Datasheet-a-Day W27 2020

5 Jul 2020electronicsreading

More datasheets/app notes/unclassifiable electronics things. I should get around to writing some blog articles about something else as well, eh?

Understand SINAD, ENOB, SNR, THD, THD + N, and SFDR so You Don’t Get Lost in the Noise Floor (Analog MT-003)

Mostly definitions and relationships for all the confusing terms used about noise in ADCs. There are a lot.

Current Sensing Circuit Concepts and Fundamentals (Microchip AN1332)

I’ve been thinking about current sensing for a couple of different applications recently, so this was useful.

The first thing they said was that current sensing resistors only good for low-to-medium current sensing applications because of $I^2 R$ losses. But they didn’t really say much about what the options are otherwise. Never mind. I don’t think I’ll ever be dealing with anything high-current enough to need something else.

So, to keep the losses low, you use low-value current sense resistors. But that means low sense voltage values, so you have to watch out that they don’t become comparable with the input offset voltage of subsequent amplification stages. And if you want to sense at high frequencies, you need a sense resistor with low inductance.

So far, so basic. Then we get on to low-side vs. high-side sensing.

Low-side sensing needs an op amp whose common mode input voltage range includes ground. (Most op amps made for this type of application have common mode headroom below ground. Somehow. It’s what it says in the datasheets. I don’t know how it works!) Low-side causes what people call “ground path disturbances”. That basically just means that the low side of the load is lifted from ground by the sense voltage, which can cause problems if the load is connected to something else in the system that’s referred to the real system ground.

High-side sensing is more complicated, for one thing because it involved dynamic common mode voltages in a way that low-side sensing doesn’t, but it does have the big advantage of being able to detect shorts to ground in the load, because the sense resistor is still in the circuit when the load shorts–in a low-side setup, the sense resistor is not in the circuit when there’s a short, so doesn’t see the high short circuit current at all.

High-side sensing seems preferred for more demanding applications, and there are a couple of approaches covered in the app note: using a single op-amp as a differential amplifier (simple); an instrumentation amplifier made from three op-amps (quite nice, and you can get integrated solutions with all nicely matched components with gain settable by a single external resistor); a two op-amp instrumentation amplifier (which has a weird-looking asymmetric circuit I don’t like much, and has worse common mode performance as a result of the asymmetry...).

Successful PCB Grounding with Mixed-Signal Chips - Follow the Path of Least Impedance (Maxim TUT5450)

This was quite a nice little app note about some EMC things. I must admit that I find all of that a little intimidating so far, and it feels like there’s really a lot to learn, so it’s nice when there’s something reasonably simple like this that even I can follow!

First thing: return paths are different for low frequencies and high frequencies. For low frequencies, we “follow the path of least resistance”, but for high frequencies, we “follow the path of least impedance”. (I guess it’s all the same really. It’s just a simplified way to think about it.)

Here’s a rule of thumb: low-frequency signals return via a straight line path across a ground plane; high-frequency signals return via a path under the outgoing signal path. There were some nice finite-element simulation results demonstrating this. I want to have a play with the Sonnet Lite EM simulation software at some point, and this seems like a good thing to explore with it.

The emphasis in this app note is on mixed signal situations, i.e. ADCs and DACs mostly, because they are the most common places where you have delicate analogue signals and noisy digital signals together.

One common approach is cutting ground planes between analogue and digital areas, but that often turns out not to be necessary: you can usually get away just with segregating signals into an analogue side and a digital side. However, it’s often useful to think of cutting the ground plane to give a single point ground, and then to lay out and route you board accordingly, pretending that the ground plane cut is there. Once that’s done right, you can fill in the ground plane cuts.

There are still a few cases where you do want ground plane cuts: you can use them to make the “path of least resistance” for DC signals avoid delicate areas, for example.

The main principle of all this: just think about where the current goes...

Toshiba Motor Solutions Guide

Not a datasheet. Closer to marketing material, but still useful to me.

It’s basically just a categorised list of Toshiba’s motor control products, but quite interesting (for me anyway) to see the range of what’s available.

Some fun things:

Using An Op Amp for High-Side Current Sensing (TI SBOA347)

More about current sensing. This is a very short but clear description of the advantages of high-side vs. low-side current sensing.

Low-side sensing lifts the load ground above the system ground, which can lead to problems with interfacing the load to other parts of the system that have their low sides connected to system ground (usually called “ground disturbances”). That’s not a problem with high-side sensing.

High-side sensing can also detect shorts to ground within the load: if there’s a short to ground in the load, the sensing resistor is still in circuit so will see the high short-circuit current. With low-side sensing, if there’s a short to ground in the load, the sensing resistor is out of the circuit, so won’t detect the high current through the short.

(I said all that for the earlier current sensing app note, but it’s worth repeating it. I’ve been finding that a lot, in this datasheet-a-day habit: a little repetition helps to reinforce the things I’m learning.)