10

OPA641

®

The third-order intercept point is an important parameter for

many RF amplifier applications. Figure 6 shows the

OPA641’s single-tone third-order intercept versus frequency.

This curve is particularly useful for determining the magni-

tude of the third harmonic as a function of frequency, load

resistance, and gain. For example, assume that the applica-

tion requires the OPA641 to operate in a gain of +2V/V and

drive 2Vp-p into 100

Ω at a frequency of 5MHz. 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:

harmonic = 2(38 – 7) = 62dB below the fundamental tone.

The OPA641’s low IMD makes the device an excellent

choice for a variety of RF signal processing applications.

The value for the two-tone third-order intercept is typically

6dB lower than the single-tone value.

Settling time, specified in an inverting gain of one, occurs in

only 18ns to 0.01% for a 2V step, making the OPA641 one

of the fastest settling monolithic amplifiers commercially

available. Settling time increases with closed-loop gain and

output voltage change as described in the Typical Perform-

ance Curves. Preserving settling time requires critical atten-

tion to the details as mentioned under “Wiring Precautions.”

The amplifier also recovers quickly from input overloads.

Overload recovery time to linear operation from a 50%

overload is typically only 30ns.

In practice, settling time measurements on the OPA641

prove to be very difficult to perform. Accurate measurement

is next to impossible in all but the very best equipped labs.

Among other things, a fast flat-top generator and high speed

oscilloscope are needed. Unfortunately, fast flat-top genera-

tors, which settle to 0.01% in sufficient time, are scarce and

expensive. Fast oscilloscopes, however, are more commonly

available. For best results, a sampling oscilloscope is recom-

mended. Sampling scopes typically have bandwidths that

are greater than 1GHz and very low capacitance inputs.

They also exhibit faster settling times in response to signals

that would tend to overload a real-time oscilloscope.

Figure 6 shows the test circuit used to measure settling time

for the OPA641. This approach uses a 16-bit sampling

oscilloscope to monitor the input and output pulses. These

waveforms are captured by the sampling scope, averaged,

and then subtracted from each other in software to produce

the error signal. This technique eliminates the need for the

traditional “false-summing junction,” which adds extra para-

sitic capacitance. Note that instead of an additional flat-top

generator, this technique uses the scope’s built-in calibration

source as the input signal.

DIFFERENTIAL GAIN AND PHASE

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

among the more important specifications for video applica-

tions. 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.

DG and DP increase with closed-loop gain and output

voltage transition. All measurements were performed using

a Tektronix model VM700 Video Measurement Set.

DISTORTION AND NOISE

The OPA641’s harmonic distortion characteristics vs fre-

quency and power output in the Typical Performance Curves.

Distortion can be further improved by increasing the load

resistance (refer to Figure 5). Remember to include the

contribution of the feedback resistance when calculating the

effective load resistance seen by the amplifier.

Although harmonic distortion may decrease with higher

load resistances (i.e., higher feedback resistors), the effec-

tive output noise will increase due to the higher resistance.

Therefore, noise or harmonic distortion may be optimized

by picking the appropriate feedback resistor.

FIGURE 5. 5MHz Harmonic Distortion vs Load Resistance.

FIGURE 6. Single-Tone Third-Order Intercept Point vs Fre-

quency.

–70

–80

–90

–100

Load Resistance (

Ω)

2f

O

3f

O

10

100

1k

10k

G = +2, V

60

50

40

30

20

10

Frequency (Hz)

10M

100M

1M

G = +2V/V