Datasheet

OPA698
18
SBOS258D
www.ti.com
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 V
L
con-
nected to ground and V
H
left floating.
SOFT-CLIPPING (Compression) CIRCUIT
Figure 13 shows a soft-clipping circuit. As soon as the input
voltage exceeds either V
CH
or V
CL
, the limiting voltages are
driven by the following equations:
VV
RV RV
RR
V
RV RV
RR
HH
CH
IN
L
CL
IN
==
×+×
+
=
×+×
+
21
12
43
34
As the amplifier is operating in the limiting mode, the output
voltage is compressed with a gain of R
1
+R
2
/R
1
for the
positive excursion above V
CH
, and by a gain of R
3
+R
4
/R
3
for
the negative excursion below V
CL
. Figure 14 shows a 5V
PP
on the input being compressed above ±1V with a compres-
sion gain of one-third.
Time (10ns/div)
V
IN
V
OUT
Input and Output Voltage (V)
0.8
0.6
0.4
0.2
0
–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.
V
H
V
L
V
O
402Ω402Ω
500Ω
200Ω
50Ω
Source
57.2Ω
OPA698
Time (50ns/div)
Output Voltage (V)
0.6
0.4
0.2
0
–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
200Ω
V
H
V
L
75Ω
300Ω
200Ω
OPA693
300Ω
50Ω
700MHz
Internal
Gain Set
50Ω Load
75Ω