AD8367 datasheet(12/24 Pages) AD

12 / 24 page

AD8367

Rev. A | Page 12 of 24

VOUT

VB1

VB2

FROM

INTEGRATOR

Figure 29. A 50 Ω resistor is added to the

output to prevent package resonance.

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

impedance 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

predicted. The input-referred noise increases in proportion to

the attenuation level. Figure 30 shows noise figure, NF, as a

function of VGAIN for the MODE pin pulled high. The minimum

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.

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

noise. Figure 30 illustrates how the third-order intercept point

at the input, IIP3, behaves as a function of VGAIN. The highest

IIP3 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 can occur, causing premature distortion.

–30

–20

–10

–30

–20

–10

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

VGAIN (V)

IIP3

Figure 30. Noise Figure and Input Third-Order Intercept vs.

Gain (RSOURCE = 200 Ω)

OUTPUT CENTERING

To maximize the ac swing at the output of the AD8367, the

output level is centered midway between ground and the supply.

This is achieved when the DECL pin is bypassed to ground via a

shunt capacitor. The loop acts to suppress deviations from the

reference at outputs below its corner frequency while not affect-

ing signals above it, as shown in Figure 31. The maximum

corner frequency with no external capacitor is 500 kHz. The

corner frequency can be lowered arbitrarily by adding an

external capacitor, CHP:

0.02

(nF)

(kHz)

(3)

A 100 Ω in series with the CHP capacitor is recommended to

de-Q the resonant tank that is formed by the bond-wire

inductance and CHP. Failure to insert this capacitor can

potentially cause oscillations at higher frequencies at high

gain settings.

MAIN

AMPLIFIER

VOUT

FROM

INPUT

HPFL

CHP

RHP

VMID

DECL

AV = 1

Figure 31. The dc output level is centered to midsupply by a control loop

whose corner frequency is determined by CHP.

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