−2.7402
−2.9544
4.99
4.99
4.99
4.99
4.99
28
22.1
20
18.2
16.9
51.45
51.68
51.61
51.73
Figure 59 through Figure 63 show these results. The compensa-
tion is simple and relies on the TMP36 precision temperature
sensor driving one side of the resistor divider as the AD8362
drives the other side. The output is at the junction of the two
resistors (see Figure 58). At 25°C, TMP36 has an output voltage
of 750 mV and a temperature coefficient of 10 mV/°C. As the
temperature increases, the voltage from the AD8362 drops and
the voltage from the TMP36 rises. R1 and R2 are chosen so the
voltage at the center of the resistor divider remains steady over
temperature. In practice, R2 is much larger than R1 so that the
output voltage from the circuit is close to the voltage of the VOUT
pin. The resistor ratio R2/R1 is determined by the temperature
drift of the AD8362 at the frequency of interest. To calculate the
values of R1 and R2, first calculate the drift at a particular input
level, −15 dBm in this case. To do this, calculate the average
drift over the temperature range from 25°C to 85°C. Using the
following equation, the average drift in dB/°C is obtained.
5V
0.1µF
AD8362
2.7nH
1nF
INHI
VOUT
VSET
CLPF
2
V
TEMP
1
1nF
TMP36F
4.7nH
R1
0.1µF
R2
INLO
5
VTGT
VREF
V
OUT
Figure 58. AD8362 with Temperature Compensation Circuit
dBError
ΔTemperature
dB/°C =
(16)
In this example, the drift of the AD8362 from 25°C to 85°C is
−2.07 dB and the temperature delta is 60°C, which results in
−0.0345 dB/°C drift. This temperature drift in dB/°C is con-
verted to mV/°C through multiplication by the logarithmic slope
(51 mV/dB at 2350 MHz). The result is −1.76 mV/°C. The
following equation calculates the values of R1 and R2:
10 mV/°C
R1 AD8362 Drift(mV/°C)
R2
=
(17)
Rev. D | Page 24 of 32
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