March 10, 2015 Richard Markley Oscilloscope Product Manager Measuring Low Voltage Signals Using an Oscilloscope
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March 10, 2015 Richard Markley Oscilloscope Product Manager
Measuring Low Voltage Signals Using an Oscilloscope
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
March 10, 2015 Measuring Low Voltage Signals 2
Why are low voltage signals so difficult to measure? Common Examples
March 10, 2015 Measuring Low Voltage Signals 3
Measurement High Resolution Applications
Power Analysis ▐Measurement of small voltage variation in presence of large voltage, i.e. Conduction Loss ▐Small current analysis on component sleep state ▐Accuracy in Ripple Voltage measurements
Medical / Biology
▐Weak cardio & neural signal with low bandwidth
Physics ▐Sensors for optical photon detectors ▐Mass Spectrometry Faraday cups or ion to photon detector ▐Radioactive Decay measurements
Wireless Communication
▐High resolution suitable for NFC, Wireless Power Charging & design using small Amplitude Shift Keying in data transmission. Typically in lower BW.
Aerospace & Defense
▐High vertical and horizontal precision to analyze ultra-low voltage wideband signal
Embedded Circuit Design
▐Low power circuit with weak signals ▐Sub threshold leakage measurements
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
March 10, 2015 Measuring Low Voltage Signals 4
… 256 Levels …
Why are low voltage signals so difficult to measure? Vertical Resolution
March 10, 2015 Measuring Low Voltage Signals 5
Zoom in
Turn on dots mode Turn on sample and hold interpolation
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
March 10, 2015 Measuring Low Voltage Signals 6
Tips Improving measurements of low voltage signals
ı Spread the signal across as much of the display as possible without clipping ı Use multiple grids to see more than one signals ı Look for oscilloscopes with as little noise as possible ı Use techniques inside the oscilloscope to increase vertical resolution
March 10, 2015 Measuring Low Voltage Signals 7
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
March 10, 2015 Measuring Low Voltage Signals 8
Tip: Scale the waveform across as much of the display as possible without clipping the signal.
2 divisions
ı 256 levels / 10 division ı 25.6 levels per division ı 2 divisions = ~50 levels ı 2^6 = 64 levels Effectively using less
than 6 bits of the ADC
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
March 10, 2015 Measuring Low Voltage Signals 9
Tip: If available, use multiple grids to view more than one waveform.
Multiple waveforms in one grid Multiple waveforms in multiple grids
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
March 10, 2015 Measuring Low Voltage Signals 10
Tip: Start with a low noise oscilloscope to begin with.
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
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
March 10, 2015 Measuring Low Voltage Signals 11
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
March 10, 2015 Measuring Low Voltage Signals 12
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
March 10, 2015 Measuring Low Voltage Signals 13
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
March 10, 2015 Measuring Low Voltage Signals 14
“HighRes” Decimation
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
March 10, 2015 Measuring Low Voltage Signals 15
Vertical System
Analog Trigger
Decimation Memory A/D Converter
8 bits
8+ bits
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
March 10, 2015 Measuring Low Voltage Signals 16
Vertical System Low Pass Filter
Digital Trigger
Decimation Memory A/D Converter
8 bits
8-16 bits
Vertical System
Analog Trigger
Decimation Memory A/D Converter
8 bits
8+ bits
Traditional Architecture
High Definition Architecture
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:
March 10, 2015 Measuring Low Voltage Signals 17
averaging is a type of filtering more resolution
125 + 1262
= 125.5
integer numbers
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)
March 10, 2015 Measuring Low Voltage Signals 18
Ways to Increase Vertical Resolution Digital Filtering – “High Definition”
March 10, 2015 Measuring Low Voltage Signals 19
High Definition inactive High Definition active
Quantization steps clearly visible. “Hidden” low level signal becomes visible. Signal characteristics can be measured.
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
March 10, 2015 Measuring Low Voltage Signals 20
Measuring Low Voltage Signals Putting the tips in to practice
March 10, 2015 Measuring Low Voltage Signals 21
Summary How to make better low voltage measurements
ı Spread the signal across as much of the display as possible without clipping ı Use multiple grids to see more than one signals ı Look for oscilloscopes with as little noise as possible ı Use techniques inside the oscilloscope to increase vertical resolution
March 10, 2015 Measuring Low Voltage Signals 22
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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