OPA643 Burr-Brown, OPA643 Datasheet - Page 13

• Manufacturer Part Number: OPA643
• Description: Wideband Low Distortion / High Gain OPERATIONAL AMPLIFIER
• Manufacturer: Burr-Brown

6dB each, even at constant output power and frequency.
This effect is due to the reduction in loop gain which
accompanies an increase in signal gain. Finally, distortion
grows as the fundamental frequency increases, due to the
rolloff in loop gain with frequency. Going the other direction,
distortion will improve at lower frequencies until the dominant
open loop pole is reached at approximately 8kHz. Starting
with the –92dBc second-harmonic for a 1MHz, 2Vp-p
fundamental into a 500
Performance Curves), the second-harmonic distortion at
20kHz will be approximately (–92dBc – 20log (1MHz/
20kHz))
much lower.
In most applications the second-harmonic will set the limit
to dynamic range. Even order nonlinearity arises from slight
asymmetries between the positive and negative halves of the
output sinusoid. This asymmetrical nonlinearity comes from
such mechanisms as voltage dependent junction capacitances,
transistor gain mismatches and imbalanced source
impedances looking out of the amplifier power pins. Once a
circuit and board layout has been determined, these
asymmetries can often be nulled out by adjusting the DC
operating point for the signal. An example of such DC
trimming is shown in Figure 7. This circuit has a DC coupled
inverting signal path to the output pin, providing gain for a
small DC offset signal applied to the non-inverting input pin.
The output is AC coupled to block off this DC operating
point and prevent it from interacting with the following
stage.
FIGURE 7. DC Adjustment for Second-Harmonic Reduction.
For a 1Vp-p output swing in the 10 to 20MHz region, an
output DC voltage in the 1.5V range will null the second-
harmonic distortion. Tests of this technique with a 200
converter input load have shown greater than 15dB
improvement in the second-harmonic component. Once the
required DC offset voltage is found for a particular board,
circuit, and signal requirement, the voltage is very repeatable
from part to part and may be fixed permanently at the non-
inverting input. Minimal degradation in second harmonic
distortion over temperature has been observed.
V
I
+5V
–5V
5k
1k
5k
–126dBc, while the third-order terms will be
R
G
0.1µF
100
load at G = +5 (from the Typical
OPA643
+V
–V
S
S
Supply Decoupling
R
F
Not Shown
V
O
13
The OPA643 has extremely low third-order harmonic
distortion. This characteristic leads to the exceptionally high
2-tone third-order intermodulation intercept as shown in the
Typical Performance Curves. The intercept curve is defined
at the 50
resistor to allow direct comparisons to RF MMIC devices.
The matching network attenuates the voltage swing from the
output pin to the load by 6dB. If the OPA643 drives directly
into the input of a high impedance device such as an ADC,
the 6dB attenuation does not exist and the intercept will
increase by at least 6dBm. The intercept is used to predict
intermodulation spurs for two closely spaced input
frequencies. If the two test frequencies, f
in terms of average and delta frequency,
the two third-order, close-in spurious tones will appear at f
tones and the intermodulation products is given by dBc =
2 • (IM3 – P
Typical Performance Curves and P
dBm at the 50
frequencies. For instance, at 10MHz the OPA643 at a gain
of +5 has an intercept of 52dBm at the matched 50
If the full envelope of the two frequencies is 2Vp-p, then
each tone will be at 4dBm. The third-order intermodulation
spurs will then be 2 • (52 – 4) = 96dBc below the test tone
power
2Vp-p two-tone envelope were delivered directly into the
input of an ADC without the matching loss or loading of the
50 /50
58dBm. With the same signal and gain conditions, but now
driving directly into a light load, the spurious tones will be
at least 2 • (58 – 4) = 108dBc below the 4dBm test tone
power levels centered at 10MHz.
NOISE PERFORMANCE
The OPA643 complements its ultra-low harmonic distortion
with low input noise terms. The input voltage noise combines
with the two input current noise terms to give low output noise
under a wide variety of operating conditions. Figure 8 shows
the op amp noise analysis model with all noise terms included.
In this model, all voltage and current noise density terms are
expressed in nV/ Hz or pA/ Hz respectively.
FIGURE 8. Op Amp Noise Analysis Model.
E
(3 • f). The difference in power between two equal test
RS
R
S
4kTR
4kT
R
network, the intercept would increase to at least
G
level
f
S
load when driven through a 50
0
0
) where IM3 is the intercept taken from the
I
BN
load for one of the two closely spaced test
(f
1
E
NI
+ f
(–92dBm).
2
)/2 and f
R
G
OPA643
OPA643
I
BI
R
0
F
|f
is the power level in
If
1
2
4kT = 1.6E –20J
and f
– f
at 290°K
4kTR
1
| /2
2
this
, are specified
F
matching
same
load.
E
O
0
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