AD8318ACPZ-R2 datasheet(14/24 Pages) AD

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AD8318

Rev. B | Page 14 of 24

Table 5 lists recommended resistors for various frequencies.

These resistors provide the best overall temperature drift based

on measurements of a diverse population of devices.

The relationship between output temperature drift and frequency

is nonlinear and is not easily modeled. Experimentation is

required to choose the correct RTADJ resistor at frequencies

not listed in Table 5.

Table 5. Recommended RTADJ Resistors

Frequency

Recommended RTADJ

900 MHz

500 Ω

1.9 MHz

500 Ω

2.2 GHz

500 Ω

3.6 GHz

51 Ω

5.8 GHz

1 kΩ

8 GHz

500 Ω

TEMPERATURE SENSOR

The AD8318 internally generates a voltage that is proportional

to absolute-temperature (VPTAT). The VPTAT voltage is multiplied

by a factor of 5, resulting in a 2 mV/°C output at the TEMP pin.

The output voltage at 27°C is typically 600 mV. An emitter

follower drives the TEMP pin, as shown in Figure 30.

1kΩ

4kΩ

CMIP

INTERNAL

VPSI

TEMP

Figure 30. Temp Sensor Interface

The internal pull-down resistance is 5 kΩ. The temperature

sensor has a slope of 2 mV/°C.

The temperature sensor output varies with output current due

to increased die temperature. Output loads less than 1 kΩ draw

enough current from the output stage causing this increase to

occur. An output current of 10 mA results in the voltage on the

temperature sensor to increase by 1.5°C, or ~3 mV.

Best precision from the temperature sensor is obtained when

the supply current to AD8318 remains fairly constant, that is,

no heavy load drive.

MEASUREMENT MODE

When the VOUT voltage, or a portion of the VOUT voltage, is

fed back to VSET, the device operates in measurement mode.

As shown in Figure 31, the AD8318 has an offset voltage, a

negative slope, and a VOUT measurement intercept greater

than its input signal range.

2.4

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–65 –60 –55 –50 –45 –40 –35 –30 –25 –20 –15 –10 –5 0

INTERCEPT

PIN (dBm)

VOUT 25°C

ERROR 25°C

RANGE OF

CALCULATION

OF SLOPE AND

INTERCEPT

Figure 31. Typical Output Voltage vs. Input Signal

The output voltage vs. input signal voltage of the AD8318 is

linear-in-dB over a multidecade range. The equation for this

function is

VOUT = X × VSLOPE/DEC × log10(VIN/VINTERCEPT)

(3)

= X × VSLOPE/dB × 20 × log10(VIN/VINTERCEPT)

(4)

where:

X is the feedback factor in VSET = VOUT/X.

VINTERCEPT is expressed in Vrms.

VSLOPE/DEC is nominally −500 mV/decade and VSLOPE/dB is

nominally −25 mV/dB.

VINTERCEPT, expressed in dBV, is the x-axis intercept of the linear-

in-dB transfer function shown in Figure 31.

VINTERCEPT is 7 dBV (20 dBm, re: 50 Ω or 2.239 Vrms) for a

sinusoidal input signal.

The slope of the transfer function can be increased to

accommodate various converter mV per dB (LSB per dB)

requirements. However, increasing the slope can reduce the

dynamic range. This is due to the limitation of the minimum

and maximum output voltages, determined by the chosen

scaling factor X.

The minimum value for VOUT is X × VOFFSET. The offset voltage,

VOFFSET, is equal to 0.5 V and is internally added to the detector

output signal.

VOUT(MIN) = (X × VOFFSET)

(5)

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