# Tag Info

38

First: What's critical isn't so much that there's a ground pin for each signal as much as that there's a ground pin near each color signal. The cross-shaped ground pin largely satisfies that requirement. Second: DVI doesn't prioritize high-quality analog video -- it's a Digital Video Interface, after all. The small loss of quality incurred by using a single ...

31

Try not to fixate too much on the fact that these are vias in a PCB. The point is that they represent impedance changes in the signal's path. These effects are not unique to vias, they can be caused by many different signal path geometries. The titles under the graphs (on page 18, slide 36 of your linked PDF) tell which impedance change is most dominant for ...

22

There are quite a number of things that will do this to you. You have not stated the length of the interface. I do direct chip to chip PCIe frequently and you really need to take this into account as you will get attenuation of roughly 0.18dB per inch due to skin effect losses and about 0.5dB per inch due to dielectric absorption on 'ordinary' FR4. I think ...

21

The maximum frequency is mostly related to the frequency-dependent loss characteristics of the cable. Eventually you get to a frequency where you simply don't get enough signal at the other end to use. Resistive losses in the conductors (including skin effect) Dielectric losses in the insulating materials Radiation losses if the cable is not fully shielded ...

19

In general, trying to push final regulated power any distance is not a good idea. In your case it clearly won't work. Yes, the return path adds to the total resistance since it is in series with the load. It is strange that you have connectors in the positive supply but not in the ground. If this is a fixed installation, then why not solder wires from ...

17

The HD-15 (aka DE-15) connector for VGA connection was compatible with hand-crimping of pins to conductors of coaxial elements of the multiwire connecting cable. That practically requires a two-pin set for each of R, G, B video signals, to accommodate a signal pin and ground (coaxial shield) pin. Those signals were not logic level, and lacked the noise ...

16

The eye you show is firmly in the category of "good" eyes. There's nothing there that should cause an error any more often than once every 100,000 years or so. Some things to look for: Are you sending real data while measuring the eye? An idle pattern or similar behavior could create an eye that looks much cleaner than the eye would be with real data. Is ...

16

No. The differential signalling is performing as intended. No. As long as the "common mode voltage" does not exceed the ratings of the transceiver. And the maximum DC ratings, which for a CAN transceiver like the MCP2551 is -42 V to +42 V. The twisted pair cable and the differential signalling mode make the single-end signals look like gibberish ...

15

Rule-of-thumb says you have to calculate transmission line effects in if your connection length is longer than 1/10th of the signal's wavelength. Transmission lines will cause reflections where they show a sudden change in impedance. The reflected signal adds to the original, may reflect again at the transmitter side, and that way go back and forth. ...

15

Noise in oscilloscope front ends is quite high, maybe 1mVp-p. Using the 1:1 probe lowers the input-referred noise floor by an order of magnitude. Still pretty crappy, but opens a few doors.

14

Use Schmitt-trigger buffers at the inputs of each board. They will clean up the signals so that any noise won't give false pulses on the clock, for instance. The 74LVC3G17 is a triple non-inverting buffer. Also, pass the buffered signals to the next board. Otherwise all inputs would be parallel and you may exceed the fan-out of the driving microcontroller ...

13

Do not confuse power transfer with voltage transfer. If the source is indeed 50 ohms, and your scope is set to 50 ohms, it becomes a voltage divider and what you see is half the voltage that you will see when the scope is set to high impedance. simulate this circuit – Schematic created using CircuitLab Power transfer has to do with the relationship ...

13

Even if ambient temperature is well controlled, air currents can cause noise if there is significant voltage across the capacitor. Capacitance degradation with time is different for different dielectrics. And the big one, voltage coefficient of capacitance. If you can use them, NP0 caps suffer from such effects much less than X5R X7R etc. parts, but they ...

13

Thanks for everyone's help. I believe Bruce Abbott has given the correct answer. I'm posting from my bed and I cannot test it yet until tomorrow, but The analysis below is confirmed, when he mentioned the word "refresh", I think the problem is already solved. I knew how Z80 refreshes the memory, but completely forgot about it in the previous days. ......

