AD8314 datasheet(12/20 Pages) AD

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AD8314

Rev. B | Page 12 of 20

APPLICATIONS

BASIC CONNECTIONS

Figure 31 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

can be added; this resistor slightly reduces the supply voltage to the

AD8314 (maximum current into the VPOS pin is approximately

9 mA when V_DN is delivering 5 mA). Its use should be

avoided in applications where the power supply voltage is very

low (that is, 2.7 V). A series inductor provides similar power

supply filtering with minimal drop in supply voltage.

OPTIONAL

(SEE TEXT)

0.1µF

OPTIONAL

(SEE TEXT)

VDN

VUP

INPUT

ENBL

RFIN

AD8314

VSET

FLTR

V_DN

VPOS

COMM

V_UP

52.3Ω

Figure 31. Basic Connections for Operation in Measurement Mode

The ENBL pin is here connected to VPOS. The AD8314 can be

disabled by pulling this pin to ground when the chip current is

reduced to about 20 μA from its normal value of 4.5 mA. The

logic threshold is around +VS/2 and the enable function occurs

in about 1.5 μs. Note, however, further settling time is generally

needed at low input levels.

The AD8314 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. Several other

coupling methods are possible, which are described in the

Input Coupling Options section.

The measurement mode is selected by connecting VSET to

V_UP, which establishes a feedback path and sets the

logarithmic slope to its nominal value. The peak voltage range

of the measurement extends from −58 dBV to −13 dBV at

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

2.5 GHz. Therefore, using the 50 Ω termination, the equivalent

power range is −45 dBm to 0 dBm. At a slope of 21.5 mV/dB,

this would amount to an output span of 967 mV. Figure 32

shows the transfer function for V_UP at a supply voltage of 3 V

and input frequency of 0.9 GHz.

V_DN, which is generally not used when the AD8314 is used in

measurement mode, is essentially an inverted version of V_UP.

The voltage on V_UP and V_DN are related by

VDN = 2.25 V − 2 VUP

While V_DN can deliver up to 6 mA, the load resistance on

V_UP 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 maximum. Figure 32 shows the logarithmic

conformance under the same conditions.

1.2

–70

INPUT AMPLITUDE (dBV)

–3

–2

–1

1.0

0.8

0.6

0.4

0.2

–60

–50

–40

–30

–20

–10

VS = 3V

RT = 52.3Ω

±1dB DYNAMIC RANGE

±3dB DYNAMIC RANGE

INTERCEPT

(–47dBm)

(+3dBm)

Figure 32. VUP and Log Conformance Error vs.

Input Level vs. Input Level at 900 MHz

TRANSFER FUNCTION IN TERMS OF SLOPE AND

INTERCEPT

The transfer function of the AD8314 is characterized in terms

of its slope and intercept. The logarithmic slope is defined as the

change in the RSSI output voltage for a 1 dB change at the input.

For the AD8314, slope is nominally 21.5 mV/dB. Therefore, a

10 dB change at the input results in a change at the output of

approximately 215 mV. Log conformance plot, Figure 32, shows

the range over which the device maintains its constant slope.

The dynamic range can be defined as the range over which the

error remains within a certain band, usually ±1 dB or ±3 dB. In

Figure 32 for example, the ±1 dB dynamic range is approximately

50 dB (from −13 dBV to −63 dBV).

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