Knockoff "Saleae" analyzer DC offset - Page 1
Forum section: electronics / testgear

author: staze
author_rank: Frequent Contributor
timestamp: July 11, 2018, 03:01:08 am
content: Hi All,Question for people. I'm trying to use one of these ancient knockoff Saleae's (actually labeled as a usbee ax pro) to look at an SMTPE signal, but I'm not triggering at all. I hooked up my scope, and I noticed there's a 3V DC offset that goes away when I unhook the LA.First I found that my previous USB cable didn't have continuity between the shell at both ends, but then I switched to another cable that did, and it STILL had the offset. Looking at the circuit, I noticed the resistor and cap that tie the LA's GND to the USB GND were also missing, so I added a 100R resistor. Same result still.I can literally see the offset if I hook a DMM between any of the channels and gnd on the analyzer.Has anyone else seen this? Is there something I'm missing? I've checked a few channels, all have the same result.Thanks!

author: hamster_nz
author_rank: Super Contributor
timestamp: July 11, 2018, 03:05:15 am
content: Might be a 'feature':Quotehttps://support.saleae.com/hc/en-us/articles/208667766-Connecting-a-Logic-Analyzer-to-My-Circuit-Appears-to-Interfere-with-Its-OperationWhat sort of signal are you looking at? Is the signal AC coupled? If so, a LA might not be of much use....

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 11, 2018, 03:39:02 am
content: Quote from: staze on July 11, 2018, 03:01:08 amthe knockoff saleae inputs are floating, you need to add pull down resistor to each input (100K - 1Meg ohm each), but when you connect the input to other low impedance pin (output) of other digital circuit during probing, it should trigger just fine even without pull down. we dont know how do you connect those pins and what type of saleae knockoff we talking about.

author: staze
author_rank: Frequent Contributor
timestamp: July 11, 2018, 04:49:07 am
content: It does appear to be AC coupled (±1V around 0V). It's Linear Timecode (SMPTE specifically). I was hoping the Saleae knockoff would be able to decode it (I found a decoder someone wrote, compiled it, and added it). Sadly, doesn't seem to work.Good info about the floating. I don't recall running into that years ago when I first tried the unit. But does make sense that it shouldn't care. It probably just isn't sensitive enough to pick up the 1V (though it'd probably only see half the data).Trying to figure out how else I can troubleshoot this.Thanks for the help!

author: Gyro
author_rank: Super Contributor
timestamp: July 11, 2018, 07:31:04 pm
content: Quote from: Mechatrommer on July 11, 2018, 03:39:02 amThe knockoff Saleae inputs are pulled up to 3V3 by 100k resistors. You'd want to pull them down with ~10k if you want them to go low.https://sigrok.org/wimg/6/67/Usbee_ax_clone_pcb_front.jpg

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 11, 2018, 11:34:42 pm
content: Mine is usb blaster knock off cypress chip iirc, modifiable to saleae knockoff. If you connect disconnect to low or hi state it will stay at low or hi, hence floating... ymmv, no picture at OP..

author: Gyro
author_rank: Super Contributor
timestamp: July 12, 2018, 09:38:50 am
content: Interesting, I hadn't seen those Cypress Ez-USB based USB Blaster clones. There also seem to be CPLD based ones in the same package (although there's no chance that they would work with Saleae or Sigrok LA). I found a Youtube teardown...https://youtu.be/M0K9FYcF-T8They do indeed seem to be missing the pullup resistors of the ('genuine') Saleae clones (including the OP's AX pro marked one), although they do retain the HC245 buffer. I'm not sure if the USB Blaster clones include direction direction control of the HC245, It's hardwired to 'input' on the Saleae clones. I don't know what other differences it has in comparison, it's obviously close enough to work, excluding the input float behavior. The original genuine 8 bit Saleae didn't include the buffer and, I think, had pulldowns instead of pullups.Ymmv indeed, if in doubt pop the lid off, you may end up with a CPLD based USB Blaster clone version instead!