Electronics > Beginners
How to improve signal integrity or signal conditioning?
slow_rider:
I have a battery operated device with a B&W LCD. I'm tapping into the video bus and inputs it into an FPGA for some video manipulation. The source signals are 5V with the highest freq. being a clk signal at about 4 MHz. The FPGA would like to see no more than 3.3V + from whatever reason the video output has its baseline at -0.5V which is not good - even with the ground connected to the FPGA board ground so it won't be "floating".
After simple resistor dividers didn't solve the problem and loaded down the bus I've decided to hookup a http://www.ti.com/lit/ds/symlink/txb0108.pdf in-line with the video bus however now the 4 MHz signal looks like cap charge / discharge cycle.
Before I go and insert some inverters in order to "square" back the signal, perhaps there is something I can do so that signal won't get loaded down so badly?
Worth mentioning that the wires I am using are all 28 AWG flat cables with connectors simple headers. Wires are kept short as possible. Maybe the wiring + connectors cause all that capacitance? However keep in mind the signal is just 4 MHz.
So any ideas are welcomed!
StillTrying:
"however now the 4 MHz signal looks like cap charge / discharge cycle."
I can't see why the 4MHz would be so bad, are VCCA, VCCB and OE all connected.
slow_rider:
Yes, all are connected.
I've disconnected the txb0108 so I'm probing straight from the video bus with a scope. No other load connected.
StillTrying:
Scope probe on X10 ?
tggzzz:
--- Quote from: slow_rider on January 23, 2019, 09:34:31 pm ---The source signals are 5V with the highest freq. being a clk signal at about 4 MHz.
--- End quote ---
No, that isn't the case. The only parameter relevant to the highest frequency is the risetime/falltime.
See the theory and measurements at: https://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/
Apart from that, you supply inadequate information, e.g. "resistors" - but where and what value, "probing with a scope" - but what type of probe and how long is the ground lead. Details matter.
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