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AD8362

Data Sheet

Rev. E | Page 24 of 32

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.

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.

Temperatur

Error

dB/

(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:

(mV/

mV/

Drift

AD8362

(17)

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

Freq.

(MHz)

Average

Drift @

−15 dBm

(dB/°C)

Slope

(mV/dB)

Average

Drift @

−15 dBm

(mV/°C)

(kΩ)

2350

−0.0345

−1.7600

4.99

2600

−0.0440

51.45

−2.2639

4.99

22.1

2800

−0.0486

51.68

−2.5102

4.99

3450

−0.0531

51.61

−2.7402

4.99

18.2

3650

−0.0571

51.73

−2.9544

4.99

16.9

INHI

INLO

CLPF

VSET

VOUT

0.1µF

VTGT

VREF

0.1µF

AD8362

1nF

2.7nH

4.7nH

1nF

TMP36F

VTEMP

Figure 58. AD8362 with Temperature Compensation Circuit

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