12

The point of this probe is that the scope has its 50Ω termination enabled. Since the scope input impedance is matched to the cable, there is no reflection from the scope. Once the load side is matched, we don't need to worry about any source-side impedance mismatch; reflections have already been suppressed. The point of the 950Ω resistor on the input side ...

11

When TX switches low-to-high, the current flows like this: Power supply Vcc -> PCB Vcc plane -> U1.Vcc pin -> U1.TX pin -> U2.RX pin -> U2.Gnd pin -> "return path" -> PCB Gnd plane -> Power supply Gnd It's great that you understand that what we call the "return path" will be the nearest plane (in this case the Vcc plane). This makes sense as the fields can'...

10

Because there is current flowing thru the ground wire between your PCB and the common ground point. The wire has a small resistance. The current time this resistance causes a offset voltage. For example, if there were 1 A flowing in the wire, then it would only need to have 23 mΩ resistance to exhibit the 23 mV offset. Another more subtle ...

10

You are confusing the DVI connector with the cable. That "single" analog ground is a big honking ground, but what goes into a cable? The simplest is a DVI-A cable, which usually has a VGA connector on the other end. Internally, it will carry 3 color channels (red, green and blue) and 2 sync: vertical and horizontal. And guess what? A DVI-A cable will ...

10

Convenience. A 1:1 probe (or the x1 setting on a switchable x10 probe) will probably have slightly lower capacitance than a 50ohm coax of the same length, and also handy clips on signal and ground. It's therefore a convenient tool for small signals where noise makes a 10:1 probe unusable, and for low frequencies where the relatively long ground lead doesn't ...

9

That's a lousy stackup. Your cores should be 0.2 mm and the balance of your thickness should be in the prepreg between layers 3 and 4 (Inner 1 and Inner 2). The reason for this is that, for any signal on a trace, there needs to be a return current path on the plane right under it. The return current in the plane will try to minimize inductance (i.e. loop ...

9

The problem that can occur is that some SR clocks before the next SR clocks, so that next SR will clock in the wrong data. A (standard?) solution for this is to wire the clock starting at the last SR. I would consider adding a (schmit-trigger?) buffer at each board for all 3 signal lines. (edit) Lowering the clock frequency won't help (unless it was far ...

9

Your first circuit is perfect... If you use a mosfet instead of a bjt. A mosfet does not pull anything low when it is on, it indeed is a capacitive load but you are going at 20kHz so that should not be an issue at all. Just be careful: your mosfet should support a quite high $V_{GS}$, i.e. 12V, that's not something all MOS are happy to do. Your circuit ...

9

It depends what you mean by 'capacitance'. If you mean two conductors separated by dielectric, then of course it doesn't. However, if you mean 'behaves like capacitor', then yes it does. a) If you take (for instance) a high value resistor from a power supply, and connect it to a capacitor, then the voltage will start at 0v and slowly ramp up. b) If you ...

9

In general, adding an inductor in series with a chip's power feed and not adding decoupling capacitors to ground (at the pins) is really bad. Just don't do this whether you split it into three or just have a common track. Even if you add capacitance you have to ensure that any changes in the current taken by the chip do not suddenly produce resonance ...

8

That's because you are actively (with the MOSFET) pulling down, but passively (with the resistor) pulling up. If you want a shorter rise time, either try a lower resistance value for R2, or use a totem-pole buffer (with two complementary MOSFETs, one to ground, and one to the supply), so that you will have a pulling effect in both levels. The easiest way to ...

8

The ringing probably isn't real. Note that you're measuring fast signals (keep an eye on the scope's timescale!), and then the inductance of the probe's ground becomes important. It doesn't help to connect the probe's ground a few mm from where you probe the signal if the wire forms a 20 cm loop. You may eliminate possible measurement problems like this ...

8

The biggest issue when chaining shift registers is ensuring that the timing relationship between the clock used by each board uses for receiving data and the change in data from the previous board is predictable. The fact that the output of the 74HC595 changes on the same edge as the clock is a little annoying in that regard. I would suggest that the clock ...

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