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Bench Presses and Cross Fits with SMUs, DMMS, and CRT Oscilloscopes


Okay, I have a confession. This article isn’t exactly concentrated on the physical exercise of doing bench presses and cross fits, but figuratively is about mental exercises of similar magnitude. So if you were lead here by googling the words “bench press” and “cross fit”, then this isn’t the reading material for you. Then again, please do read it anyway since you’ve gotten through the effort of opening it on your browser. Please…


“Why that choice of words for the title?” you may ask. Well, anyone who has sat in front of an electronics test bench will attest that, aside from the physical effort of bringing your instruments there, an auxiliary mental analysis is required for proper measurement. For example, you are checking the output of an IC you recently purchased. The datasheet claimed -60 dB PSRR, but when you sweep the input with a sine wave amplitude from ground potential to the maximum operating voltage, you observe significant noise. With a wrinkled forehead and disgruntled expression, you storm the manufacturer all guns blazing, ignorant of the possibility that the SMU could be at fault. After all, with all those fancy digits, it’s hard to find room for doubt. No?

Digital Multimeters


Did you know that the Agilent 34401/10/11 series will be discontinued on December 1 this year? They will be replaced by the new 34461/65 series, sporting a brighter and more colorful LCD display. I personally haven’t appreciated the advantages of all this “TrueVolt” and “TrueFrom” stuff, more so because I don’t fully understand how it helps (What? Have the older equipment been giving False voltage readings all this time?). For those who need more time to migrate, Agilent will still support the old series for 5 more years.

Setting the DMM to the correct measurement range is important. Choosing the appropriate resolution goes with it too (4.5, 5.5, or 6.5 digits). Imagine trying to measure leakage current at a range of 1A or higher (and at 4.5 digits). The current reading will be way above the actual current, which is typically in the order of nA/pA, because of the preset lower limit. As for large currents, say with RON measurements of power MOS devices or “short-source” currents, there are select DMMs to use. Common DMMs with good voltage reading capabilities would most likely not have a very wide current range (and vice versa). There are few exceptions (like the 34411 mentioned above), which is both superior in voltage and current measurements. These exceptions, of course, fairly necessitate exceptions with your budget.

The integration time [PLC], raised when readings are too unstable, can serve as a double-edged sword. I remember a friend who wanted to make the most out of a DMM’s accuracy. He maxed out the PLC (usually at 100PLC) while taking the load regulation of an IC at peak temperature. Unfortunately, the output shut down at a current value way below the rated load.

Why?

An increased integration time consequently increased the time the IC was subjected to a certain load. At high loads, the IC heated up. So the temperature rose above the peak and hit thermal shutdown (assuming the IC had one - if not, then the chip may have ended up in a pile of smoke).

Resistance measurements that discount wire and contact resistance are at times desirable. I think many are already familiar with how 4-wire terminal readings compensate for them.

By the way, DMMs usually weigh around 3-4 kg, an excellent starting point for beginners. Their fixated handles conveniently give the arms and shoulders many degrees of freedom. 3 sets of lifting (8 reps per set) for each arm will tone your biceps (and maybe gain you arms stronger than Tarzan).


Source-Measure Units


Some old SMUs [those produced in the 90’s] yield noisier current measurements compared to newer ones. I had a personal experience with this, when I tried to measure the feedback current of an amplifier. Setting an older version sweep instrument to VSIM mode, I saw so much spiky and wavy lines at the I-V plot. As carefully and cautiously as possible, I swapped the old instrument with a newer one and… WALLAH! The I-V plot smoothened out.

The difference in function between 2-wire and 4-wire connections in an SMU is more crucial than with DMMs. 4-wire connections compensate for wire and contact resistance in parameters that demand high load, but where you place them is also a big part of the story. When the sense lines are floating, the SMU will compensate a max of around 1V, before defaulting to a 2-wire connection. I haven’t tested the timing limitations so it’s hard to give an exact limit for the allowable pulse width.

The use of SMUs with a 4-wire connection is not always an attractive option, as it introduces minute noisy currents which affect very low current measurements. Does anyone know if it comes from the sense lines? I know there should practically be no current through those wires but the internal blocking elements must have leakage, right?



SMUs have different settable voltage and current limits. Setting the current limit and voltage limit are trivial procedures, but the unwary engineer may fall victim to pitfalls either way. Testing an IC’s overcurrent protection mechanism with the source limit set to a few tens of mA can shed a false sense of hope. “Look everyone! I’ve designed an IC that never gets broken no matter how many times I short the output terminals to ground! Wee!” Deplorable is thy faith when you get another look at the SMU you used.

Then again, using the maximum limits for all your sources isn’t advisable either. Imagine, a friend of yours who playfully nudged your arm as you relocated a shorting wire connected to ground, causing that said wire to accidentally come in tangent with the output pin of your IC’s internal regulator. Imagine, the lights of your SMU turn red and the letters “LIM” flashing on the LED display. Imagine, your face turning red as the lights of your SMU as you slowly grab your friend by the neck, ears fuming with whistling steam. Yes, such circumstances are indeed, disastrous.

Source-Measure Units are quite heavy, and do compete with the weight of instruments that use CRTs. Their weights commonly range from 14-18 kg., and are more ideal for bench presses. To conduct a bench press, lay flat on your test bench (with the front of your body facing the ceiling), and firmly hold both ends of the SMU by the edges. Be careful NOT to lose your grip as you steadily and slowly lower the SMU to your chest.

There is another supply worth mentioning, the DC supply, which does not support a 4-wire feature, and cannot sink any current. They are beneficial through cost and noise injection [in particular, the one that has a low maximum voltage range]. Some are lighter than SMUs, 9-10 kg. in fact, and can be used if one is already overwhelmed by the weight of an SMU.


Oscilloscopes

With all the new oscilloscopes emerging in the market today, I have seriously been tempted to patent a new design that uses already existing oscilloscopes, then adding Arduino and GSM modules to communicate measured data and plots by smartphone, and calling it the “SCOPE-O-RAMA 5000”. Unfortunately, I may be too late. Browsing the catalog of our local electronics shop, I found a “touch-screen” oscilloscope that did just that, and it looked like an obese IPad.

In my opinion, an oscilloscope’s bandwidth sits at the highest tier in the decision-making process. There is a long formula for determining the sole bandwidth required for an oscilloscope/probe to successfully measure and display a signal with minimal loss in integrity. But I prefer the brusque form: 0.35/rise time. The bandwidth demanded by the entire system is just the vector magnitude of the individual parts.

Propagation delay, slew rate, and other timing parameters of an IC are affected by the oscilloscope’s bandwidth. A bandlimited signal, like a pulse with stretched rising/falling edges (and ringing perhaps?), will result in miscalculated reference levels. An erroneous propagation delay can trick the designer into thinking that the specifications of the customer have been met.

On jitter and eye patterns, the jumbo scopes come in, as their CRT displays can show which graticules have been lighted more frequently. Agilent does not have any cataloged CRT oscilloscopes because they implement the function by modulating the RGB pixel’s luminous intensity.

Inexpensive oscilloscopes can weigh as light as DMMs, around 3 kg., while the “Ferrari” scopes can weigh as heavy as SMUs, or even more! Have you ever tried lifting a Tektronix DPO?

Personally, my instrument weight-lifting experience gifted me with bulging biceps, bulging abs, and bulging eyes (when I try to raise one of those heavy SMUs). Unfortunately, none of these have improved my chances of getting a date.

What about your experience? Please do share and tell us about your body-building experience with these instruments.















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