Spike Detector For Oscilloscopes


Dynamic flip-flops ignore
pulses at their inputs that are shorter than 40 ns or do not have TTL levels. This means that TTL
flip-flops are poorly suited to capturing noise pulses having unknown
durations and magnitudes. Anyone who has ever tried to observe very
short laser pulses (15–25 ns) is familiar with this problem. By
contrast, this circuit can detect impulses with widths less than 8 ns
and amplitudes between +100 mV and +5 V. The heart of this circuit is
formed by a MAX903, a very fast comparator with internal memory. The IC
has separate supply pins for its analogue and digital portions. The
analogue portion is powered by a symmetrical ±5-V supply.

This allows the detector to also handle input voltages that are
negative relative to ground. The internal memory and output stage
operate from a single-ended +5-V supply, so the output signal has proper
TTL levels. The MAX903 (IC1) has a special
internal memory circuit (latch). The latch either connects the output of
the internal comparator directly to the signal output or stores the
most recent TTL level and blocks the output of the internal comparator, causing the most recent TTL
level appears at the output. This allows short input pulses to be
stretched to any desired length. Despite its extremely short switching
times, the MAX903 consumes only a modest 18 mW.

Spike Detector Circuit For Oscilloscopes

Spike Detector Circuit Diagram For Oscilloscopes

In the quiescent state, the voltage on the Latch input (pin 5) is at 1.75 V. This reference voltage is provided by LED
D1, which draws its current via R2. In this state the latch is
transparent, and a positive edge at the input appears will appear as a
negative transition on the output after a propagation delay of 8 ns
(tPD). This only happens if the peak voltage on the input is more
positive than ground potential. C1 passes this change in the output
voltage level to the Latch input (pin 5). As soon as the voltage on the
Latch input drops below 1.4 V, the internal latch switches to the Hold
state. In this state, the output is no longer connected to the
comparator, and the output remains low for the duration of the latch
hold time, regardless of what happens with the input signal.

The latch hold time is determined by the time constant of the C3/R1
network; it has an adjustment range of 100–500 ns. Pulses of this length
can be readily observed using practically any oscilloscope. This latch
function in this circuit is only triggered if the input signal has a
rising edge that crosses the zero-voltage level. The internal latch
remains transparent for signals in the range of –5 V to 0 V, so such
pulses will not be stretched. If only positive input voltages are
anticipated, the negative supply voltage is not necessary and the
circuit can be powered from a single +5-V supply. A fast circuit such as
this requires a carefully designed circuit board layout. All
connections to the IC must be kept very short.

Decoupling capacitors C1 and C2 should preferably be placed
immediately adjacent to the supply pins. Pin 3 of the IC can be bent
upward and soldered directly to a length of coax or twisted-pair cable
(air is still the best insulator). If a coax cable is used, the
unbraided screen must not be formed into a long pigtail. It’s better to
peel back a short length of the screen, wrap a length of bare wire
around it and solder it directly to the ground plane. The supply traces
for the analogue and digital portions must be well separated from each
other, and each supply must be well decoupled, even if only a single
supply voltage (+5 V) is used. The preferred solution is to use two
independent voltage regulators.


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