Hello everybody...

What we are building... :
We are building a mini laptop by assembling a Single board computer (SBC), DC-DC converter (24V i/p 5V and 12 V o/p ), LCD, mouse, 4x4 matrix keyboard etc.. for a particular application.
We have one more module(small PCB) which is connected to SBC through PC104 and line-in. Also it shares same power supply.
We have designed a PCB (called as base board) which holds this module and SBC together. Base board is used for routing lines of pc104, audio, mouse, usb, vga, ethernet etc. It is multi layered and one layer is ground (reference line) itself.
We have used 2W audio amplifier ST TDA7268 which takes i/p from line-out and feeds to a 8 ohm speaker.
computer is loaded with Windows XP.
All the modules work fine when tested separately..

Problem: When the computer is on there is a constant humming noise (loud enough) in the speaker. Also while moving the mouse (no matter usb or ps2) some other noise adds to it. Same is the case if you keep pressing a key in the keyboard.
Since our final product is simply 'a laptop + our module' connected to it through PC104 and line in, detaching the module simply leaves a laptop. (laptop in the sense every part is separately bought and integrated)
Case1: Completely detach 'our module' (testing the laptop built by us)
observations:
- Constant loud humming back ground noise
- PS/2 mouse adds noise while moving
- USB mouse adds noise while moving
- Keyboard (both external and our 4x4 keyboard) causes noise if a key i keep pressing a key.
- In windows we have muted all the input audio except line-out (line-in, cd-in etc). It has no effect.
our conclusion:
- No idea why this loud back ground humming is appearing. Could not conclude.
- mouse/usb/keyboard clock is causing noise. May be problem is with dc grounding. (But we have one complete layer of base board PCB grounded (reference), and all grounding connections are made to that. It will give a star like connection. So i hope ground loops cannot happen).
( What else can cause this kind of problem?)
Case2: Our module connected (It shares power supply, PC104 with SBC, also connected to line-in)
observations:
- Same humming, mouse, keyboard noises.
- When in windows line-in is muted, no noise. Every thing works perfectly.
- When line-in connection from the module to SBC is removed (same as muting line-in), no noise.
Conclusions:
Couldn't come to any conclusion because in previous case line-in had no effect, now it is completely depending on line-in.

Other observations:
- With proper capacitors added, no fluctuations in power supply lines.
- When checked separately audio amplifier is working fine.
- Please note that we have connected dc ground to chassis of the laptop.
Constraints:
- We cannot mute line-in while operating. It is very much needed. Just for checking we muted line in.
- We are using complete audible frequency range. So filters cannot be added.
- We are using 24V DC battery for power supply. (given to DC-DC converter to get 12V and 5V). So there should not be ground loops which can be caused from earthing line (third line in ac supplies; usually connected to the body of an electronic device. Also called as ground).

What I want...:
- There should not be noise from mouse/keyboard.
- Background humming noise should be minimal.
- I want to know whether it is grounding issue.
- I am stuck;not being able to suspect on anything. So, what do you think? wha is causing noise? Can anyone give some probable causes?
- Can anyone suggest me how to proceed my troubleshooting process?
- I may not be able to give schematics. Since it is a normal laptop, I request you to 'guess' the problem with your experience and help me with some suggestions.

Thank you for reading my problem patiently...
Waiting for your response.
KTHURST.

 

You are probably picking up electrical noise from the processor and other devices. It sounds like your case and power supply are not well shielded. A lot of time and money is spent on computers & laptops to insure they do not radiate noise.

Battery power is no guarantee, by the way. I once ran a noisy device from external batteries and discovered that the battery leads were radiating so much noise that other devices were affected.

 

We have used one more amplifier. ST TS4984. And output of this is given to headphone.
In that same kind of noise exists, but it is not noticeable.
(One of the two o/p audio channels(left and right channels) is given to 1W audio amplifier(-to headphone-)
and other channel is given to 2W amplifier(-given to speaker- )). We cannot use that 1W audio amplifier for speaker because audio gain is insufficient (audio is not loud enough).
I have attached 2W audio amplifier (TDA7268) datasheet. In that I have shorted signal ground and power ground.
Will that cause any problem?
Can audio isolation transformer help?

 

The noise is inherent to the switching of transistors. Some time ago, we had a music program. It was mostly a set of shift commands. You opened the doors of the computer and held a radio in front- the AM tuner picked up the switching noise. The organized shifting sounded like different tones, so you could compose simple melodies.

Because the noise is ubiquitous, it is very hard to suppress. Look at the metal cases with internal shields and grounding fingers. It is hard to keep the noise inside the case.

 

You basicly need to separate the audio ground from the digital ground. Best approach is star-grounding, in the way that these two grounds meet a single point in the circuits, preferably at the decoupling capacitors in the on-board (i guess?) power supply.

If that is not possible, you could try inserting for example 10ohm resistor between the central ground and the audio-part ground. Also you can add decoupling capacitors or CRC filter in the audio part power supply.

You just have to try to separate the audio components from the digital as much as possible.

 

Thanks for your reply...

One thing I must mention which I have observed while troubleshooting. I tried with different audio amplifiers. All have noise problem. But one IC (TS 4984) has clear audio quality which we were expecting our system to have.
The difference between this IC and other ICs is that it has got differential output. So power ground will not come into picture at the output stage. (??)
I am not being able to understand it properly. Can anyone now guess what would be the problem? Please give a try to explain.

Regards,
Kthurst.