OPA643PB datasheet(13/18 Pages) TI1

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OPA643

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

Ω 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.

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

Ω 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,

0 ≡ (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 50

Ω 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 50

Ω 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).

this

same

2Vp-p two-tone envelope were delivered directly into the

input of an ADC without the matching loss or loading of the

Ω/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.

Supply Decoupling

Not Shown

0.1µF

OPA643

–V

+5V

–5V

100

Ω

FIGURE 8. Op Amp Noise Analysis Model.

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

Ω

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.

FIGURE 7. DC Adjustment for Second-Harmonic Reduction.

4kT

OPA643

4kT = 1.6E –20J

at 290°K

4kTR

√

4kTR

√

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