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We appreciate your patience as we improve our online experience.
12-03-2005 03:11 AM
Digital systems are immune to noisy signals in a certain range and will tolerate grounding schemes which are not optimized, but it is better to avoid trouble just from the beginning.
Here you mend digtial circuit component in fixture and CB68LP can slove ground problem and noise problem? Am my understanding correct?
What do you mean by better to avoid trouble just from the beginning? Could you pls describe?
BTW in analogue data acquisition systems you can ruin signal integrity completely by wrong grounding. It is never a fault to take as much care of grounding digital signals as you would do with analogue signals
Are you talking about analog and digital signal in one board as CB68LP? need to take care the ground of the analog signal
If so, how can I reslove the same ground issue?
12-03-2005 03:57 AM
12-05-2005 01:06 AM
12-05-2005 01:13 AM
12-05-2005 05:30 AM
Hi Buechsens:
I would like to make it simple. I have the following questions that need your direct answer, otherwise I don't understand and I can not finish the project. Would you please explain to me?
On the Original setting you simply connected the digital input you wanted to test to BOTH dig output AND DGND. This, of course, will short the output (what do you mean by shorting the output? Becasue I know the picture of this setup, I can not get what you meant. Could you please describe to me or draw a simple picture) , and no test signal is generated. (I believe if you expalin to me about shorting the output, I may see why you said the test signal is not generated)
The 'Setting is right' seems to be correct. However, I would suggest to connect DGND directly to the neg terminal of the 9VDC power supply. This will avoid noise generated by excessive long and winding GND connections. (Did you mean the lead cable wire from the groud of the push buttons in the fixture to the DGND will contain noise? How come the wire put in the negative terminal of the 9V DC using the same wire will have no noise? Would you pls explain?
The third setting will only work (and probably not very good) if there is parasitic GND connection via the mains supply earth (here you meant computer ground connected to the earth, right?). Similar to a supply circuit (supply circuit means a signal source with a power supply and a resistor in a close loop circuit, right) , a signal circuit must always be closed, i.e. both need a return path. In this setting the return path is open (you meant if the DGND is not connected to the ground of the fixture, it is connected to the computer earth. It is not a return path. Why? Becaure the ground of the fixture and the DGND is connected right now, why computer ground can replace for a reture path in ths case. Would you pleaase explain? I don't understand.and you very probably will never get proper test signals ( I think you have the experience because I am exactly having this appearance.)
12-05-2005 05:43 AM
12-05-2005 06:14 AM
Hi Buechsens:
>(Did you mean the lead cable wire from the groud of the push buttons in the fixture to the DGND will contain noise? How come the wire put in the negative terminal of the 9V DC using the same wire will have no noise? Would you pls explain?
There is some risk that supply current flows through the internal connection between the GND of the IC in question and a randomly selected GND terminal point. You can reduce this risk by using the reference point of the power supply. Anyhow, these are general statements, and selection of grounding method depends on actual setup, including subtleties as pc board layout etc. If you find a solution which works correctly, without any noise signals, you are on the right path.
In that case, you meant the current flows through the internal connection between GND of the IC and a randonly selected GND terminal point are in questions. That will generated noise? You said noise is caused by a current changed. According to what you said, when the current flow throung the test fixure, the current is in question and the current is changed, is that correct?
>(you meant if the DGND is not connected to the ground of the fixture, it is connected to the computer earth. It is not a return path. Why?
As I understand, your test fixture does NOT have a connection between GND (neg supply) and earth. So there is no connection between DGND/earth of the pc and GND of the fixture, and the loop is open.
I am sorry. I meant why it MUST take DGND/earth connected to GND of the fixture to complete the path, How can not use take DGND/earth.
I don't understand. They both seems like a ground.
>
12-05-2005 06:35 AM
12-05-2005 08:03 AM
Hi Buechsens:
As mentioned, it is hard to tell from the distance where to locate the optimum grounding point. Any IC will have variations in supply current when any of its states (input or output) changes, so the supply current will never be constant. Also, do NOT consider copper tracks, wires etc as a pure ohmic resistance. When there are fast changes in supply current the parasitic inductance comes into play - and this may cause much higher voltage spikes than expected.
So, in conclusion, it is better to put a DGND (earth) to the negative terminal of the power supply, since wire has a noise, IC have variations that cause noise, in queston to serch ground. Once you put it in the negative terminal of the power supply, it is optimum to reduce troubles, am I correct?
I am sorry. I meant why it MUST take DGND/earth connected to GND of the fixture to complete the path, How can I not just use DGND to computer ground without wiring GND of the fixture I don't understand. They both seems like a ground. I chnage it here.
Yes, this is a very common mistake. You never know where supply current(s) flow in a pc system (it is difficult enough to tell in a self designed system). So you never know whether there will be any voltage shifts between earth and DGND of the PC. And you never know what voltage shifts occur between earth terminals of two different mains outlets. (I am confuse here because DGND and the computer ground, don't you know they are the same if you use DMM, why you said different here, I don't understand , could you pls explain?
12-05-2005 08:05 AM
(i don't want to out of my topic, my topic is if I put wire from DGND to computer ground, I mean not connect DGND to test fixture ground, why can't it set this up to complete the circuit, could you please explain?)
Please give your answer directly. It is much easier