how to connect the analog ground to digital ground ?

[kimjin]

connecting analog and digital ground

in my design ,there are two ad converter, they both used single 5v supply,how to connect the analog ground plane to digital ground plane?i'm puzzled for days.should i use a plane for analog and digital ground?
the design is a at96 6u eurocard,the bus can supply double 12v and single 5v supply, should i divide the power analog supply from digital supply at where? and how should i decouple the analog power and ground to digital power and plane?
i have been learnd that 3 cap should be placed in the analog power entrance,and 2 caps with a beam between them is need,how should i organize the power filter circuit ?
should i connect the annlog ground to digital at the power entrance or under the ad coverter?
should the analog power and ground be divided from the digital's with 2 inductance used?
i'm a newbie ,who can explaine these questions?thank you very much.

 

[CMOS]

analog digital ground

Generally analog and digital parts are placed seperately on the PCB. Grond planes of both analog and digital are kept separate and are connected to each other only at a single point on the PCB either using a jumper (for single sided PCB) or using a thin-small trace. This jumper/thin trace acts as inductor and avoids high frequency ground interference from one side to other.

 

[IanP]

connecting digital and analog ground

This is the best discussion I have seen of why a common ground point for mixed analogue/digitial circuits is needed ..
..
The best way to handle the ground problem is to have different ground systems
that connect at only one point. The precision analog REFERENCE ground should
always be designed so that an absolute minimum of current actually flows through
it. In practice this is accomplished by having all reference ground connections
terminating at a SINGLE POINT
..
The general solution to successfully mixing analog and digital probably doesn't
exist, but everyone has a dozen or so partial solutions, and there are "very
long" threads about this on piclist - if you can find them. This whole area is
probably more art than science.

For more goto: **broken link removed**

Regards,
IanP

 

[ranier]

separating analog and digital ground

I think you can put ferrite bead between analog and digital ground to reduce interference.

thank you.

 

[vale]

how to connect analog ground and digital ground

Hi all,
A different opion that AGND and DGND grounds should not be seperated is presented in the following article:
http://www.hottconsultants.com/pdf_files/june2001pcd_mixedsignal.pdf
How do you think about it?

 

[visualart]

analog ground plane

Generally analog and digital parts are placed seperately on the PCB. If connect them together, You must connect them by the capacitance

 

[CMOS]

connecting ground planes

Generally analog and digital parts are placed seperately on the PCB. If connect them together, You must connect them by the capacitance

Capacitor? I don't agree with that. Capacitor will block all the DC content and will allow only high frequency AC. This way both analog and digital will not have a common reference ground potential.

Can you clarify your point?

 

[keenboyee]

connecting digital ground to analog ground

use a small bread to connect them

 

[House_Cat]

connecting digital and analog grounds

All of the back-and-forth discussion above misses the real issue. It doesn't make any difference whether or not you use one continuous ground plane, a split plane, or two independent planes.

What you are really trying to do is keep the return currents for analog and digital separated. You don't want a digital signal to share a return path with an analog signal. If the two share the same path, you will modulate one with the other thus causing 'noise'.

Keeping the two return paths apart is as simple as routing the signal traces adjacent to the ground plane in such a way as to keep them separated. The return side of the signal loop will follow the path of least inductance (which in most cases is also the path of least impedance). So you can assume with a fairly high degree of accuracy that the ground current for your trace follows like a mirror image on the plane directly below the signal trace. Most AD and DA converters are bonded out in such a way that the analog and digital signals are on separate sides of the package. This makes it easy to run the signal traces away from one another, and use just one solid ground plane for reference.

Keep in mind that you have to look for physical obstructions, such as blind via holes and slots in the plane, that will force the return current to flow around them. Such physical obstructions make the return side of the loop larger since the return signal can't follow directly below the trace.

