Preliminary Technical Data

AD8312

Rev. PrA| Page 11 of 13

APPLICATIONS

BASIC CONNECTIONS

Figure 22 shows connections for the basic measurement mode.

A supply voltage of 2.7 V to 5.5 V is required. The supply to the

VPOS pin should be decoupled with a low inductance 0.1 µF

surface mount ceramic capacitor. A series resistor of about 10 Ω

may be added; this resistor will slightly reduce the supply

voltage to the AD8312 (maximum current into the VPOS pin is

approximately 9 mA when VOUT is delivering 5 mA). Its use

should be avoided in applications where the power supply

voltage is very low (i.e., 2.7 V). A series inductor will provide

similar power supply filtering with minimal drop in supply

voltage.

Figure 22. Basic Connections for Operation in Measurement Mode

The AD8312 has an internal input coupling capacitor. This

eliminates the need for external ac-coupling. A broadband

input match is achieved in this example by connecting a

52.3 Ω resistor between RFIN and ground. This resistance

combines with the internal input impedance of approximately

3 kΩ to give an overall broadband input resistance of 50 Ω.

The measurement mode is selected by connecting VSET to

VOUT, which establishes a feedback path and sets the

logarithmic slope to its nominal value. The peak voltage range

of the measurement extends from −45 dBm to 0 dBm at

0.9 GHz, and only slightly less at higher frequencies up to

2.5 GHz. At a slope of 21.5 mV/dB, this would amount to an

output span of 967 mV. Figure 23 shows the transfer function

for VOUT at a supply voltage of 3 V, and input frequency of 0.9

GHz.

The load resistance on VOUT should not be lower than 10

kΩ in order that the full-scale output of 1 V can be generated

with the limited available current of 200 µA max. Figure 23

shows the logarithmic conformance under the same conditions.

Figure 23. VOUT and Log Conformance Error vs. Input Level vs. Input Level

EVALUATION BOARD

Figure 24 shows the schematic of the AD8312 evaluation board.

The layout and silkscreen of the component and circuit sides

are shown in Figure 25 to Figure 28. The board is powered by a

single supply in the range, 2.7 V to 5.5 V. The power supply is

decoupled by a single 0.1 µF capacitor.

Table 4 details the various configuration options of the

evaluation board.

Figure 24. Evaluation Board Schematic