AD8367
60
50
60
50
40
30
20
10
0
NF
V
V
B1
B2
40
50Ω
FROM
INTEGRATOR
V
OUT
30
20
IIP3
10
0
Figure 29. A 50 Ω resistor is added to the
output to prevent package resonance.
–10
–20
–30
–10
–20
–30
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.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
V
(V)
GAIN
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
NOISE AND DISTORTION
external capacitor, CHP
:
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.
10
HP (nF) + 0.02
f
HP (kHz) =
(3)
C
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
FROM
INPUT
VOUT
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.
g
m
V
MID
HPFL
A
= 1
V
C
HP
DECL
R
HP
Figure 31. The dc output level is centered to midsupply by a control loop
whose corner frequency is determined by CHP
.
Rev. A | Page 12 of 24
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