People tend to think that there is some sort of magic about using separate analog and digital planes. It is only the lazy man's way of ensuring that the two types of return current (analog and digital) don't mix. It is physically impossible for a logic level digital signal return path on a ground plane to infulence in any way the analog signal return path that is an inch away. In general, if the two types of signals are kept greater than 3 or 4 trace widths apart, the coupling between the two will be at least -50db.

The bottom line - do a careful layout. Keep analog and digital signal traces separated by as far as possible. Use a solid ground plane, and route the critical signals as directly as possible.

Remember that there is no magic or mystic way that analog and digital signals will find each other on a ground plane. Simply sharing the same plane cannot in any way cause mutual interferrence. In order for one type of signal to cause a problem with the other, the two signal loops have to be mixed physically and/or electronically. You control the fields around the traces with the size and location of the traces in your layout. Those fields are the only way signals can interact.

 

[Kral]

analog and digital grounds

Kimjin,
You should route the sensitive analog gorunds in such a way that the only current that flows in the analog ground path is current that is associated with the analog signal. Providing separate analog and signal ground planes aids in accomplishing this task. The analog, digital, power ground should be connected together at one point only. Ideally, this point should be physically close to the most sensitive analog circuit in the system. Frequently this is the A/D converter. You should route the analog grounds to a separate pins on the board connector (one for each analgol input. The only signal grounds from the outside world that connect to this board ground connection should be the grounds) associated with analog input signals.
Regards,
Kral

 

[AndrzejM]

ferrite bead analog digital ground

The whole discussion in above posts are related to problem HOW to connect the ground but there is only one remark concerning the question WHY the gruond should be separated.

The answer for the second question is in my opinion more important.
1. If the ground is ideal there is no difference between the separated and the common grounds.
2. In practice however the pcb tracks have resistances idnuctances and capacitances.
From the Ohms low one can calculate how strong digital signal (and other noise signals) are added to the analog input.
U (of common GND origin noice) = R (of track resistance ) * I (of digital pulses flowing through the part of the ground path between tha reference level for analog input and the "zero" analog input ).
If one have a strong analogue level (in the range of single volts) , the mentioned above "digital noise" is neglactable and the "ground problem" do not exist.

If the analogue input lays in uV or mV level, every accidental (undefined) current flowing through the common part of the ground path will be added to useful input signal and of course will appear at the analog output as a noise (the usual components of this noise are: "digital pulses", noise from rectifier 50Hz or 100Hz ,...........)
In case of very low input signal even the separate wounding in the mains transformer is added (and rectifier and stabilisator) for TOTAL insulation of the the analog amplifier input stage.

Also additional shield for the input stage is used. This shield is insulated from the common ground and connected by the special wire to the common ground in one point only.

More complicated solutions are also used. The special amplifier to generate the "virtual groud" level could be a good example of this.

Three wires are used to connect the analog input signal: one for HIGH one for LOW and one for GND.

As a general rule , using the separate grounds for analog and digital (connected in one point) should be used as a common, good practice (It costs nothing).

 

[ptmsl]

analog and digital ground

If you don't have time to experiment and your systems specifications are not very tight use a single plane for analog and digital ground. Using seperate planes is usually looking for trouble. The routing of the signal traces is more important than seperating the two grounds. Besides if your pcb is not large the resistance of the plane is very low and if your digital signals are of lower frequency than a few MHz the inductance of a ground plane is negligible. You can use seperate supplies for analog and digital power as long as the voltage stays within the limits specified by the manufacturer of the IC. Decouple the power supplies as close to the input pins of the IC as possible and if needed use two capacitors one low value (Better use NPO/COG types) for high frequency and one bigger (>=100nF X7R dielectric type) for lower frequencies. If you use common supplies you can seperate them localy (maybe with a local plane or thick traces) using a ferrrite bead.

 

[ashokgak]

I'm not understand, why we are connecting analog and digital ground by ferrite bead or resistor or jumper? what is the use?
If we connect both grounds, analog section will take digital grounds return path and digital section will take analog grounds return path...
Then what is the use of separating these two grounds...
Please help with your answer.

Thanks in advance
Ashok