AD8318ACPZ-R2 datasheet(15/24 Pages) AD

15 / 24 page

AD8318

Rev. B | Page 15 of 24

The maximum output voltage is 2.1 V × X, and cannot exceed

400 mV below the positive supply.

VOUT(MAX) = (2.1 V × X) when X < (VPOS − 400 mV)/(2.1 V)

(6)

VOUT(MAX) = (VPOS − 400 mV) when X ≥ (VPOS − 400 mV)/ (2.1 V)

(7)

When X = 1, the typical output voltage swing is 0.5 V to 2.1 V.

The output voltage swing is modeled using Equation 5 to

Equation 7 and restricted by Equation 8:

VOUT(MIN) < VOUT < VOUT(MAX)

(8)

When X = 4 and VPOS = 5 V,

(X × VOFFSET) < VOUT < (VPOS − 400 mV)

(4 × 0.5 V) < VOUT < (2.1 V × 4)

2 V < VOUT < 4.6 V

For X = 4, slope = −100 mV/dB; VOUT can swing 2.6 V, and the

usable dynamic range is reduced to 26 dB from 0 dBm to –26 dBm.

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

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

output voltage per decade of input power. One decade is equal

to 20 dB; VSLOPE/DEC/20 = VSLOPE/dB represents the output voltage

slope in V/dB.

As noted in Equation 3, the VOUT voltage has a negative slope.

This is the correct slope polarity to control the gain of many

power amplifiers and other VGAs in a negative feedback

configuration. Because both the slope and intercept vary

slightly with frequency, refer to Table 1 for application-specific

values for the 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 corresponding power levels.

Beginning with the definitions of dBm and dBV,

P (dBm) = 10 × log10(Vrms2/(Z0 × 1 mW))

(9)

V (dBV) = 20 × log10(Vrms/1 Vrms)

(10)

When Equation 9 is expanded

P (dBm) = 20 × log10(Vrms) − 10 × log10(Z0 × 1 mW)

(11)

and given Equation 10, Equation 11 can be rewritten as

P (dBm) = V (dBV) − 10 × log10(Z0 × 1 mW)

(12)

For example, PINTERCEPT for a sinusoidal input signal, expressed

in terms of dBm (decibels referred to 1 mW), in a 50 Ω system is

PINTERCEPT (dBm) = VINTERCEPT (dBV)

− 10 × log10(Z0 × 1 mW) =

(13)

7 dBV − 10 × log10(50 × 10−3) = 20 dBm

For further information on the intercept variation dependence

upon waveform, refer to the AD8313 and AD8307 data sheets.

DEVICE CALIBRATION AND ERROR CALCULATION

The measured transfer function of the AD8318 at 2.2 GHz is

shown in Figure 32. The figure shows plots of both output

voltage vs. input power and calculated log conformance error

vs. input power.

As the input power varies from −65 dBm to 0 dBm, the output

voltage varies from 2 V to about 0.5 V.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

2.5

2.0

1.5

1.0

0.5

–0.5

–1.0

–1.5

–2.0

–65 –60 –55

–45 –40 –35 –30 –25 –20 –15

–5

PIN (dBm)

INTERCEPT

PIN1

PIN2

VOUT +25°C

VOUT –40°C

VOUT +85°C

ERROR +25°C

ERROR –40°C

ERROR +85°C

VOUT2

VOUT1

VOUTIDEAL = SLOPE × (PIN – INTERCEPT)

SLOPE = (VOUT1 – VOUT2)/(PIN1 – PIN2)

INTERCEPT = PIN1 – (VOUT1/SLOPE)

ERROR (dB) = (VOUT ×VOUTIDEAL)/SLOPE

Figure 32. Transfer Function at 2.2 GHz

Because the slope and intercept vary from device to device,

board-level calibration is performed to achieve high accuracy.

The equation can be rewritten for output voltage, from the

Measurement Mode section, using an intercept expressed

in dBm.

VOUT = Slope × (PIN – Intercept)

(14)

In general, the calibration is performed by applying two known

signal levels to the AD8318 input and measuring the corre-

sponding output voltages. The calibration points are generally

chosen to be within the linear-in-dB operating range of the

device (see Figure 32). Calculation of the slope and intercept is

done by:

Slope = (VOUT1 − VOUT2)/(PIN1 − PIN2)

(15)

Intercept = PIN1 − VOUT1/Slope

(16)

Once the slope and intercept are calculated, an equation can be

written to allow calculation of an (unknown) input power based

on the output voltage of the detector.

PIN(unknown) = VOUT (measured)/Slope + Intercept

(17)

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