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Old 21st August 2010, 05:00 PM   #1
jethari is offline jethari  India
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Default Digital Potentiometers

Hi
Anyone tried using a DS1267 digital pot? I'm experimenting with one, driving it with a PIC16F628. The problem is, moment the chip is powered, the resistance value goes haywire. Even the end to end resistance is nowhere near the actual (100K). It fluctuates between 20k and 30k. With the power off, the end to end resistance shows 98.2K. I'm doing the measurements with a DMM.

Hari
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Old 21st August 2010, 07:24 PM   #2
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I haven't tried this chip, but the behaviour you describe is odd.

Things to try:

Check the supply voltage is correct and not noisy.

Check that $RST$ is tied low.

Check that the DMM is not misbehaving. Try another meter, preferably a conventional one.

Check that you're actually measuring across the correct pins. Looking at the block diagram the end-end resistance should be unaffected by the supply voltage, so there's got to be something odd going on.

w
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Old 22nd August 2010, 05:12 AM   #3
jethari is offline jethari  India
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I've checked the supply voltages and the other connections. In fact I disconnected the CLK and DQ lines from the PIC and connected RST to ground. Still no luck. BTW I have two pieces and both of them are exhibiting the same phenomenon.
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Old 24th August 2010, 03:22 PM   #4
jethari is offline jethari  India
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I've done some further checking, there is a positive voltage on the wiper pins - about 2V. I have the RST pin pulled to ground.

Hari
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Old 24th August 2010, 09:41 PM   #5
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Hmm. What voltage do you have on pin 1, Vb, the substrate bias?

If you are not intending to apply negative voltages to the resistor inputs, tie this pin to ground, 0V.

w

Last edited by wakibaki; 24th August 2010 at 09:53 PM.
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Old 25th August 2010, 01:40 AM   #6
jethari is offline jethari  India
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Vb is tied to ground.

Hari
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Old 25th August 2010, 05:57 PM   #7
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OK, I would try clocking a value into the shift register using switches.

The wiper value should be 50% on power-up, so this is a last resort.

You need 3 switches. Pull RST, CLK & DQ down to ground using resistors, say 5k. Connect the switches to +VCC and the I/O pins so that you can control the state of the pins using the switches.

Set RST low, CLK high and DQ to high or low as you choose.

Set RST high and toggle CLK low then high. Repeat 16 times, setting a value for DQ as you choose, until you have clocked 17 values into the shift register. (Could be all '1's.) Set RST low.

Try measuring the end-to-end and wiper resistances again. If this fails, then the chips are faulty, for whatever reason.

w
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