Texas Instruments | High Speed Comparators for Triggering in Oscilloscopes | Application notes | Texas Instruments High Speed Comparators for Triggering in Oscilloscopes Application notes

Texas Instruments High Speed Comparators for Triggering in Oscilloscopes Application notes
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Oscilloscope Triggering with High Speed Comparators
Jaskaran Atwal, Analog Signal Chain
The oscilloscope is one of the most useful test and
measurement tools for electronic engineers.
Oscilloscopes are used to analyze electrical signals for
the purposes of testing, debugging, and
troubleshooting electronic circuits. Without this tool
many technological developments would not be
possible. Oscilloscopes have come a long way from
when they were first invented before WWII. The
original analog oscilloscopes have been replaced in
the majority of applications by digital oscilloscopes.
The trigger system can be implemented in software or
hardware. An analog trigger system typically works by
monitoring the input waveform with an amplifier and
passing this input to a high speed comparator to detect
if the trigger threshold set by the user has been
reached. The comparator will fire and alert the
Acquisition System when the trigger threshold has
been met. Only now will the data from the output of the
ADC be processed by the Acquisition System and be
displayed on the screen.
Over the years, many advancements have been made
to oscilloscopes with the continuing development of
software and analog and digital devices, but the basic
functionality has remained the same. The oscilloscope
operates by taking an electrical signal as an input and
displaying this waveform on a screen. To display a
repeating waveform, a trigger is required. Without this
trigger, multiple copies of the repeating waveform
would be drawn all over the display. This would lead to
a jumble rather than a clean centered display of the
waveform. An example block diagram of the triggering
circuit is shown in Figure 1.
To properly detect and trigger at any input signal, the
comparator must be high speed. High speed is
important when the input signal may contain a glitch or
very small pulse width. Without a high speed
comparator, the system may not be able to trigger on
this input signal. The propagation delay of the
comparator is part of what makes this triggering
possible, but the specific comparator parameters to
look for are toggle rate and pulse width. Toggle rate
and pulse width for comparators can be compared to
the bandwidth parameter for amplifiers. Along with
that, it is important that the trigger system is immune
to noise to prevent any false triggering. Hysteresis is a
key feature that can help comparators with minimizing
the effects of noise. The LMH7322 is a good fit for this
application with its extremely small pulse width of 260
ps, toggle rate of 3.9 Gb/s, and 75 mV of
programmable hysteresis. Size constraints of the
overall system can also be important when selecting
components for this application. The LMH7322 is
available in a small 24-pin WQFN package.
Scope
Input
+
Amp
_
ADC
Acquisition
System
+
Comp
_
Trigger
Threshold
Figure 1. Oscilloscope Triggering Block Diagram
Now that we have introduced the importance of
triggering in oscilloscopes, let’s discuss how the
triggering function is designed in oscilloscopes. The
triggering function tells the oscilloscope what data from
the input waveform it needs to care about and when
the Acquisition System must start processing and
displaying this data. The point at which the
oscilloscope triggers, or the Trigger Threshold, is
configurable by the user and typically based on a
falling or rising edge and voltage level of the input
waveform.
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Scope
Input
+
Amp
_
ADC
Acquisition +
Trigger System
Figure 2. Software Trigger Block Diagram
Software triggers on the other hand work by constantly
sampling data from the ADC and determining when
the trigger threshold has been reached. The input
signal is not split into parallel paths as shown in
Figure 2. Once the ADC and software determines the
threshold has been reached, the waveform is
processed and displayed on the screen.
Oscilloscope Triggering with High Speed Comparators Jaskaran Atwal, Analog Signal
Copyright © 2018, Texas Instruments Incorporated
Chain
1
www.ti.com
Scope
Input
Input
Coupling
+
Amp
_
DC
ADC
Acquisition
System
AC
DC
+
Comp
_
AC
Trigger
Coupling
Trigger
Threshold
Figure 3. Input and Trigger Coupling Block
Diagram
The introduction of analog and software based trigger
systems give us the opportunity to discuss why one
could be preferred over the other. The AC and DC
coupling options on the inputs of the oscilloscope are
well known, but less known are the coupling options
on the trigger shown in Figure 4. As the software
implementation of the trigger does not split up the
input signal, the trigger does not have the option to be
AC or DC coupled. The trigger instead relies on the
input coupling and triggers based on this coupled
signal. The downside to this is that you cannot display
the DC coupled input waveform while triggering on the
AC coupled input waveform.
One example of this drawback is shown when
analyzing a spike or glitch on a power line. In this
scenario you may want to see both the DC component
of the signal and the spike or glitch on top. The
resolution of the ADC can be taken up by the DC
component of this input signal. The software trigger,
without AC coupling, may not have enough remaining
resolution to detect the spike or glitch and display the
full input signal. On the other hand, Analog trigger
systems split up the input of the oscilloscope as shown
in Figure 3. This allows for the freedom to have input
coupling and trigger coupling independent from each
other and avoid the drawback described.
2
Figure 4. Trigger Coupling Options
As discussed, the triggering system is essential to
properly display an electrical signal on an oscilloscope.
An analog trigger system however can offer the most
freedom in these applications. High speed
comparators are at the center of these analog trigger
systems. The LMH7322 is a high speed comparator
that is a good fit for this application with its small pulse
width, high toggle rate, programmable hysteresis, and
small size package.
Table 1. Device Recommendations
Device
Toggle
Rate
Min
Pulse
Width
Hysteresis
Supply
LMH7322
3.9 Gb/s
260 ps
75 mV
2.7 V - 12 V
LMH7324
3.72 Gb/s
290 ps
22.5 mV
5 V - 12 V
LMV7220
1.8 Gb/s
-
External
2.7 V - 12 V
Table 2. Other High Speed Comparators
Device
Propagation Hysteresis
Delay
Supply
TlV3501
4.5 ns
6 mV
2.7 V - 5.5 V
LMV7219
7 ns
7 mV
2.7 V - 5 V
TL3016
7.6 ns
External
±5V
Oscilloscope Triggering with High Speed Comparators Jaskaran Atwal, Analog Signal
Chain
Copyright © 2018, Texas Instruments Incorporated
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