11

®

OPA2658

CAPACITIVE LOADS

The OPA2658’s output stage has been optimized to drive

low resistive loads. Capacitive loads, however, will decrease

the amplifier’s phase margin which may cause high fre-

quency peaking or oscillations. Capacitive loads greater than

5pF should be buffered by connecting a small resistance,

usually 10

Ω to 35Ω, in series with the output as shown in

Figure 5. This is particularly important when driving high

capacitance loads such as flash A/D converters.

In general, capacitive loads should be minimized for opti-

mum high frequency performance. Coax lines can be driven

if the cable is properly terminated. The capacitance of coax

cable (29pF/foot for RG-58) will not load the amplifier

when the coaxial cable or transmission line is terminated

with its characteristic impedance.

COMPENSATION

The OPA2658 is internally compensated and is stable in

unity gain with a phase margin of approximately 62

°, and

approximately 64

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

recommended feedback resistor value. Frequency response

for other gains are shown in the Typical Performance Curves.

The high-frequency response of the OPA2658 in a good

layout is very flat with frequency.

DISTORTION

The OPA2658’s Harmonic Distortion characteristics into a

100

Ω load are shown versus frequency and power output in

the Typical Performance Curves. Distortion can be further

improved by increasing the load resistance as illustrated in

Figure 6. Remember to include the contribution of the

feedback resistance when calculating the effective load re-

sistance seen by the amplifier.

Narrowband communication channel requirements will ben-

efit from the OPA2658’s wide bandwidth and low

intermodulation distortion on low quiescent power. If output

signal power at two closely spaced frequencies is required,

third-order nonlinearities in any amplifier will cause spuri-

ous power at frequencies very near the two funda-

are specified in terms of average and delta frequency,

f

FIGURE 5. Driving Capacitive Loads.

C

L

R

L

50

Ω

R

S

10

Ω to 35Ω

402

Ω

402

Ω

1/2

OPA2658

tone, third-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 OPA2658

provides much greater spurious free range than what a two-

tone third-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.

–55

–60

–65

–70

–75

–80

–85

5MHz HARMONIC DISTORTION vs

LOAD RESISTANCE (G = +2)

Load Resistance (

Ω)

10

100

1k

G = +2, V

3f

O

2f

O

FIGURE 6. 5MHz Harmonic Distortion vs Load Resistance.

–65

–70

–75

–80

–85

–90

–18 –16 –14 –12 –10

–8

–6

–4

–2

0

2

4

TWO TONE, THIRD-ORDER SPURIOUS LEVELS

Single Tone Power (dBm)

20MHz

10MHz

5MHz

CROSSTALK

Crosstalk is the undesired result of the signal of one channel

mixing with and reproducing itself in the output of the other

channel. Crosstalk occurs in most multichannel integrated

circuits. In dual devices, the effect of crosstalk is measured by

driving one channel and observing the output of the undriven

channel over various frequencies. The magnitude of this effect

is referenced in terms of channel- to-channel isolation and

expressed in decibels. "Input referred" points to the fact that

there is a direct correlation between gain and crosstalk, there-

fore at increased gain, crosstalk also increases by a factor

equal to that of the gain. Figure 8 illustrates the measured

effect of crosstalk in the OPA2658U.

FIGURE 7. Third-Order Intercept Point vs Frequency.