DAC8775 Quad-Channel DAC EVM - part 5: HART Interface

RE: DAC8775 Quad-Channel DAC EVM - part 5: HART Interface

Jan Cumps — Sun, 05 Aug 2018 13:51:30 GMT

The same measurement with a 2.2 kHz signal.

[View:/resized-image/__size/806x486/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-1c9abc00-6d28-42ce-9d4f-927ed24f3ce2/0638.contentimage_5F00_188133.png:806:486]

The amplitude of the HART output dropped with 76 mVpp ... That's 0.13 mA ...

At 3 kHz, we're at 364 mV

[View:/resized-image/__size/804x484/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-1c9abc00-6d28-42ce-9d4f-927ed24f3ce2/8233.contentimage_5F00_188134.png:804:484]

edit: and 10 kHz. This is an order of magnitude above the operational range for this chip. Just for kicks ...

[View:/resized-image/__size/806x486/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-1c9abc00-6d28-42ce-9d4f-927ed24f3ce2/4024.contentimage_5F00_188135.png:806:486]

for all these captures, I've inverted the HART signal to show the delay better ...

RE: DAC8775 Quad-Channel DAC EVM - part 5: HART Interface

DAB — Sat, 04 Aug 2018 17:46:39 GMT

Nice update Jan.

Thanks for showing how to use this embedded communications port.

DAB

RE: DAC8775 Quad-Channel DAC EVM - part 5: HART Interface

jc2048 — Sat, 04 Aug 2018 10:03:09 GMT

This HART stuff is interesting. Wonder why they chose a modem standard from 1960-something?

Just like in the previous blog, I used an EEVBlog µCurrent as a load. It translates the DAC's output current into a voltage.

Why not just use a resistor to convert the current to a voltage?

If you used a 500R resistor (quarter watt or better - a pair of 1k resistors in parallel, maybe?), you could use a voltage probe and you'd see a voltage signal the same size as the one you're feeding in.

I'm presuming here that the output is floating and the current is sourced rather than sunk, so it doesn't matter if you have the grounds on both the input side and the current output side connected to your 'scope ground (but you're now the expert on this chip, so you decide if that's correct or not).

That way you'll probably see a better signal and can properly measure the current amplitude (if you have a reasonably accurate resistor); you'll be able to measure the delay; and, if your generator has a sweep capability, you can see what the frequency response is like (not essential, but what kind of engineer are you if you're not curious about these kind of things?)

Forgot, we're meant to be doing [+] as well as [-]. Really good 'not a Road Test' you're doing here. Keep up the good work. Have a star.