AD8318

Rev. B | Page 17 of 24

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

0

5

2.5

–2.5

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

TEMPERATURE DRIFT AT DIFFERENT

TEMPERATURES

Figure 37 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.

When using a reduced temperature range, higher measurement

accuracy is achievable for all frequencies.

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

0

5

2.5

–2.5

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

ERROR –10°C

ERROR –20°C

ERROR +25°C

ERROR 0°C

ERROR +85°C

ERROR –40°C

ERROR +70°C

Figure 35. Error vs. Temperature with Respect to Output Voltage at 25°C

(Does Not Take Transfer Function Nonlinearities at 25°C into Account)

VARIATION IN TEMPERATURE DRIFT FROM

DEVICE TO DEVICE

Figure 36 shows a plot of output voltage and error for multiple

AD8318 devices measured at 5.8 GHz. The concentration of

black error plots represents the performance of the population

at 25°C (slope and intercept are calculated for each device).

The red and blue curves 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.

Figure 37. Typical Drift at 5.8 GHz for Various Temperatures

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

5

15

2.0

–2.0

–1.6

–1.2

–0.8

–0.4

0

0.4

0.8

1.2

1.6

SETTING THE OUTPUT SLOPE IN MEASUREMENT

MODE

To operate in measurement mode, VOUT is connected to VSET.

This yields the typical logarithmic slope of −25 mV/dB. The

output swing corresponding to the specified input range is then

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

increased by placing a resistor divider between VOUT and

VSET (that is, one resistor from VOUT to VSET and one

resistor from VSET to common).

As an example, if two equal resistors, such as 10 kΩ/10 kΩ, are

used, the slope doubles to approximately −50 mV/dB. The input

impedance of VSET is approximately 500 kΩ. Slope setting

resistors should be kept below ~50 kΩ to prevent this input

impedance from affecting the resulting slope. When increasing

the slope, the new output voltage range cannot exceed the

output voltage swing capability of the output stage. Refer to the

Measurement Mode section for further details.

Figure 36. Output Voltage and Error vs. Temperature (+25°C, −40°C, and

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

VOUT

AD8318

–50mV/dB

VSET

10kΩ

10kΩ

Figure 38. Increasing the Slope