AN-1026
APPLICATION NOTE
The HD2 at 50 MHz is approximately −88 dBc, relative to a
2 V p-p input signal. To compare the harmonic distortion level
to 1 ENOB LSB, this level must be converted to a voltage as shown
in Equation 33.
HARMONIC DISTORTION
Low harmonic distortion in the frequency domain is important
in both narrow-band and broadband systems. Nonlinearities in
the drivers generate single-tone harmonic distortion and multitone,
intermodulation distortion products at amplifier outputs.
−88
20
⎛
⎞
⎜
⎟
⎟
HD2 =
(
2 V p - p
)
10
≈ 80 ꢁV p - p
(33)
⎜
The same approach used in the noise analysis example can be
applied to distortion analysis, comparing the harmonic distortion
of the ADA4939 with 1 LSB of the AD9445’s ENOB of 12 bits
with a 2 V full-scale output. One ENOB LSB is 488 μV in the
noise analysis.
⎝
⎠
This distortion product is only 80 μV p-p, or 16% of 1 ENOB
LSB. Thus, from a distortion standpoint, the ADA4939 is a
good choice to consider as a driver for the AD9445 ADC.
Because ADC drivers are negative feedback amplifiers, output
distortion depends on the amount of loop gain in the amplifier
circuit. The inherent open-loop distortion of a negative feedback
amplifier is reduced by a factor of 1/(1 + LG), where LG is the
available loop gain.
The distortion data in the specifications table of the ADA4939
is given for a gain of 2, comparing second and third harmonics
at various frequencies. Table 3 shows the harmonic distortion
data for a gain of 2 and differential output swing of 2 V p-p.
Table 3. Second and Third Harmonic Distortion of the ADA4939
The input (error voltage) of the amplifier is multiplied by a large
forward voltage gain, A(s), then passes through the feedback
factor, β, to the input, where it adjusts the output to minimize the
error. Therefore, the loop gain of this type of amplifier is A(s) × β;
as the loop gain (A(s), β, or both) decreases, harmonic distortion
increases. Voltage feedback amplifiers, such as integrators, are
designed to have large A(s) at dc and low frequencies, and then
roll off as 1/f toward unity at a specified high frequency. As A(s)
rolls off, loop gain decreases and distortion increases. Therefore,
the harmonic distortion characteristic is the inverse of A(s).
Parameter
Harmonic Distortion (dBc)
HD2 @ 10 MHz
HD2 @ 70 MHz
HD2 @ 100 MHz
HD3 @ 10 MHz
HD3 @ 70 MHz
HD3 @ 100 MHz
−102
−83
−77
−101
−97
−91
The data shows that harmonic distortion increases with frequency
and that HD2 is worse than HD3 in the bandwidth of interest
(50 MHz). Harmonic distortion products are higher in frequency
than the frequency of interest, so their amplitude can be reduced
by system band-limiting. If the system had a brick-wall filter at
50 MHz, then only the frequencies higher than 25 MHz are of
concern because all harmonics of higher frequencies are eliminated
by the filter. Nevertheless, the system was evaluated up to 50 MHz
because any filtering that is present may not sufficiently suppress
the harmonics, and distortion products can alias back into the
signal bandwidth. Figure 16 shows the harmonic distortion vs.
frequency of the ADA4939 for various supply voltages with a
2 V p-p output.
Current feedback amplifiers use an error current as the feedback
signal. The error current is multiplied by a large forward
transresistance, T(s), which converts it to the output voltage,
then passes through the feedback factor, 1/RF, which converts
the output voltage to a feedback current that tends to minimize
the input error current. The loop gain of an ideal current feedback
amplifier is therefore T(s) × (1/RF) = T(s)/RF. Like A(s), T(s) has
a large dc value and rolls off with increasing frequency, reducing
loop gain and increasing the harmonic distortion.
Loop gain also depends directly upon the feedback factor, 1/RF.
The loop gain of an ideal current feedback amplifier does not
depend on a closed-loop voltage gain; therefore, harmonic
distortion performance does not degrade as the closed-loop
gain increases. In a real current feedback amplifier, loop gain
does have some dependence on the closed-loop gain but not
nearly to the extent that it does in a voltage feedback amplifier.
This makes a current feedback amplifier, such as the ADA4927,
a better choice than a voltage feedback amplifier for applications
requiring high closed-loop gain and low distortion.
–60
V
= 2V p-p
OUT, dm
–65
–70
HD2, V (SPLIT SUPPLY) = ±2.5V
S
HD3, V (SPLIT SUPPLY) = ±2.5V
S
HD2, V (SPLIT SUPPLY) = ±1.65V
S
HD3, V (SPLIT SUPPLY) = ±1.65V
S
–75
–80
–85
HD2 ≈ –88dBc @ 50MHz
–90
–95
–100
–105
–110
1
10
100
FREQUENCY (MHz)
Figure 16. Harmonic Distortion vs. Frequency
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