AD8314

Rev. B | Page 13 of 20

The intercept is the point at which the extrapolated linear

response would intersect the horizontal axis (see Figure 32).

Using the slope and intercept, the output voltage can be

calculated for any input level within the specified input range by

where:

reference level (either dBm or dBV in this case).

the same reference level.

For example, at an input level of −40 dBV (−27 dBm), the

output voltage is

dBV VS. dBm

The most widely used convention in RF systems is to specify

power in dBm, that is, decibels above 1 mW in 50 Ω. Specification

of log amp input levels in terms of power is strictly a concession

to popular convention; they do not respond to power (tacitly

power absorbed at the input), but to the input voltage. The use

of dBV, defined as decibels with respect to a 1 V rms sine wave,

is more precise, although this is still not unambiguous because

waveform is also involved in the response of a log amp, which,

for a complex input (such as a CDMA signal), does not follow

the rms value exactly. Since most users specify RF signals in

terms of power (more specifically, in dBm/50 Ω), both dBV and

dBm are used in specifying the performance of the AD8314

showing equivalent dBm levels for the special case of a

50 Ω environment. Values in dBV are converted to

dBm re 50 Ω by adding 13.

FILTER CAPACITOR

The video bandwidth of both V_UP and V_DN is

approximately 3.5 MHz. In CW applications where the input

frequency is much higher than this, no further filtering of the

demodulated signal is required. Where there is a low frequency

modulation of the carrier amplitude, however, the low-pass

corner must be reduced by the addition of an external filter

()

F

C

Bandwidth

Video

+

×

×

=

pF

5

.

3

kΩ

13

π

2

1

OPERATING IN CONTROLLER MODE

Figure 33 shows the basic connections for operation in the

controller mode, and Figure 34 shows a block diagram of a

typical controller mode application. The feedback from V_UP

to VSET is broken and the desired setpoint voltage is applied to

rails high (2.2 V on a 3.3 V supply, and 1.9 V on a 2.7 V supply)

when the applied power is less than the value corresponding to

the setpoint voltage. When the input power slightly exceeds this

amplifier gain pin, decrease rapidly toward ground. In the

amplifier to reduce its output. This restores a balance between

the actual power level sensed at the input of the AD8314 and

the demanded value determined by the setpoint. This assumes

that the gain control sense of the variable gain element is

positive, that is, an increasing voltage from V_DN tends to

increase gain. The output swing and current sourcing capability

of V_DN are shown in Figure 22 and Figure 25.

0.1µF

INPUT

VSET

1

2

3

4

ENBL

RFIN

AD8314

8

7

6

5

VSET

FLTR

V_DN

VPOS

COMM

V_UP

52.3Ω

Figure 33. Basic Connections for Operation in Controller Mode

DAC

FLTR

V_UP

VSET

AD8314

DIRECTIONAL

COUPLER

POWER

AMPLIFIER

RF INPUT

GAIN

CONTROL

VOLTAGE

RFIN

V_DN

52.3Ω

Figure 34. Typical Controller Mode Application

The relationship between the input level and the setpoint

voltage follows from the nominal transfer function of the device

1 V on VSET demands a power level of 0 dBm at RFIN. The

corresponding power level at the output of the power amplifier

is greater than this amount due to the attenuation through the

directional coupler.