Electronics > Projects, Designs, and Technical Stuff
EMF pickup from amplifier in I2C line causing glitches. (Now with scope trace!)
Starlord:
http://www.ti.com/lit/an/sloa119b/sloa119b.pdf
On Pg 15, why does it state "Zcm at 250 kHz represents the impedance seen by the amplifier at the switching frequency" when the point on the graph being referenced appears to be at 3.5KHz, and furthermore is not at 51 ohms as indicated, but 45 ohms instead? Is the X axis of the graph labeled incorrectly? And did they get the impedance wrong?
[edit]
Nevermind, I see what they did. I thought there were two red lines because there's a sharp point at the base of the graph, and I assumed they'd be pointing towards the point of interest and the point they were pointing to was close to 51 ohms. But the point they're talking about is on the opposite side of the graph, unlabeled.
T3sl4co1l:
Yeah, it just looks weird because the semi-log plot.
Tim
Starlord:
So on the subject of I2C over long cables again, I've got an idea.
The issue with using less than 2K pullups on my I2C lines is that I have one chip, an ADC, which I'm not sure can withstand the higher currents.
I was looking through the I2C spec sheet, and I saw mention of series resistors in there. And it got me thinking... What do you suppose would happen if I used stronger pullups on the microcontroller end of the cable, but put series resistors at the other end?
For example, what if I reduced the 2K pullups to 500 ohms, and then put 1.5K series resistors at the end of my cable, so the total series resistance between the chips and the 5V bus is still 2K?
Would that in any way improve the rise times, because all that cable capacitance is connected to the strong pull ups? Or would it make no difference because to discharge the cable current has to flow through the 1.5K resistors?
dmills:
Steady state, with the output on that remote chip pulling down, how low will the voltage seen by your controller go (Think potential divider)?
Regards, Dan.
Starlord:
--- Quote from: dmills on July 09, 2016, 07:35:13 pm ---Steady state, with the output on that remote chip pulling down, how low will the voltage seen by your controller go (Think potential divider)?
Regards, Dan.
--- End quote ---
Ah. So if the remote chip is trying to pull the line down to 0V, the voltage divider formed with the pullup will only allow it to go down to 3.75V :(
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