Datasheet

LT1016
7
APPLICATIO S I FOR ATIO
WUUU
The circuit shown in Figure 1 is the best electronic means
of generating a known fast, clean step to test comparators.
It uses a very fast transistor in a common base configura-
tion. The transistor is switched “off” with a fast edge from
the generator and the collector voltage settles to exactly
0V in just a few nanoseconds. The most important feature
of this circuit is the lack of feedthrough from the generator
to the comparator input. This prevents overshoot on the
comparator input that would give a false fast reading on
comparator response time.
To adjust this circuit for exactly 5mV overdrive, V1 is
adjusted so that the LT1016 output under test settles to
1.4V (in the linear region). Then V1 is changed –5V to set
overdrive at 5mV.
The test circuit shown measures low to high transition on
the “+” input. For opposite polarity transitions on the
output, simply reverse the inputs of the LT1016.
High Speed Design Techniques
A substantial amount of design effort has made the LT1016
relatively easy to use. It is much less prone to oscillation
and other vagaries than some slower comparators, even
with slow input signals. In particular, the LT1016 is stable
in its linear region, a feature no other high speed compara-
tor has. Additionally, output stage switching does not
appreciably change power supply current, further enhanc-
ing stability. These features make the application of the
50GHz gain-bandwidth LT1016 considerably easier than
other fast comparators. Unfortunately, laws of physics
dictate that the circuit environment the LT1016 works in
must be properly prepared. The performance limits of high
speed circuitry are often determined by parasitics such as
stray capacitance, ground impedance and layout. Some of
these considerations are present in digital systems where
designers are comfortable describing bit patterns and
memory access times in terms of nanoseconds. The
LT1016 can be used in such fast digital systems and
Figure 2 shows just how fast the device is. The simple test
circuit allows us to see that the LT1016’s (Trace B)
response to the pulse generator (Trace A) is as fast as a
TTL inverter (Trace C) even when the LT1016 has only
millivolts of input signal! Linear circuits operating with
this kind of speed make many engineers justifiably wary.
Nanosecond domain linear circuits are widely associated
with oscillations, mysterious shifts in circuit characteris-
tics, unintended modes of operation and outright failure to
function.
–
+
PULSE
IN
0V
0V
–100mV
–3V
–5V
–5V
5V
0.1µF
50Ω
25Ω
25Ω
10Ω
400Ω
130Ω
750Ω
10k
2N3866
V1
†
0.01µF
0.01µF**
LT1016
L
Q
Q
10 SCOPE PROBE
(C
IN
≈ 10pF)
10 SCOPE PROBE
(C
IN
≈ 10pF)
* SEE TEXT FOR CIRCUIT EXPLANATION
** TOTAL LEAD LENGTH INCLUDING DEVICE PIN.
SOCKET AND CAPACITOR LEADS SHOULD BE
LESS THAN 0.5 IN. USE GROUND PLANE
†
(V
OS
+ OVERDRIVE) • 1000
1016 F01
Figure 1. Response Time Test Circuit