11

®

OPA4658

–50

–60

–70

–80

–90

–100

Load Resistance (

Ω)

10

100

1k

2f

O

3f

O

G = +2

The third-order intercept is an important parameter for many

RF amplifier applications. Figure 6 shows the OPA4658’s

two tone, third-order intercept vs frequency. This curve is

particularly useful for determining the magnitude of the

third harmonic as a function of frequency, load resistance,

and gain. For example, assume that the application requires

the OPA4658 to operate in a gain of +2V/V and drive

2Vp-p into 100

Ω at a frequency of 10MHz. Referring to

Figure 6 we find that the intercept point is +38dBm. The

magnitude of the third harmonic can now be easily calcu-

lated from the expression:

P

Harmonic = 2(38 – 7) = 62dB below the fundamental. The

OPA4658’s low distortion makes the device an excellent

choice for a variety of RF signal processing applications.

CROSSTALK

Crosstalk is the undesired result of the signal of one channel

mixing with and reproducing itself in the output of another

channel or channels. Crosstalk is inclined to occur in most

multichannel integrated circuits. In quad devices, the effect of

crosstalk is measured by driving three channels 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, therefore at increased gain, crosstalk also in-

creases by a factor equal to that of the gain. Figure 7 illustrates

the measured effect of crosstalk in the OPA4658U.

FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance.

FIGURE 6. Third Order Intercept Point vs Frequency.

70

60

50

40

30

20

100k

1M

10M

100M

Frequency (Hz)

G = +2

(G = +2, R

FIGURE 8. Configuration for Testing Differential Gain/Phase.

OPA4658

75

Ω

75

Ω

402

Ω

402

Ω

75

Ω

75

Ω

TEK TSG 130A

TEK VM700A

1/4

DIFFERENTIAL GAIN AND PHASE

Differential Gain (DG) and Differential Phase (DP) are criti-

cal 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 subcarrier of 4.43MHz.

All NTSC measurements were performed using a Tektronix

model VM700A Video Measurement Set.

DG and DP of the OPA4658 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

video analyzer. The signal averaging feature of the analyzer

FIGURE 7. Channel-to-Channel Isolation (three active channels).

Frequency (Hz)

10

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

1M

10M

100M

G = +2