[SOLVED] Generic Advice needed on Grounding

Hello Everyone,

I am working on a sensor data aggregation system that have analog signal conditioning circuits and I-V converter circuits built from Op-Amps. Analog feed from each of this sensors will be taken to the on-chip ADC of the ARM based processor for further analysis.

I would like to know if this would be a good idea to connect all the ground points of these Op-Amps to say "Analog GND" and the processor ground points to say "Digital GND" and finally short these grounds "somewhere" on the PCB with a "resistor" ?

I am seeing some noise in the first proto of this design, so your advice on separating the two grounds will help me in designing a "cleaner" circuit.

Looking forward for your advices..

Hi,

I´d say the problem is to find the source of the noise.

Is it the digital section, then your solution is fine.

If the noise comes from the sensors, then the noise (current) needs to travel across the complete analog section and results in an increased noise level across the analog ground plane.

In case you have shielded cables to the sensors with additional analog ground wires (inside), Then I´d connect the shielding not to the analog ground plane, but the analog signals wires to the analog ground plane.

It depends on the application.
There is no general valid solution.

Klaus

KlausST said:

Hi,

In case you have shielded cables to the sensors with additional analog ground wires (inside), Then I´d connect the shielding not to the analog ground plane, but the analog signals wires to the analog ground plane.

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Thank you KlausST .. just to add one more point here, cables attached to the sensor are not shielded. I feel the source of noise is from the digital section because these sensors are extremely low power and low speed, am I correct in assuming such signals might only get affected by the noise, but cannot be a source of noise themselves ? In that case do you suggest that I can have two ground islands (connected later on the PCB) ?

In general you keep the analog and digital grounds separate, and combine that at a single point near where the analog signal is converted to digital (in this case the ARM processor).
The analog and digital power should also be kept separate.
You can tie the two grounds together with a low resistance ferrite bead to further minimize noise coupling between the two grounds.

I feel the source of noise is from the digital section

Click to expand...

A lot of other factors may also affect the quality of the signal, such as :

• Distance between sensors and the board.
• Supply voltage of sensors and the A/D references.
• Level of EMI at the surrounding environment.

So, it is hard to say from where comes the most significant part of the noise.

Hi,

Low power sensors maynot generate much noise.
But the signals of unshielded cables may be influenced by noise signals in the environment.
Mains frequency, inductive or calacjtive coupling of noisy signals nearby, or the wires to the sensor may act like antennas.

In either case decide to use RC, LC (damped) and/or common mode signal filters at the signal input.

With a true anti aliasing filter you may use software filters, they are more flexible.

Sometimes there are known sources of noise, like SMPS, motor controllers...then you may focus on these frequencies.

Klaus

KlausST said:

Hi,

In either case decide to use RC, LC (damped) and/or common mode signal filters at the signal input.

Klaus

Click to expand...

Yes, this would be really good. let me explore the options..

KlausST said:

Hi,

Sometimes there are known sources of noise, like SMPS, motor controllers...then you may focus on these frequencies.

Klaus

Click to expand...

SMPS sounds like a very good possibility.. I will try powering the circuit with a 9V battery and check the results.

Thanks a lot again