Download Measuring Low Voltage Signals Using an Oscilloscope

Measuring Low Voltage Signals
Using an Oscilloscope
March 10, 2015
Richard Markley
Oscilloscope Product Manager
Agenda
ı Why are low voltage signals so difficult to measure?
Common Measurements
 What makes them difficult to measure?
ı Tips and suggestions to better measure low voltage signals
ı Ways to increase vertical resolution
 Averaging
 High-Resolution
 Digital Filtering
ı Live Walkthrough

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Why are low voltage signals so difficult to measure?
Common Examples
Measurement
High Resolution Applications
▐ Measurement
of small voltage variation in presence of large voltage, i.e. Conduction Loss
current analysis on component sleep state
▐ Accuracy in Ripple Voltage measurements
Power Analysis
▐ Small
Medical /
Biology
▐ Weak
cardio & neural signal with low bandwidth
▐ Sensors
Physics
for optical photon detectors
▐ Mass Spectrometry Faraday cups or ion to photon detector
▐ Radioactive Decay measurements
Wireless
Communication
▐ High
Aerospace &
Defense
▐ High
Embedded
Circuit Design
▐ Low
resolution suitable for NFC, Wireless Power Charging & design using small
Amplitude Shift Keying in data transmission. Typically in lower BW.
▐ Sub
vertical and horizontal precision to analyze ultra-low voltage wideband signal
power circuit with weak signals
threshold leakage measurements
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Why are low voltage signals so difficult to measure?
Vertical Resolution
ı Small signal in the presence of a larger
Most oscilloscopes use 8-bit ADC
 256 levels to place samples in to
 2^8 levels/buckets
 256 levels are spread across the
display
 500mV/div = 5V across the
display
 5V / 256 levels = 19.5mV
ı Very small signal
 Smaller signals allow you to adjust
the front end gain
 But can sometimes be overwhelmed
by the inherent noise of the
oscilloscope

…
256 Levels …
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Why are low voltage signals so difficult to measure?
Vertical Resolution
Zoom in
Turn on dots mode
Turn on sample and hold interpolation
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Agenda
ı Why are low voltage signals so difficult to measure?
Common Measurements
 What makes them difficult to measure?
ı Tips and suggestions to better measure low voltage signals
ı Ways to increase vertical resolution
 Averaging
 High-Resolution
 Digital Filtering
ı Live Walkthrough

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Tips
Improving measurements of low voltage signals
ı
ı
ı
ı
Spread the signal across as much of the display as possible without clipping
Use multiple grids to see more than one signals
Look for oscilloscopes with as little noise as possible
Use techniques inside the oscilloscope to increase vertical resolution
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Tips
Spread the signal across as much of the display as possible without clipping
ı Using just a few divisions limits the
number of levels the ADC can use
ı Don’t make the signal too big as it
can saturate the front end and
impact measurements as well
2 divisions
ı
ı
ı
ı
256 levels / 10 division
25.6 levels per division
2 divisions = ~50 levels
2^6 = 64 levels
 Effectively using less
than 6 bits of the ADC
Tip: Scale the waveform across as much of the display as possible without clipping the signal.
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Tips
Use multiple grids to view more than one signal
ı Viewing multiple signals simultaneously can be tough
ı If you scale them across just a few divisions you end up with the issue
discussed in the previous slide
ı Using multiple grids allows you to maintain the full ADC resolution for each
waveform and easily view them
Multiple waveforms in one grid
Multiple waveforms in multiple grids
Tip: If available, use multiple grids to view more than one waveform.
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Tips
Look for low noise oscilloscopes
ı All oscilloscopes have noise, and we’ll discuss ways to lower that
ı Oscilloscope noise can hide low amplitude signals
ı Starting with low noise to begin with will help improve measurements
ı While not ideal, a quick and dirty
way to see peak to peak noise of a
scope is remove any probes and
zoom in to 1mV/div and see how
many divisions of noise there are
ı Some manufacturers also spec rms
noise which is more accurate
ı Note: Some scopes bandwidth limit
at lower volt/div settings
Tip: Start with a low noise oscilloscope to begin with.
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Agenda
ı Why are low voltage signals so difficult to measure?
Common Measurements
 What makes them difficult to measure?
ı Tips and suggestions to better measure low voltage signals
ı Ways to increase vertical resolution
 Averaging
 High-Resolution
 Digital Filtering
ı Live Walkthrough

