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Precision signal viewing

Tue, 10/08/2013 - 12:16pm
Joel Woodward, Agilent Technologies

The need for seeing additional current and voltage detail is increasing.  Many scope users want better ability to see small signal changes on a large signal (high dynamic range measurements). Power rails, biomedical technology developed to interact with human physiology, high-energy physics experiments that produce small pulses, and mobile devices where power consumption in sleep mode is critical, drive the need to see signal detail as small as just a few millivolt or milliamps. 

Viewing signal detail of this amplitude can be challenging due to resolution levels and noise.  Traditionally scopes have been designed with 8 bits of resolution that yield 256 quantization levels (28).  This limits the smallest vertical resolution to full scale divided by 256.  Seeing changes less than amount is impossible.    In addition, signals coming into the scope include front-end noise that is added to the signal with the result stored to the scope’s memory.  This noise masks signal detail making it impossible to see changes that are smaller than the scope’s noise.  Need precision signal viewing?  Consider one or more of the following to help you see additional signal detail.   

Get a scope with 12 bits of resolution
Several oscilloscope vendors now offer scopes with 12-bits of resolution and range in bandwidth from 250 MHz up to 2 GHz.   For a scope set to a full scale vertical value of 800 mV for example, an 8-bit scope will achieve a resolution of 3mV while a scope with 12 bits of yield resolution 195 uV.  The increased resolution enables the oscilloscope to more precisely plot signal detail. 

As scope vendors increase resolution levels, they also need to reduce noise in order to allow the scope to take advantage of the additional resolution.  Today’s scopes with 12-bits of resolution deliver roughly three times lower noise than 8-bit scopes of equivalent bandwidth.    

Put your scope in high-res mode
High-res mode causes the scope to oversample and digitally filter the output of the ADC to achieve more bits of resolution.    Perhaps the most important attribute of high-res mode is its ability to reduce overall scope noise in a single acquisition.  For this reason, some scope vendors now have scopes natively locked into high-res mode to achieve higher bits of resolution and lower noise.  For High-res mode to the work, the ADC must sample dramatically faster (up to ten times more) than what is necessary to satisfy Nyquist (typically 2.5x the bandwidth of the osciiloscope).  These samples are typically called hypersamples.  Each group of hypersamples is averaged and stored to acquisition memory.  As hypersampling runs much faster than the needed sample rate, this technique allows the scope to achieve up to 12-bits of resolution with 3 times lower noise than 8-bit oscilloscopes of equivalent bandwidth.

Use probes with higher dynamic range
Probes and cables add another dimension to viewing small signals.   Active probes typically use the 50 Ω signal paths for scope channels and these paths have slightly lower noise than their 1 MΩ signal path counterparts.  New probe technology goes further to enable precision viewing of small currents.  Recently released current probes allow users see current down to 50 uA.  Such probes achieves this result by an architecture that uses sense resistor technology with 2 gain paths from the probe’s amplifier.  One path shows the normal current view a user would see while the other circuit clamps around a small vertical area and enables the scope to use its full vertical range on a small clamped zoom area.

 

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