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ShortSniffer Voltage Probe - What is it and why should I care?

The ShortSniffer was designed to locate short circuits on electrical circuit boards.  It contains a signal generator and a high-gain amplifier, which provide the stimulus and gain for the remarkable performance of its short circuit locating abilities.  Sometimes a high-gain amplifier can be used for other things . . .

ShortSniffer Voltage Probe - Background:

The ShortSniffer Voltage probe evolved as an effective troubleshooting tool for a small set of problems, but has been quite useful in a variety of non-obvious circuit design, development, troubleshooting, and optimization efforts.  Let me tell you about the characteristics of the system and of a few of the successes with the non-traditional uses, and maybe you will find this tool useful in your design or troubleshooting efforts.

ShortSniffer Voltage Probe - Hear the input voltage with a lot of gain:

The ShortSniffer high-gain amplifier was specifically optimized for the primary ShortSniffer task of locating short circuits, but the resulting circuit will let you hear audio signals down in the 1 microvolt range.  Since there are non-linear elements in the amplifiers, a 1 Volt signal will be much louder than a 1 microvolt signal, but not a million times louder. 

ShortSniffer Voltage Probe - If your circuit is already working, why should you keep messing around with it? 

OK, your new design seems to be working . . . are you done, yet?  If you have a little more time until the "final" design is released to production, why not "look around" to see if any minor changes might make things even better?  There may be minor noise issues in your circuits that are "good enough" for the specifications, but maybe they can be made better.  By poking around the board with the ShortSniffer Voltage Probe, you might be surprised at what you can hear!  For instance, are there unexpected audio  noise pulses coming from your regulated power supply?  They may be caused by various cyclic load variations cause by software loops or I/O loading, but there might be another reason.  Can you hear noise on your "quiet analog ground" net?  It might be OK, or maybe there is an unwanted current path from an improperly routed trace to your blinking LED. 

ShortSniffer Voltage Probe - Claims, Cautions, and Usage:

The standard ShortSniffer short circuit locating functions are designed to be used on unpowered circuits.  The ShortSniffer Voltage Probe deviates from this mode, requiring you to work on your circuit while power is applied.  The input capacitor is rated at 50V, setting the maximum applied voltage.  If your circuit has any voltages higher than 50V, you risk damage to yourself, the probe and to the ShortSniffer if you make contact with these higher voltages.  Be Careful! Since the probe has a .1uF capacitor on its input, the capacitor can be charged to whatever voltage it touches.  Moving the probe to other nets in your circuit will transfer charge from the input capacitor (limited by the 100 Ohm series resistor) to the next net it touches.  Beware of the possibility of damage under these conditions.

You may be able to gain significant understanding of your circuit and layout if you poke around your new design (or even an old design) with a high-gain amplifier.  There will be sounds that will show up obvious problems, and other sounds that may be an interesting characteristic of a properly operating system.  You find that comparing "normal" sounds between systems will indicate a "sick" system with different characteristic sounds.

Not all signals in your systems will have variations in the audio range, so this method will not indicate all types of problems. Also, not all noises will indicate an actual problem.  I spent some time reducing the very loud power supply noise in a differential amplifier system, only to find that there was no improvement to the noise in the output of the system, since the amplifier had excellent power supply rejection.   You can think of this method of troubleshooting as using an audio spectrum analyzer with really strange specifications. 

Although becoming familiar with the audio noises in your system can be beneficial and educational, don't waste large quantities of time making things more quiet than necessary. 

ShortSniffer Voltage Probe - Circuit loading:

The ShortSniffer Voltage Probe was designed to couple the probed voltages from a live circuit into the ShortSniffer high-gain amplifier, but to also provide protection against overloads that might damage the ShortSniffer.  Unfortunately, this is not a high impedance design like your voltmeter or oscilloscope probes, so it does not allow the flexibility of probing high impedance notes without possibly disrupting the normal circuit functions.  For larger amplitude signals, the ShortSniffer Voltage Probe input impedance is about 0.1uF, in series with 100 Ohms, feeding a diode clamp to protect the ShortSniffer input amplifier. For small signals, the load is about .05uF in series with 200 Ohms.  Probing a high impedance node in an audio chain may give a good audio signal to the headphones, but it may significantly (temporarily) affect the resultant signal further down the signal chain.

ShortSniffer Voltage Probe - Usage:

Turn on the ShortSniffer.  Adjust the ShortSniffer Drive and Gain controls to minimum (full CCW).  Attach the headphones and place them over your ears.  Connect the ground clip of the Voltage probe to the reference point, typically a quite ground on your circuit.  Probing the attachment point should give no sounds, but Probing any other point on the board will allow you to hear the difference between these two points.  If you are probing a circuit ground, try touching other grounds in the system, noticing the sounds you hear.  If needed, you can increase the Gain by rotating the Gain knob on the ShortSniffer, providing up to 100 times the minimum gain.

ShortSniffer Voltage Probe - Example Success Stories:

  • Touching the output of the 3.0V LDO regulator produced a loud 3.5kHz tone.  This was unexpected, since casual probing of the circuit with the digital scope showed only a little noise, but nothing alarming.  Switching the scope to AC coupling for a better look at the noise, and tweaking the vertical sensitivity, triggering and horizontal sweep showed an unexpected clean 50mV P-P triangle wave that had gone undetected in previous verification passes.  Adding a 10uF Tantalum cap to the output of the regulator brought stability back to the system, and eliminated the unwanted signal and tone, leaving only a gentle background hiss in the headphones.
  • The annoying 800 Hz noise in the Bluetooth stethoscope headphones was visible on the oscilloscope probing the output of the amplifier chain of the system, but not noticeable on the earlier stages.  Working backwards through the system, one stage at a time with the ShortSniffer Voltage Probe, I could hear the tone on the output of each amplifier, even though the output of the previous stage had been shorted out.  I know, it's not polite to short outputs of amplifiers, but I didn't do permanent damage.  I found that the Bluetooth 2.4GHz 800 packet bursts were being detected by the input protection diodes at each amplifier input.  Adding 10pF caps across each amplifier eliminated the problem, without affecting the desired signals.
  • While developing the current generation of ShortSniffers, I used the voltage probe to determine the necessary size of the power supply capacitors, along with additional R-C power supply stage decoupling to keep the signal generator current surges from affecting the high-gain amplifier power supply.
  • While developing the current generation of ShortSniffers, I used the voltage probe to discover improper grounding paths and to determine the necessary ground islands to keep the common mode current paths of the signal generator and the headphone and speaker amplifier circuits from affecting the high-gain input stage.
  • Working with the OSU Solar Car team, we found ways to identify cold solder joints on the Solar arrays by probing the array outputs and then tapping on all of the solder joints.  Cold solder joints made a lot of audio noise, identifying the required rework areas.