OPA658 datasheet(11/18 Pages) TI1

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OPA658

SBOS045A

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appear at fO ± 3 × Df. The 2-tone, 3rd-order spurious plot

shown in Figure 7 indicates how far below these two equal

power, closely-spaced tones the intermodulation spurious

will be. The single-tone power is at a matched 50

Ω load. The

unique design of the OPA658 provides much greater spuri-

ous free range than what a 2-tone, 3rd-order intermodulation

intercept specification would predict. This can be seen in

Figure 7 as the spurious-free range actually increases at the

higher output power levels.

COMPENSATION

The OPA658 is internally compensated and is stable in unity

gain with a phase margin of approximately 62

°, and approxi-

mately 64

° in a gain of +2V/V when used with the recom-

mended feedback resistor value. Frequency response for

other gains are shown in the Typical Characteristics.

The high-frequency response of the OPA658 in a good

layout is very flat with frequency.

DISTORTION

The OPA658’s Harmonic Distortion characteristics into a

100

Ω load are shown versus frequency and power output in

the Typical Characteristics. Distortion can be further im-

proved by increasing the load resistance as illustrated in

Figure 6. Remember to include the contribution of the feed-

back resistance when calculating the effective load resis-

tance seen by the amplifier.

Narrowband communication channel requirements will ben-

efit from the OPA658’s wide bandwidth and low

intermodulation distortion on low quiescent power. If output

signal power at two closely spaced frequencies is required,

3rd-order nonlinearities in any amplifier will cause spurious

power at frequencies very near the two fundamental frequen-

cies. If the two test frequencies, f1 and f2, are specified in

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

Df =

Ωf

2 – f1Ω, the two, 3rd-order, close-in spurious tones will

DIFFERENTIAL GAIN AND PHASE

Differential Gain (dG) and Differential Phase (dP) are among

the more important specifications for video applications. dG

is defined as the percent change in closed-loop gain over a

specified change in output voltage level. dP is defined as the

change in degrees of the closed-loop phase over the same

output voltage change. Both dG and dP are specified at the

NTSC sub-carrier frequency of 3.58MHz and the PAL sub-

carrier of 4.43MHz. All NTSC measurements were performed

using a Tektronix model VM700A Video Measurement Set.

dG/dP of the OPA658 were measured with the amplifier in a

gain of +2V/V with 75

Ω input impedance and the output

back-terminated in 75

Ω. The input signal selected from the

generator was a 0V to 1.4V modulated ramp with sync pulse.

With these conditions the test circuit shown in Figure 8

delivered a 100IRE modulated ramp to the 75

Ω input of the

videoanalyzer. The signal averaging feature of the analyzer

was used to establish a reference against which the perfor-

FIGURE 5. Driving Capacitive Loads.

FIGURE 6. 5MHz Harmonic Distortion vs Load Resistance.

FIGURE 7. 3rd-Order Spurious Level vs Frequency.

FIGURE 8. Configuration for Testing Differential Gain/Phase.

OPA658

Ω

ISO

Ω to 35Ω

402

Ω

402

Ω

–55

–60

–65

–70

–75

–80

–85

5MHz HARMONIC DISTORTION vs

LOAD RESISTANCE (G = +2)

Load Resistance (

Ω)

100

G = +2

O = 2VPP

O = 5MHz

–65

–70

–75

–80

–85

–90

–18 –16 –14 –12 –10

–8

–6

–4

–2

2-TONE, 3RD-ORDER SPURIOUS LEVELS

Single-Tone Power (dBm)

20MHz

10MHz

5MHz

OPA658

Ω

402

Ω

402

Ω

TEK TSG 130A

TEK VM700A

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