Weird problem with DIT4192 SPDI/AES/EBU Transmitter - diyAudio
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Old 16th January 2009, 12:26 PM   #1
Salar is offline Salar  Germany
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Default Weird problem with DIT4192 SPDI/AES/EBU Transmitter

Hello,
3 years after installing a digital transmitter DIT4192 in my Nakamichi OMS-5EII,
see this
diyaudio-thread

I found out, that the circuit radiates heavily.
(i.e the image on my analog TV was noisy, when the Nak was switched on)
To make a long story short, (besides the fact that the circuit was not properly grounded), I installed a switch, to interrupt power, both +5V and ground.
BUT:
The DIT4192 is ALWAYS working, nevertheless when being switched on or off!

With ground and +5V Voltage disconnected and only the signal inputs connected, I can always measure around 3 V between the +5V input of the DIT4192 circuit and chassis ground. Even connecting the DIT4192 circuit ground to chassis/player ground will still give +3V in in the DIT4192 +5V input.
When chassis ground is diconnected, I also have some small voltage on the DIT4192 circuit ground.
So, this thing works (no audible distortions), even without being
powered...

The only explanation I have is that voltage from the DIT4192 signal inputs (Oscillator Inputs, Bit Clock, Word Clock/L-RClock, they are about 2V) is leaking through the circuit.
I checked the circuit several times, but it seems to be O.K, no leakage because of shorting wires and connections.
The tentlabs oscillator and the DIT4192 have their own power supplies, but their grounds are connected to the player´s power supply ground. So I guess no problems there

So, is this a normal behaviour that the DIT4192 could run from
signal voltage only? Or did I blow the DIT4192 to some unstable state?
All the best,
Sal
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Old 17th January 2009, 02:10 AM   #2
dmills is offline dmills  United Kingdom
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Location: High Wycombe
That sort of thing is certainly not unheard of, I had a entire cmos logic board do that to me once, dozens of 4000 series chips and it would work 'most of the time....', eventually I found that there was a condition that occured about once a week or so where there would be insufficient inputs at logic one and some of the logic would reset.

Mr muppet here had forgotten to hook the 5V rail up to the board!

I would solve your RFI problem rather then adding a switch (may be board layout, decoupling or grounding).

Regards, Dan.
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Old 17th January 2009, 07:34 AM   #3
Salar is offline Salar  Germany
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Join Date: Feb 2006
Location: Germany
Hello Dan!
Thank you very much for the reply!
I will rebuild the board with the DIT4192 only + output transfomer, using a SSOP to
DIP converter board. I could build/glue it in a sandwich-like style,the board with chip, one isolated thin copper plane for positive voltage connections, one plane for negative voltage.
But are planes better for RFI? Or should i keep the common connections for supply voltage and GND as physically as small as possible?
All the best,
Salar
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Old 17th January 2009, 08:09 AM   #4
jahonen is offline jahonen  Finland
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Location: Kuopio, Finland
Quote:
Originally posted by Salar
But are planes better for RFI? Or should i keep the common connections for supply voltage and GND as physically as small as possible?
All the best,
Salar

I'd say that plane is a necessity if you are targetting to low EMI. Basic rule to robust and clean digital circuit is basically to use tons of ground. It is the ground/signal inductance that matters above all.

But, you'll also need to ensure that the trace is close enough to the plane for image currents to effectively cancel the trace currents. This means that trace distance through dielectric must not be larger than width of the trace (this makes about 75 ohms impedance traces). For digital circuits this means that you'll need to use very thin PCB or (more practical) use at least 4 layer PCB (not so expensive nowadays anymore).

Also, where you hook digital signals around, always ensure that each signal has current return path (ground) very close to signal itself. Otherwise, it is an invitation for EMI failure.

Regards,
Janne
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