Electronics > Beginners
How to improve signal integrity or signal conditioning?
slow_rider:
Scope was on x1, setting it to x10 had a dramatic effect - the signal actually looks much more like square wave now (without any load but the scope).
The probe is passive (not sure what capacitance).
The voltage divider I had tried to make is 10k-20k however that is now not part of the circuit.
Ground loop is about 10cm long.
tggzzz:
--- Quote from: slow_rider on January 23, 2019, 11:46:55 pm ---Scope was on x1, setting it to x10 had a dramatic effect - the signal actually looks much more like square wave now (without any load but the scope).
--- End quote ---
I think you mean the "scope probe". Details matter. In this case it was unimportant, but poor information runs the risk of wasting other people's time.
In future, in order allow us to help you, have a look at https://entertaininghacks.wordpress.com/library-2/good-questions-pique-our-interest-and-dont-waste-our-time-2/
--- Quote ---The probe is passive (not sure what capacitance).
--- End quote ---
It should say so somewhere on the probe.
--- Quote ---The voltage divider I had tried to make is 10k-20k however that is now not part of the circuit.
--- End quote ---
So the Thevenin equivalent resistance is 6.6kohm. With a 15pF tip capacitance the measurement would be limited by the RC time constant of 100ns, or with 100pF it would be 660ns.
Compare that with your original trace.
--- Quote ---Ground loop is about 10cm long.
--- End quote ---
kg4arn:
@slow_rider
The scope probe always loads the signal test point. The trick is knowing if the loading is significant or not. x10 setting on the probe typically has 10-15pF capacitive load to the ground point. x1 setting will have 10 times that amount. Using a long grounding lead from the probe to the test point causes other issues.Download the ABCS of probes document from Tektronix. The whole document is very good and I think the issues you were having are discussed around page 32.
https://www.tek.com/document/primer/abcs-probes-primer
spec:
Hi slow_rider
The problems you are encountering are common when driving displays.
As has been said by tggzzz in replay#4, 4MHz is not a slow signal, and especially when you take into account rise time, jitter, etc.
The other thing is that the TXB0108, and the resistor, resistor/BJT, resistor/MOSFET translators you see floating around on the net, are only good for slow signals, and even then they are best avoided as they do not meet the edge speed specification or logic level specification for MOS logic and fall apart hopelessly as the frequency increases due to parasitic capacitive loading.
You also need to decouple the chips with 100nF X7R ceramic capacitor across the supply pins.
It is not clear, but do you need bi-directional comms?
bson:
The LVC logic family will both convert 5V to 3.3V and give you fast transitions on the output. It does have a maximum transition time of 10ns/V (16.5ns for a 1.65V threshold) on its inputs. Basically it can be used as 3.3V logic that's 5V tolerant.
For the opposite, going from a 3.3V logic domain to 5V, consider AHC powered by the 5V domain.
http://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf
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