OPA643 Datasheet, PDF (13/17 Pages) Burr-Brown (TI) – Wideband Low Distortion, High Gain OPERATIONAL AMPLIFIER

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 500Ω load at G = +5 (from the Typical
Performance Curves), the second-harmonic distortion at
20kHz will be approximately (–92dBc – 20log (1MHz/
20kHz)) ≅ –126dBc, while the third-order terms will be
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.
+5V
5kΩ
+V_S
1kΩ
100Ω
Supply Decoupling
Not Shown
0.1µF
OPA643
V_O
5kΩ
–5V
R_G
V_I
R_F
–V_S
FIGURE 7. DC Adjustment for Second-Harmonic Reduction.
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 50Ω load when driven through a 50Ω matching
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, f₁ and f₂, are specified
in terms of average and delta frequency,
f₀ ≡ (f₁ + f₂)/2 and ∆f ≡ |f₂ – f₁| /2
the two third-order, close-in spurious tones will appear at f₀
± (3 • ∆f). The difference in power between two equal test
tones and the intermodulation products is given by ∆dBc =
2 • (IM3 – P₀) where IM3 is the intercept taken from the
Typical Performance Curves and P₀ is the power level in
dBm at the 50Ω load for one of the two closely spaced test
frequencies. For instance, at 10MHz the OPA643 at a gain
of +5 has an intercept of 52dBm at the matched 50Ω load.
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 level (–92dBm). If this same
2Vp-p two-tone envelope were delivered directly into the
input of an ADC without the matching loss or loading of the
50Ω/50Ω network, the intercept would increase to at least
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.
E_NI
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 200Ω
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.
OPA643
E_O
R_S
I_BN
E_RS
√4kTR_S
4kT
R_G
R_F
√4kTR_F
R_G
I_BI
4kT = 1.6E –20J
at 290°K
FIGURE 8. Op Amp Noise Analysis Model.
®
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OPA643