OPA692 datasheet(22/31 Pages) BURR-BROWN

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DISTORTION PERFORMANCE

OPA3693

SBOS353 – DECEMBER 2006

OPA3693 can be very susceptible to decreased

configuration it is just RF (see Figure 44). Also,

stability and may give closed-loop response peaking

providing an additional supply decoupling capacitor

when a capacitive load is placed directly on the

(0.01

µF) between the supply pins (for bipolar

output pin. When the amplifier open-loop output

operation) improves the 2nd-order distortion slightly

resistance

considered,

this

capacitive

load

(3dB to 6dB).

introduces an additional pole in the signal path that

The OPA3693 has an extremely low 3rd-order

can decrease the phase margin. Several external

harmonic distortion. This feature also produces a

solutions to this problem have been suggested.

high two-tone, 3rd-order intermodulation intercept.

When the primary considerations are frequency

Two graphs for this intercept are given in the in the

response flatness, pulse response fidelity, and/or

Typical Characteristics; one for

±5V and one for +5V.

distortion, the simplest and most effective solution is

The lower curve shown in each graph is defined at

to isolate the capacitive load from the feedback loop

the 50

Ω load when driven through a 50Ω matching

by inserting a series isolation resistor between the

resistor, to allow direct comparisons to RF MMIC

amplifier output and the capacitive load. This resistor

devices. The higher curve in each graph shows the

does not eliminate the pole from the loop response,

intercept if the output is taken directly at the output

but rather shifts it and adds a zero at a higher

pin with a 500

Ω load, to allow prediction of the

frequency. The additional zero acts to cancel the

3rd-order spurious level when driving a lighter load,

phase lag from the capacitive load pole, thus

such as an ADC input. The output matching resistor

increasing the phase margin and improving stability.

attenuates the voltage swing from the output pin to

The Typical Characteristics show a Recommended

the load by 6dB. If the OPA3693 drives directly into

RS vs Capacitive Load curve (Figure 15) to help the

the input of a high-impedance device, such as an

designer pick a value to give < 0.5dB peaking to the

ADC, this 6dB attenuation is not taken and the

load. The resulting frequency response curves show

intercept increases, as shown in the 500

Ω load

a 0.5dB peaked response for several selected

typical characteristic.

capacitive

loads

and

recommended

The intercept is used to predict the intermodulation

combinations. Parasitic capacitive loads greater than

spurious levels for two closely-spaced frequencies. If

2pF can begin to degrade the performance of the

the two test frequencies (f1 and f2) are specified in

OPA3693. Long PCB traces, unmatched cables, and

terms of average and delta frequency, fO = (f1 + f2)/2

connections to other amplifier inputs can easily

and

∆f = |f2 – f1|/2, then the two, 3rd-order, close-in

exceed this value. Always consider this effect

spurious tones appear at fO ±3 × ∆f. The difference

carefully, and add the recommended series resistor

between two equal test tone power levels and these

as close as possible to the OPA3693 output pin (see

intermodulation spurious power levels is given by

the Board Layout Guidelines section).

∆dBc = 2 × (IM3 – P

O), where IM3 is the intercept

The criterion for setting this RS resistor is a maximum

taken from the Typical Characteristics and PO is the

bandwidth, flat frequency response at the load

power level in dBm at the 50

Ω load for one of the

(< 0.5dB peaking). For the OPA3693 operating at a

two closely-spaced test frequencies. For instance, at

gain of +2V/V, the frequency response at the output

50MHz, the OPA3693 at a gain of +2 has an

pin is very flat to begin with, allowing relatively small

intercept of 47dBm at a matched 50

Ω load. If the full

values of RS to be used for low capacitive loads.

envelope of the two frequencies needs to be 2VPP at

this load, this requires each tone to be 4dBm (1VPP).

The 3rd-order intermodulation spurious tones will

then be 2

× (47 – 4) = 83dBc below the test tone

The OPA3693 provides good distortion performance

power level (–79dBm). If this same 2VPP two-tone

into a 100

Ω load on ±5V supplies. Relative to

envelope were delivered directly into a lighter 500

Ω

alternative solutions, the OPA3693 holds much lower

load, the intercept would increase to the 48dBm

distortion at higher frequencies (> 20MHz) than

shown in the Typical Characteristics. With the same

alternative solutions. Generally, until the fundamental

output signal and gain conditions, but now driving

signal reaches very high-frequency or power levels,

directly into a light load with no matching loss, the

the 2nd-harmonic dominates the distortion with a

3rd-order spurious tones will then be at least 2

× (48

negligible 3rd-harmonic component. Focusing then

– 4) = 92dBc below the 4dBm test tone power levels

on the 2nd-harmonic, increasing the load impedance

centered on 50MHz (–88dBm). We are still using a

improves distortion directly. Remember that the total

4dBm for the 1VPP output swing into this 500Ω load.

load

includes

the

feedback

network—in

the

While not strictly correct from a power standpoint,

noninverting configuration (see Figure 42), this value

this does give the correct prediction for spurious

is the sum of RF + RG, while in the inverting

level. The class AB output stage for the OPA3693 is

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