Datasheet

OPA698
19
SBOS258D
www.ti.com
VERY HIGH-SPEED SCHMITT TRIGGER
Figure 15 shows a very high-speed Schmitt Trigger. The
output levels are precisely defined, and the switching time is
exceptional. The output voltage swings between V
H
and V
L
.
The circuit operates as follow. When the input voltage is less
than V
HL
then the output is limiting at V
H
. When the input is
greater than V
HH
then the output is limiting at V
L
, with V
HL
and
V
HH
defined as the following:
V
RR R
R
V
RR R
R
V
HL HH REF
OUT
,
|| || || ||
=×+×
12 3
1
12 3
2
Due to the inverting function realized by the Schmitt Trigger,
V
HL
corresponds to V
OUT
= V
H
, and V
HH
corresponds to
V
OUT
= V
L
.
Time (100ns/div)
Input and Output Voltage (V)
3
2
1
0
–1
–2
–3
V
IN
V
OUT
OPA698
R
2
402Ω
R
1
200Ω
R
3
200Ω
V
REF
V
OUT
V
H
V
L
+2V
–2V
V
IN
FIGURE 13. Soft-Clipping Circuit.
FIGURE 14. Soft Clipping with a Gain of 1/3 above the clamp
level (±1V).
Figure 16 shows the Schmitt Trigger operating with V
REF
=
+5V. This gives us V
HH
= 2.4V and V
HL
= 1.6V. The propa-
gation delay for the OPA698 in a Schmitt Trigger configura-
tion is 6ns from high-to-low, and 5ns from low-to-high.
OPA698
V
H
V
L
R
4
2kΩ
R
3
1kΩ
24.9Ω
R
2
2kΩ
V
CL
–1V
V
CH
+1V
V
OUT
V
IN
R
1
1kΩ
UNITY-GAIN BUFFER
Figure 17 shows a unity-gain voltage buffer using the OPA698.
The feedback resistor (R
F
) isolates the output from the input
capacitance at the inverting input. R
F
= 24.9Ω is recom-
mended for unity-gain buffer applications. R
C
is an optional
compensation resistor that reduces the peaking typically
seen at G = +1. Choosing R
C
= R
S
+ R
F
gives a unity-gain
buffer with approximately the G = +2 frequency response.
The frequency response for this circuit is shown in the
electrical characteristics curves.
Time (10ns/div)
Input and Output Voltage (V)
4
3
2
1
0
–1
–2
–3
–4
V
IN
V
OUT
FIGURE 16. Schmitt Trigger Time Domain Response for a
10MHz Sinewave.
OPA688
V
O
R
S
V
S
R
F
24.9Ω
R
C
FIGURE 17. Unity-Gain Buffer.
FIGURE 15. Very High-Speed Schmitt Trigger.