| ??? 04/17/08 22:22 Read: times |
#153644 - re: hmm Responding to: ???'s previous message |
Kai Klaas said:
Andy said:
There's no need to have separate analog and digital ground planes, in spite of what most ADC and DAC data sheets tell you. You must be smarter than these developement engineers. I can't find the article on ADI's website, but even Barrie Gilbert has come out and said that most mixed-signal device data sheets are simply wrong when they recommend separate ground planes. And Henry Ott has some things to say (pdf) about mixed-signal layout. Have you tested this with the AD7730? No, nothing that slow. But, yes, every design we do here at work has a single ground plane, with great care taken to ensure that things are quiet. Do you think that high frequency currents run on a straight line from point A to B on a ground plane or do they spread and cover more an area than a straight line? If so, do you still think that having only one ground plane is advantageous? Actually, neither ... the signal return current follows the path of least impedance. For high-speed (digntal) signals, this means that it follows the path of least inductance, which is to say that the return current is in the plane directly underneath the signal trace. The low-frequency stuff tends to spread out and take the path of least resistance. Hence one needs to ensure that the digital traces stay out of the analog "area" of the PCB. One can certainly put a slit in the ground plane, which has the effect of keeping the circulating currents on each side of the slit separate. But it's still the same plane. Andy said:
Consider a system with more than one ADC. Where do you connect the ground planes together? A system with more than one ADC usually gives poorer results. Why using a technique suited for more than one ADC if he has only one? Well, the system I'm working on now has 16 ADCs because it deals with sixteen channels. Another board I recently did was four channels of 16 bit ADCs clocked at 4 MHz. So Mahmood's example is the simple one, whereas I typically deal with several converters per board. Andy said:
A shield is an extension of the chassis, and as such should be connected to the enclosure. Then make sure your enclosure connects to the circuit common. No! The shield of a cable can be part of the chassis, but need not. Think only about what currents might flow along the cable shield when being improperly connected to the chassis: Ground return currents of power supply, ESD currents from chassis to protective earth, secondary ESD, etc. I am thinking of a cable shield, which is intended to shield the signal wires from external interference (RFI, whatever). The LAST thing you want this shield to do is to dump that RFI noise into your sensitive circuit ground reference. By connecting it to the chassis, you do that. Of course if you have an unbalanced system, where the shield is also the signal return, you are necessarily limited in what you can do. but the point remains that you do NOT want to drain the noise into your circuit. Very sensitive circuits can contain several different shields, connected to several different local analog grounds, none of them necessarily being connected to the chassis. Perhaps your definition of shield differs from mine? -a |
| Topic | Author | Date |
| Load cell shield | 01/01/70 00:00 | |
| Earth ground | 01/01/70 00:00 | |
| where is Earth ground ... | 01/01/70 00:00 | |
| yes but.. | 01/01/70 00:00 | |
| Why Ain- is grounded... | 01/01/70 00:00 | |
| read datasheet of AD7730 | 01/01/70 00:00 | |
| it's correct | 01/01/70 00:00 | |
| try Yourself and tell us result | 01/01/70 00:00 | |
| Connect the shield to analog ground! | 01/01/70 00:00 | |
| I tend to forget | 01/01/70 00:00 | |
| Good idea! | 01/01/70 00:00 | |
| Analog gnd | 01/01/70 00:00 | |
| Exactly! | 01/01/70 00:00 | |
| Ferrite beads trick! | 01/01/70 00:00 | |
| Loadcell Shield | 01/01/70 00:00 | |
| ground planes | 01/01/70 00:00 | |
| Hhm... | 01/01/70 00:00 | |
| re: hmm | 01/01/70 00:00 | |
| interesting | 01/01/70 00:00 | |
| RF | 01/01/70 00:00 | |
| RF GSM | 01/01/70 00:00 | |
| I use use this | 01/01/70 00:00 | |
Walkie talkies | 01/01/70 00:00 | |
| Load Cell Screens | 01/01/70 00:00 | |
| JUST FOR RECORD | 01/01/70 00:00 | |
| Two enclosures, on safety class I requirements | 01/01/70 00:00 | |
| No proof, of course... | 01/01/70 00:00 | |
| Load cell cable problem | 01/01/70 00:00 | |
| fix it | 01/01/70 00:00 | |
| Problem is | 01/01/70 00:00 | |
| so make Digital Load Cell | 01/01/70 00:00 | |
| good idea | 01/01/70 00:00 | |
| give picture | 01/01/70 00:00 | |
| Screens and cables. | 01/01/70 00:00 | |
| "Fixed Structure" | 01/01/70 00:00 | |
| Fixed? | 01/01/70 00:00 |