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Strange behaviour from the power connecitons to my prototyping board

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Aussie Susan

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OK, it's dumb question time.

I have built up a circuit using an AD9958 DDS chip on a SchmartBoard as a prototyping platform. The board also holds the crystal, a few capacitors, resistors etc that are needed dot make the chip work, but most of the pins are just connected to the +1.8V power source or ground wires or the signal/control wires that connect the board to a PIC controller.

The PIC controller works OK on its own but it has the power regulators to the DDS prototyping board.

The two boards can be unplugged and the following describes the behaviour of the prototyping board on its own.

My problem is that, using a DMM, there appears to be a short between the +1.8V rail and ground, but after a second or so, the meter shows the "resistance between the power and ground rails as increasing to about 3 or 4 ohms over a 5 (or so) second period and then stabilising at that reading.

My first thought it that there is a short on the board but 1) I can't see it (that doesn't mean it's not there....) and 2) why does the short become a low valued resistance after a few seconds.

My next thought is that I'm seeing a bypass capacitor charging up, but I can't see why it stops after a few seconds and shows a couple of ohms?

Before I strip the board down and try to recover my DDS IC, can anyone suggest what might be causing the effect I'm seeing?

Thanks

Susan

(PS in detail: there is a 26.9MHz crystal between 2 of the pins with 39pF capacitors to ground on each side for the internal oscillator to use. There is a 1K91 ohm resistor from another pin to ground to set the reference current for the DAC's. There is a 680pf capacitor to ground in the PLL loop filter. All of these are as per the data sheet. All of the analog and digital groups are connected together [I know - but for my initial purposes I should be able to get away with this]; ditto the analog and digital 1.8V connections. There is also a 3.3v source being fed to the chip that shares the ground connection).
 
You should check that the DMM shows a short as 0 ohms, by touching the leads together. If not, fix the DMM first.

It could be that you are reading the resistance with the polarity reversed. If the bypass capacitors are charged, then it will take a few seconds for the DMM to discharge the capacitors. Until then, the resistance will read a zero. The rising value could well be the bypass capacitors charging up, possibly in reverse. The couple of ohms could be the diodes inside the DDS chip that stop the power supply going negative.
 
Thank you for the comment. I'll follow it up when I get home tonight.

I was discussing this with a friend of mine as well and he suggested that the DMM may be supplying enough voltage to the chip to be (partially?) operating it - it only needs 1.8V! In that case I could be seeing just about anything.

Susan
 
Just to finish off this thread....

Last night I tested the DMM by shorting the leads together while on the "Ohm" setting. The DMM read all zeros, but after a second or so (longer than I would normally have held the leads together to test for zero) it began to climb in exactly the same way as I saw with my circuit board. So Diver300 was absolutely right - it was the DMM leading me astray.

Now all I have to do is find the short circuit on the board!!!!!!

Thanks

Susan
 
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