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Data Sheet

AD8364

Rev. C | Page 31 of 44

TEMPERATURE COMPENSATION ADJUSTMENT

The AD8364 has a highly stable measurement output with

respect to temperature. However, when the RF inputs exceed a

frequency of 600 MHz, the output temperature drift must be

compensated for using ADJ[A, B] for optimal performance.

Proprietary techniques are used to compensate for the temper-

ature drift. The absolute value of compensation varies with

frequency, balun choice, and circuit board material. Table 5

shows recommended voltages for ADJ[A, B] to maintain a

temperature drift error of typically ±0.5 dB or better over the

entire rated temperature range with the recommended baluns.

Table 5. Recommended Voltages for ADJ[A, B]

Frequency (MHz)

450

880

1880

2140

2500

ADJ[A, B] (V)

0.5

0.65

0.85

1.10

Compensating the device for temperature drift using ADJ[A, B]

allows for great flexibility. If the user requires minimum temper-

ature drift at a given input power or subset of the dynamic range,

the ADJ[A, B] voltage can be swept while monitoring OUT[A, B]

over temperature. Figure 67 shows the result of such an exercise

with a broadband balun, one that is not the recommended balun

at 1880 MHz. The value of ADJ[A, B] where the output has

minimum movement (approximately 0.77 V for the example in

Figure 67) is the recommended voltage for ADJ[A, B] to achieve

minimum temperature drift at a given power and frequency.

ADJA (V)

2.50

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

1.70

1.65

1.60

1.55

1.50

1.45

1.40

+85°C

+65°C

+45°C

+25°C

+10°C

–40°C

–20°C

Figure 67. OUTA vs. ADJA over Temp. Pin = −30 dBm, 1.9 GHz

The ADJ[A, B] input has high input impedance. The input can

be conveniently driven from an attenuated value of VREF using

a resistor divider, if desired.

Figure 68 shows a simplified schematic representation of the

ADJ[A, B] interface.

VPSR

VREF/2

ADJ[A, B]

COMR IADJ[A, B]

INTERNAL

CURRENT

Figure 68. ADJ[A, B] Interface Simplified Schematic

DEVICE CALIBRATION AND ERROR CALCULATION

The measured transfer function of the AD8364 at 2.14 GHz is

shown in Figure 69. The figure shows plots of both output

voltage vs. input power and calculated error vs. input power. As

the input power varies from −50 dBm to 0 dBm, the output

voltage varies from 0.4 V to about 2.8 V.

–2.0

2.0

1.2

1.6

0.8

0.4

–0.4

–0.8

–1.2

–1.6

PIN MEAS (dBm)

–60

–45 –40 –35

–55 –50

–25 –20 –15

–30

–5

–10

3.50

2.80

3.15

2.10

2.45

1.40

1.75

0.70

0.35

1.05

PIN1

VOUT1

VOUT2

PIN2

ERROR CW +25°C

ERROR CW –40°C

ERROR CW +85°C

INTERCEPT

BLUE = –40°C

GREEN = +25°C

RED = +85°C

Figure 69. Transfer Function at 2.14 GHz.

Because slope and intercept vary from device to device, board-

level calibration must be performed to achieve high accuracy.

The equation for output voltage can be written as

VOUT = Slope × (PIN − Intercept)

Where Slope is the change in output voltage divided by the

change in power (dB), and Intercept is the calculated power at

which the output voltage is 0 V. (Note that Intercept is a theoretical

value; the output voltage can never achieve 0 V).

In general, the calibration is performed by applying two known

signal levels to the AD8364 input and measuring the corresponding

output voltages. The calibration points are generally chosen to

be within the linear-in-dB operating range of the device (see the

Specifications section for more details).

Calculation of the slope and intercept is done using the

equations:

Slope = (VOUT1 − VOUT2)/(PIN1 − PIN2)

Intercept = PIN1 − (VOUT1/Slope)

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