AD8318 datasheet(17/24 Pages) AD

17 / 24 page

AD8318

Rev. 0 | Page 17 of 24

output voltage at ambient. This is a key difference in comparison

to the previous plots. Up to now, all errors have been calculated

with respect to the ideal transfer function at ambient.

When we use this alternative technique, the error at ambient

becomes by definition equal to 0 (see Figure 33)

This would be valid if the device transfer function perfectly

followed the ideal VOUT = Slope × (Pin-Intercept) equation.

However since a log amp in practice will never perfectly follow

this equation (especially outside of its linear operating range),

this plot tends to artificially improve linearity and extend the

dynamic range. This plot is a useful tool for estimating

temperature drift at a particular power level with respect to the

(non-ideal) output voltage at ambient. However, to achieve this

level of accuracy in an end application would require calibration

at multiple points in the device’s operating range.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

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

PIN (dBm)

2.5

–2.5

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

VOUT +25°C

VOUT –40°C

VOUT +85°C

ERROR +25

°C wrt V

OUT

ERROR –40

°C wrt V

OUT

ERROR +85

°C wrt V

OUT

Figure 33. Error vs. Temperature with respect to Output Voltage at 25 °C Does

Not Take into Account Transfer Functions’ Nonlinearities at 25°C

VARIATION IN TEMPERATURE DRIFT FROM DEVICE

TO DEVICE

Figure 34 shows a plot of output voltage and error for multiple

AD8318 devices, measured in this case at 5.8 GHz. The

concentration of black error plots represents the performance

of the population at 25°C (slope and intercept has been

calculated for each device). The red and blue plots of error

indicate the measured behavior of a population of devices over

temperature. This suggests a range on the drift (from device to

device) of 1.2 dB.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

–65

–55

–45

–35

–25

–15

–5

PIN (dBm)

2.0

–2.0

–1.6

–1.2

–0.8

–0.4

0.4

0.8

1.2

1.6

Figure 34. Output Voltage and Error vs. Temperature (+25°C, –40°C, and

+85°C) of a Population of Devices Measured at 5.8 GHz

TEMPERATURE DRIFT AT DIFFERENT

TEMPERATURES

Figure 35 shows the log slope and error over temperature

for a 5.8 GHz input signal. Error due to drift over

temperature consistently remains within ±0.5 dB, and only

begins to exceed this limit when the ambient temperature

drops below −20°C. For all frequencies when using a

reduced temperature range higher measurement accuracy is

achievable.

2.2

2.0

1.8

1.6

1.4

1.2

1.0

0.8

0.6

0.4

0.2

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

PIN (dBm)

2.5

–2.5

–2.0

–1.5

–1.0

–0.5

0.5

1.0

1.5

2.0

VAPC +25°C

VAPC 0°C

ERROR –10°C

VAPC –40°C

VAPC +70°C

ERROR –20°C

VAPC +85°C

ERROR +25°C

ERROR 0°C

VAPC –20°C

ERROR +85°C

VAPC –10°C

ERROR –40°C

ERROR +70°C

Figure 35. Typical Drift at 5.8 GHz for Various Temperatures

SETTING THE OUTPUT SLOPE IN

MEASUREMENT MODE

To operate in measurement mode, VOUT must be

connected to VSET. This yields the nominal logarithmic

slope of approximately −25 mV/dB. The output swing

corresponding to the specified input range will then be

approximately 0.5 V to 2.1 V. The slope and output swing

AD8318 Datasheet(PDF) 17 Page - Analog Devices

AD8318 Datasheet(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL