We've been using a little 8-bit, 48-MHz USB logic analyzer--a Chinese knockoff of a Saleae Logic 8. It's been pretty handy for characterizing the ADC in a bathymetric lidar chip we've been helping design.
It'll stream data forever at 16 MS/s. However, at higher sample rates it quits after ~100k samples. Sheesh, and we spent eight bucks on it. ;)
The USB form factor is nice, because it's the sort of instrument you want to pull out and attach down in the guts of some gizmo, and with USB you can just use a longer cable, which is convenient.
Sooo, I'm considering buying something a bit better than our $8 Chinesium knockoff. Suggestions?
Up front note: I am a believer in high end logic analyzers after I was part of a team to evaluate (at HP's request) their two products (1602A, 1610 a )logic analyzers back in the day. Fast fwd to 2023.... Without knowing some of your requirements (speed, channels, etc.) hard to say. Still, I'd point you to Keysight 16861A - a 34 channel LA or the 16862a (currently have one in my lab). May not be in the budget you have. I'd consider a older used unit.
If you are drawn to PC-USB configurations, I've used the Saleae Pro 16 and thought it was pretty good - a bit higher in price than some of the chinesium products. Fairly good sampling rates: 500 MS/s and 100 MHz for digital, and analog of 50 MS/s and 5 MHz.
I have seen but not used a Digilent product, which intrigues me: Digilent Digital (or Analog) discovery.
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A colleague of mine recently got this for his lab, which he likes (after doing a fair bit of searching). It may be more functionality than you need but the DA +LA functions are intriguing...
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general, I am not a big fan of having test gear tied to a PC ....You are tied into continuous vendor support to keep up with OS updates and changes, unless one freezes their environment.
This one is just a dongle with a single chip in it, a copy of Saleae's original Logic (originally $149), which they no longer sell. So it relies pretty heavily on the USB bandwidth and latency of the computer.
Simon is using it with Sigrok, I think. Certainly we're not using the Saleae software.
It's not that simple; write speed to flash is limited, and there's little RAM to buffer. A spinning-rust hard drive, with a few dozen MB of RAM cache, outpaces thumb drives for speed, and a short RAID stack can go faster yet. Your logic analyzer must deliver specified write speeds that match a logic device, cannot generally tolerate the slow approximations of a USB data stream.
The Chinesium ones use a Nordic 8-bit-parallel-to-USB3 chip, and the rest is up to Sigrok.
Turns out that you can get 16 bits by using two of them at once. You have to stitch the decoded data files together yourself, but that's not too hard. (Simon's a Python whiz.)
They're both looking at the clock and at bit 6, so it's easy to align the time stamps.
The ADC clock is coming from a Highland P400 DDG, so we just wired it up, hit the green button to start the clock, and it just works. (BTW the Ali one has the label on straight, and the Amazon one doesn't.)
Well, we're going to be in the market for a real 16-bit one, it looks like. Turns out that this 12-bit ADC has a _gigantic_ glitch at the bit
10 carry--see
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and
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. These are single frames from a cell phone video summary, so they aren't perfect, but they get the point across.
The red curve is the ADC samples, plotted against the right hand scale, and the blue curve is the input ramp signal, plotted against the left hand scale in volts. The horizontal scale is seconds. The two curves were roughly aligned by eye, which accounts for the minor time scale error. The ADC clock was 1 MHz, and the logic analyzer's was 16 MHz (give or take).
As you can see, it's not just a normal sort of high-order carry glitch, or even really just a DNL issue at all, because it takes many samples to recover.
The sawtooth is pretty blameless. It's based on a bootstrap ramp generator. (I got the idea from JL long ago.)
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Differentiating the ramp with a 20 nF / 1k ohm RC, it's linear to within
10 ppm or thereabouts. There's a small soakage tail lasting about 2 ms after the reset, because I actually used a mylar cap rather than polyprop, but after that the differentiated output is basically just the noise of the TCA0372 (22 nV in 1 Hz). And of course the TCA0372 output looks like a car battery (1.4 A).
We'll try it again with another chip, because it's possible that we blew this one up in some artistic manner.
I was chatting with one of my colleagues in our lab and asked him for input on lower cost USB based LAs. He pointed to this one which he said he purchased for his personal use (The $150USD version).
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thought the software was well done and was an updated design that has a good amount of memory. In looking at the offerings, the U3 Pro looks like a worthwhile feature upgrade, e.g. 1GHz sampling rate/8 channels, 500MHz/16 ch, Stream mode withUSB3 @ $300 USD
USB may be quite fragile when running continuous at high data rates - I have that, too, both with original Saleae and with clones. Good quality cables, clamp-on ferrites and a different USB host may help. A separate USB BUS with no other devices on it is also a good idea.
We have the original Salea at work (the USB3 Logic Pro 16). Not cheap, but works quite nice.
At home, I have a chinese Logic16 clone - that is based on the USB2 FX2 chip with a FPGA in front. Works fine with Sigrok (which the newer Logic16 Pro did not when I bought it).
I'm a huge fan of boat anchors--I have a whole lab full of top-of-the-line HP and Tek gear from the '90s and early '00s. I bought much of it when I first went out on my own, in 2009. That was a really wonderful time to be buying equipment--on average I paid about 4 cents on the dollar.
The PC-USB thing isn't something I'd consider for expensive gear, because as you say it's brittle and not very future-proof. (Not as bad as oscilloscopes that run Windows, but don't let me get started on that old rabbit trail.)
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I've seen the Analog Discovery 2, because my younger daughter had one for her EE class at the community college. (She now does PCB layout for Cirrus Logic in Austin.) They're pretty cool gizmos, especially for the dough, but to my eye the signal integrity wasn't thought out as well as the main board. Plastic BNC connectors, yecch. I'll check out the Digital Discovery.
That's not that hard to do with Sigrok, I don't think--we run stuff like that in Docker containers sometimes. Dunno if that would trash the USB speed.
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