LT1016
7
1016fc
applicaTions inForMaTion
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 ap-
preciably
change
power supply current, further enhancing
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 design
-
ers are comfortable describing bit patterns and memory
access times in terms of nanoseconds. The LT1016 can
be used in such fast digital systems and Figure2 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, mysteri
-
ous shifts in circuit characteristics, unintended modes of
operation and outright failure to function.
Figure 1. Response Time Test Circuit
–
+
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
10X SCOPE PROBE
(C
IN
≈ 10pF)
10X 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
