OPA698 datasheet(18/27 Pages) TI

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OPA698

SBOS258B

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If the negative excursion of the rectified signal is not desired,

it can easily be removed by replacing the OPA693 with the

OPA698 configured as a difference amplifier with VL con-

nected to ground and VH left floating.

SOFT-CLIPPING (Compression) CIRCUIT

Figure 13 shows a soft-clipping circuit. As soon as the input

voltage exceeds either VCH or VCL, the limiting voltages are

driven by the following equations:

×+

As the amplifier is operating in the limiting mode, the output

voltage is compressed with a gain of R1+R2/R1 for the

positive excursion above VCH, and by a gain of R3+R4/R3 for

the negative excursion below VCL. Figure 14 shows a 5VPP

on the input being compressed above

±1V with a compres-

sion gain of one-third.

Time (10ns/div)

OUT

0.8

0.6

0.4

0.2

–0.2

–0.4

–0.6

–0.8

FIGURE 12. 10MHz Sinewave Rectified.

(1)

(2)

High-Speed Full Wave rectifier #1

The circuit shown in Figure 9 uses only one amplifier, in an

inverting gain of –1 configuration. The upper limiting voltage

is left open, resulting in an upper limiting voltage of +3.5V.

The lower limiting voltage is connected to the input signal,

resulting in the following behavior. When the input voltage is

negative, the amplifier is not limiting, resulting in the inversion

of the input sinewave to the output. During the positive

excursion of the input signal, the output signal is being driven

by the limiting input pin. Since the output is driven from the

limiter input pin from positive inputs, the lower slew rate in the

input path restricts the application of this approach to lower

amplitude and/or frequencies. A 2MHz fully rectified sinewave

is shown in Figure 10.

402

Ω

402

Ω

500

Ω

200

Ω

Source

57.2

Ω

OPA698

Time (50ns/div)

0.6

0.4

0.2

–0.2

–0.4

–0.6

FIGURE 9. High-Speed Full Wave Rectifier #1.

FIGURE 10. 2MHz Sinewave Rectified.

FIGURE 11. High-Speed Full Wave Rectifier #2.

In order to reach higher frequencies, a second method is

recommended.

High Speed Full Wave rectifier #2

The circuit shown in Figure 11 combines a half-wave rectifier

driving the OPA693 in an inverting configuration, while the

input signal drives the noninverting input of the fixed gain

amplifier OPA693, resulting in a full wave rectifier function.

Results are shown in Figure 12.

OPA698

200

Ω

300

Ω

200

Ω

OPA693

300

Ω

700MHz

Internal

Gain Set

Ω Load

Ω

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