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Ways to Increase Vertical Resolution
ı Maximizing vertical resolution has multiple benefits
Lowers noise
 Increases resolution and quantity of levels to place sample points in
ı There are three key ways to increase vertical resolution
 Averaging
 High Resolution
 Filtering
ı Each of these increases the signal to noise ratio thereby giving more signal
resolution

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Ways to Increase Vertical Resolution
Averaging
ı Averaging is a very common technique to
increase vertical resolution
 Takes a user defined number of
waveforms and averages them in to a
single waveform, thereby increasing the
signal to noise ratio
ı Benefits
 Maintains bandwidth and sample rate
ı Drawbacks
 Requires a repetitive waveform
 Post processed so the trigger never sees
the increased resolution
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Ways to Increase Vertical Resolution
High Resolution
ı High resolution is another common technique to increase vertical resolution


Takes extra samples and averages them together to get a single point
This lowers noise and increases bits of resolution
“HighRes”
Decimation
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Ways to Increase Vertical Resolution
High Resolution
ı Benefits
Works in single shot – doesn’t require multiple waveforms
ı Drawbacks
 Filter bandwidth is not explicitly known
 Depends on the decimation factor
 Requires effective sample rate reduction
 Increases the chances of aliasing
 Post processed so the trigger never sees the increased resolution

Vertical System
A/D Converter
8
bits
8+
bits
Decimation
Memory
Analog Trigger
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Ways to Increase Vertical Resolution
Digital Filtering – “High Definition”
ı Digital filtering reduces noise and
increases signal to noise ratio
ı Extremely low noise front end and
monolithic ADC work together to give
high spurious free dynamic range
(SFDR)
ı Digital filtering is done in the backend
ASIC prior to the digital trigger so the
trigger sees the increased resolution
Vertical System
A/D Converter
8+
bits
8
bits
Decimation
Memory
Analog Trigger
Traditional Architecture
Vertical System
A/D Converter
8
bits
Low Pass Filter
8-16
bits
Decimation
Digital Trigger
High Definition Architecture
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Memory
Ways to Increase Vertical Resolution
Digital Filtering – “High Definition”
ı How does digital filtering increase the ADC’s resolution?



Works very similar to an audio ADC
Many audio ADCs are 1-bit ADCs, but through filtering and oversampling they
can reach 24-bits of resolution
Simplified Example:
integer numbers
125 + 126
= 125.5
2
averaging is a type of filtering
March 10, 2015 Measuring Low Voltage Signals
more resolution
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Ways to Increase Vertical Resolution
Digital Filtering – “High Definition”
ı Benefits
Works in single shot – doesn’t require multiple waveforms
 Known bandwidth – it can be adapted to application
 High acquisition rate and all functions still available
 Increased resolution is seen by the trigger
 Digital trigger has 0.04div sensitivity
 At least 10x better than analog trigger path
 Data path is designed to handle up to 16-bit resolution – 65,536 levels
 500uV/div setting allows even more scaling of the waveform
ı Drawbacks
 Low pass filter (but you choose the bandwidth cutoff)

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Ways to Increase Vertical Resolution
Digital Filtering – “High Definition”
High Definition inactive
High Definition active
Quantization steps clearly visible.
“Hidden” low level signal becomes visible.
Signal characteristics can be measured.
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Agenda
ı Why are low voltage signals so difficult to measure?
Common Measurements
 What makes them difficult to measure?
ı Tips and suggestions to better measure low voltage signals
ı Ways to increase vertical resolution
 Averaging
 High-Resolution
 Digital Filtering
ı Live Walkthrough

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Measuring Low Voltage Signals
Putting the tips in to practice
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Summary
How to make better low voltage measurements
ı
ı
ı
ı
Spread the signal across as much of the display as possible without clipping
Use multiple grids to see more than one signals
Look for oscilloscopes with as little noise as possible
Use techniques inside the oscilloscope to increase vertical resolution

High Definition Mode
 Benefits
 Works in single shot – doesn’t require multiple
waveforms
 Known bandwidth – it can be adapted to
application
 High acquisition rate and all functions still available
 Increased resolution is seen by the trigger
 Digital trigger has 0.04div sensitivity
 Data path is designed to handle up to 16-bit
resolution – 65,536 levels
 500uV/div setting allows even more scaling of the
waveform
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