REV. 0

AD8367

–10–

Power and Voltage Metrics

Although power is the traditional metric used in the analysis of

cascaded systems, most active circuit blocks fundamentally

respond to voltage. The relationship between power and voltage

is defined by the impedance level. When input and output imped-

ance levels are the same, power gain and voltage gain are identical.

However, when impedance levels change between input and

output, they differ. Thus, one must be very careful to use the

appropriate gain for system chain analyses. Quantities such as

OIP3 are quoted in dBV rms as well as dBm referenced to 200

Ω.

The dBV rms unit is defined as decibels relative to 1 V rms. In a

200

Ω environment, the conversion from dBV rms to dBm requires

the addition of 7 dB to the dBV rms value. For example, a

+2 dBV rms level corresponds to +9 dBm.

Noise and Distortion

Since the AD8367 consists of a passive variable attenuator

followed by a fixed gain amplifier, the noise and distortion

characteristics as a function of the gain voltage are easily pre-

dicted. The input-referred noise increases in proportion to the

attenuation level. Figure 4 shows noise figure, NF, as a func-

NF of 7.5 dB occurs at maximum gain and increases 1 dB for

every 1 dB reduction in gain. In receiver applications, the

minimum NF should occur at the maximum gain where the

received signal presumably is weak. At higher levels, a lower

gain is needed, and the increased NF becomes less important.

0

1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

60

50

–30

10

0

–10

–20

30

20

40

60

50

–30

10

0

–10

–20

30

20

40

NF

IIP3

Figure 4. Noise Figure and Input Third Order Inter-

200

Ω)

The input-referred distortion varies in a similar manner to the

noise. Figure 4 illustrates how the third-order intercept point at

of 20 dBV rms (27 dBm re 200

Ω) occurs at minimum gain. The

IIP3 then decreases 1 dB for every 1 dB increase in gain. At lower

levels, a degraded IIP3 is acceptable. Overall, the dynamic range,

represented by the difference between IIP3 and NF, remains

reasonably constant as a function of gain. The output distortion

and compression are essentially independent of the gain. At low

gains, when the input level is high, input overload may occur,

causing premature distortion.

Output Centering

The output level is centered midway between ground and the

supply if the DECL pin is left floating. Alternatively, the out-

put level may be set by driving the DECL pin with the desired

reference level. As shown in Figure 5, the loop acts to suppress

deviations from the reference at outputs below its corner

frequency while not affecting signals above it. The maximum

corner frequency with no external capacitor is 500 kHz. The

corner frequency can be lowered arbitrarily by adding an exter-

f

C

HP

HP

(kHz)

(nF)+ .

=

10

002

(3)

A capacitor at pin DECL is recommended to decouple the

reference level to which the output is centered.

HPFL

DECL

1

VOUT

FROM

INPUT

MAIN

AMPLIFIER

Figure 5. The dc output level is centered to mid

supply by a control loop whose corner frequency is

RMS Detection

The AD8367 contains a square-law detector that senses the output

signal and compares it to a calibrated set-point of 354 mV rms

which corresponds to a 1 V p-p sine wave. Any difference between

the output and set-point generates a current which is integrated by

ground, to provide an AGC control voltage. There is also an

internal 5 pF capacitor on the DETO pin.

The resulting voltage is used as an AGC bias. For this appli-

cation, the MODE pin is pulled low and the DETO pin is

tied to the GAIN pin. The output signal level is then regu-

lated to 354 mV rms. The AGC bias represents a calibrated

rms measure of the received signal strength (RSSI). Since in

the AGC mode the output signal is forced to the 354 mV rms

set-point (–9.02 dBV rms), Equation 2 can be recast to

VV

IN RMS

DETO

−

=−

+

×

(dBV rms)

54 02

50

.

(4)

where –54.02 dBV rms

–45 dB

9.02 dBV rms.

characteristic single-pole time constant,

τµ

AGC

AGC

C

( s)

(nF)

=×

10

(5)

where the internal 5 pF capacitor has been lumped with the