OPA621 datasheet(14/17 Pages) TI1

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OPA621

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 as shown in the Typical Performance

Curves. All measurements were performed using a Tektronix

model VM700 Video Measurement Set.

DISTORTION

The OPA621’s Harmonic Distortion characteristics into a

Ω load are shown vs frequency and power output in the

Typical Performance Curves. Distortion can be further

improved by increasing the load resistance as illustrated in

Figure 8. Remember to include the contribution of the

feedback resistance when calculating the effective load

resistance seen by the amplifier.

FIGURE 9. Two-Tone Third-Order Intermodulation Inter-

cept vs Frequency.

FIGURE 8. 10MHz Harmonic Distortion vs Load Resistance.

Two-tone, third-order intermodulation distortion (IM) is an

important parameter for many RF amplifier applications.

Figure 9 shows the OPA621’s two-tone, third-order IM

intercept vs frequency. For these measurements, tones were

spaced 1MHz apart. This curve is particularly useful for

determining the magnitude of the third-order IM products as

a function of frequency, load resistance, and gain. For

example, assume that the application requires the OPA621

to operate in a gain of +2V/V and drive 2Vp-p (4dBm for

each tone) into 50

Ω at a frequency of 10MHz. Referring to

Figure 9 we find that the intercept point is +47dBm. The

magnitude of the third-order IM products can now be easily

calculated from the expression:

Third IMD = 2(OPI3P – PO)

where OPI3P = third-order output intercept, dBm

O = output level/tone, dBm/tone

Third IMD = third-order intermodulation ratio

below each output tone, dB

For this case OPI3P = 47dBm, P

O = 4dBm, and the third-

order IMD = 2(47 – 4) = 86dB below either 4dBm tone. The

OPA621’s low IMD makes the device an excellent choice

for a variety of RF signal processing applications.

NOISE FIGURE

The OPA621’s voltage and current noise spectral densities

are specified in the Typical Performance Curves. For RF

applications, however, Noise Figure (NF) is often the

preferred noise specification since it allows system noise

performance to be more easily calculated. The OPA621’s

Noise Figure vs Source Resistance is shown in Figure 10.

SPICE MODELS

Computer simulation using SPICE is often useful when

analyzing the performance of analog circuits and systems.

This is particularly true for Video and RF amplifier circuits

where parasitic capacitance and inductance can have a major

effect on circuit performance. A SPICE model using

MicroSim Corporation’s PSpice is available for the OPA621.

This simulation model is available through the Burr-Brown

web site at www.burr-brown.com or by calling the Burr-

Brown Applications Department.

NOISE FIGURE vs SOURCE RESISTANCE

100

10k

100k

Source Resistance ( )

Ω

NFdB = 10log 1 +

en2 + (inRS)2

4kTRS

FIGURE 10. Noise Figure vs Source Resistance.

V = 2Vp-p

10MHz HARMONIC DISTORTION

vs LOAD RESISTANCE

–40

–50

–60

–70

–80

–90

100

200

300

400

500

Load Resistance ( )

Ω

G = +2V/V

G = +5V/V

OUT

250

Ω

250

Ω

–

G = +2V/V

10203040

5060708090

100

Frequency (MHz)

R = 50

Ω

R = 100

Ω

R = 400

Ω

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