AD8317ACPZ-R7 datasheet(12/19 Pages) AD

12 / 19 page

AD8317

Data Sheet

Rev. C | Page 12 of 19

The slope is given by

−ID × 2x × 1.5 kΩ = −22 mV/dB × x

For example, if a resistor divider to ground is used to generate a

VSET voltage of VOUT/2, x = 2. The slope is set to −880 V/decade

or −44 mV/dB.

TEMPERATURE COMPENSATION OF OUTPUT

VOLTAGE

The primary component of the variation in VOUT vs. temperature,

as the input signal amplitude is held constant, is the drift of the

intercept. This drift is also a weak function of the input signal

frequency; therefore, provision is made for the optimization of

internal temperature compensation at a given frequency by

providing Pin TADJ.

COMM

ICOMP

VINTERNAL

TADJ

RTADJ

1.5k

Ω

AD8317

Figure 26. TADJ Interface

RTADJ is connected between TADJ and ground. The value of

this resistor partially determines the magnitude of an analog

correction coefficient, which is used to reduce intercept drift.

The relationship between output temperature drift and

frequency is not linear and cannot be easily modeled. As a

result, experimentation is required to choose the correct

TADJ resistor. Table 4 shows the recommended values for

some commonly used frequencies.

Table 4. Recommended RTADJ Values

Frequency

Recommended RTADJ

50 MHz

18 kΩ

100 MHz

18 kΩ

900 MHz

18 kΩ

1.8 GHz

8 kΩ

1.9 GHz

8 kΩ

2.2 GHz

8 kΩ

3.6 GHz

8 kΩ

5.3 GHZ

500 Ω

5.8 GHz

500 Ω

8 GHz

Open

MEASUREMENT MODE

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

back to the VSET pin, the device operates in measurement

mode. As seen in Figure 27, the AD8317 has an offset voltage,

a negative slope, and a VOUT measurement intercept at the high

end of its input signal range.

0.25

0.50

0.75

1.00

1.25

1.50

2.00

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

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

10 15

PIN (dBm)

RANGE FOR

CALCULATION OF

SLOPE AND INTERCEPT

VOUT IDEAL

VOUT 25°C

ERROR 25°C

1.75

INTERCEPT

Figure 27. Typical Output Voltage vs. Input Signal

The output voltage vs. input signal voltage of the AD8317 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.

VSLOPE/DEC is nominally −440 mV/decade, or −22 mV/dB.

VINTERCEPT is the x-axis intercept of the linear-in-dB portion of

the VOUT vs. PIN curve (see Figure 27).

VINTERCEPT is 2 dBV for a sinusoidal input signal.

An offset voltage, VOFFSET, of 0.35 V is internally added to

the detector signal, so that the minimum value for VOUT is

X × VOFFSET; therefore, for X = 1, the minimum VOUT is 0.35 V.

The slope is very stable vs. process and temperature variation.

When base-10 logarithms are used, VSLOPE/DECADE represents the

volts/decade. A decade corresponds to 20 dB; VSLOPE/DECADE/20 =

VSLOPE/dB represents the slope in volts/dB.

As noted in Equation 3 and Equation 4, the VOUT voltage has a

negative slope. This is also the correct slope polarity to control

the gain of many power amplifiers in a negative feedback con-

figuration. Because both the slope and intercept vary slightly

with frequency, it is recommended to refer to the Specifications

section for application-specific values for slope and intercept.

Although demodulating log amps respond to input signal

voltage, not input signal power, it is customary to discuss the

amplitude of high frequency signals in terms of power. In this

case, the characteristic impedance of the system, Z0, must be

known to convert voltages to their corresponding power levels.

The following equations are used to perform this conversion:

P [dBm] = 10 × log10(VRMS2/(Z0 × 1 mW))

(5)

P [dBV] = 20 × log10(VRMS/1 VRMS)

(6)

P [dBm] = P [dBV] − 10 × log10(Z0 × 1 mW/1 VRMS2)

(7)

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