Digitial POT AC response?

[mrblueblue1234]

Hello everyone!

1.) I am using a digital potentiometer (POT) in my project and having a problem with the waveform being distorted through he part. But before I do anything I set the digital POT to ~1k and I measure the resistance through the POT with my multimeter and I measure about 1k. This is good but it is the DC resistance.


2.) But when I apply an AC signal ~230 Hz through the POT the waveform that I receive through the device is extremely distorted.

What I am expecting is a smooth uniform sinewave but the waveform looks something like this:

3.) If I remove the digital POT and replace with a physical through hole 1k resistor I get an excellent waveform that looks similar to the below image. I don't have a photo of the perfect waveform that I get but the below shows a representation of how it looks.

3.) There sees to be an issue when I apply an AC signal to this part. I was wondering if any one had some ideas of explanations or possible fixes?

This is the part number:

Datasheet:

https://ww1.microchip.com/downloads/en/DeviceDoc/22096b.pdf

Thank you for any ideas!

-MrBlue

 

[wwfeldman]

2.) But when I apply an AC signal ~230 Hz through the POT the waveform that I receive through the device is extremely distorted.

What I am expecting is a smooth uniform sinewave but the waveform looks something like this:

View attachment 163584


3.) If I remove the digital POT and replace with a physical through hole 1k resistor I get an excellent waveform that looks similar to the below image. I don't have a photo of the perfect waveform that I get but the below shows a representation of how it looks.

View attachment 163585

What do you mean: "looks something like this:"
and "I don't have a photo of the perfect waveform that I get but the below shows a representation of how it looks."

You clearly have screenshots(?) of something.
Maybe you should show the actual waveforms instead of representations

What is attached to the output of the digital pot?

a quick run through the data sheet only showed a test circuit
where an op amp follower is connected to the output of the wiper
what is connected to your digital pot?

 

[Easy peasy]

how many volts are you applying to the pot ... ?

 

[mrblueblue1234]

.....
What is attached to the output of the digital pot?

The output of the POT is connected to ground.


a quick run through the data sheet only showed a test circuit
where an op amp follower is connected to the output of the wiper
what is connected to your digital pot?

One side of the POT is connect to the output of a DAC that is generating a sine wave and the other side of the POT is connected to ground

 

[KlausST]

Hi,

Most probably you did not keep on
Resistor Terminal Input Voltage Range | (Terminals A, B and W) | VA,VW,VB: Vss — VDD

Edit: Just realized that you wrote 230Hz, not 230V. Thus the folowing sentence is invalid
(If you connect 230V AC to the pot I xpect it to immediately explode.)

Show your schematic. Complete related to all pot connections.

Klaus

 

[FvM]

The requested schematic circuit should also indicate the supply voltage. Negative potentiometer voltage vas shown in post #1 is surely exceeding the accepted device vltage.

 

[mrblueblue1234]

1.) Here is the how the POT is being used:

Vin is a sine wave with a peak-to-peak voltage of 3.3V. The digital POT is programmed for 1k. Measuring the DC resistance across the POT shows 1k and all is well at DC. Vout then goes to our analog signal chain that includes amplification and filtering.

2.) In the divider above "z" is the digital POT and the connections on this daughter board are shown below.
If I measure the DC resistance from the point S1 to ground I measure 1k. From point S2 to ground also 1k. If I then start driving an AC signal "Vin" to pins 2 and 3 of J11 this is when I start seeing the measured waveform being distorted. If I remove the POT U6 from the board and replace with normal resistors (wired up some through hole resistors) the waveforms look fine. Jumper JP5 is installed in this situtaion.

3.) Could it be a layout issue with the part? Reflections? Some bias issues within the chip when the waveform changes?


4.) Here is a picture of a good waveform using a through hole resistors. Driven frequency is 230 Hz for all images.

5.) Now with the digital POT:

The amplitude is larger than expected here and also dipping below my 0V point. Maybe these are reflections?

Also when I am using the digital POT there is an external wall wart power supply that is used that is connected to the common ground. This is then regulated down to power the digital POT and other electronics.

When I am getting the clean waveform, bypassing the digital POT with the through hole resistor the external power supply is still connected in this case. To me this seems to indicate this is not a power supply, grounding issues but an issue with the POT.

Thanks for any suggestions!

-MrBlue

 

[stenzer]

Hi,

The amplitude is larger than expected here and also dipping below my 0V point. Maybe these are reflections?

I doubt that, because the wavelength for 230 Hz frequency signal would be about 1000 km.

I don't know how the DC voltage ratings of your circuitry looks like (i.e. input voltage of your 3.3 V LDO) neither the power consumption, but you may test your circuitry with a battery e.g. 9 V. This would be an easy way to identify if the SMPS is the problem source.

BR

 

[KlausST]

Hi,

"Vin is a sine wave with a peak-to-peak voltage of 3.3V."

What does this mean?
from -10V to -6.7V = 3.3V pp
from -1.65V to +1.65V = 3.3V pp
from 100V to 103.3V = 3.3V pp
--> give absolute min and max values.

Klaus

 

[mrblueblue1234]

All,

Thanks for all the posts. This turned out to be a grounding issues. When I connected my daughter board ground directly to my external system ground the problem was resolved!

-MrBlue

 

[RCinFLA]

Looks like part uses T-gate analog switches. Signal has to stay within bounds of power supply voltage.