Do I have to buffer a digital audio pot?

Thread Starter

szabikka

Joined Sep 3, 2014
94
Hello everyone!

I have ventured into a part of electronics I am not familiar at all - namely digital audio potentiometers. I'm planning a project that interfaces a DS1801 digital audio pot in between a computer's speaker output and a TEA2025 power amplifier, which itself drives two 4 ohm speakers. Other than the datasheet I have not found too much on the internet about the DS1801 (I was hoping to find a general app note). I found however a design note where the wiper outputs are buffered with rail-to-rail op amps. The schematic of the design note only shows that the buffered output is going to some connectors, so I can't deduce if the need for buffering applies to my own case too. The TEA2025 has a relatively high input impedance of 30k, based on that information only, I would hesitate to use any kind of buffering because I think it's irrelevant in the case of such a high input impedance and I'm afraid it would cause unwanted signal distortion. I'm attaching datasheets for the TEA2025 and the DS1801 and I also attach the design note I found.
I have to add I'm planning to connect the DS1801 and it's controlling MCU to a 5V supply while the TEA2025 would be fed from a 12V supply.
My questions are: Do I have to use a buffering op amp in my case and if yes, should I connect it to the 12V or the 5V supply?

Thank you for the help in advance!
 

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KeithWalker

Joined Jul 10, 2017
1,919
The most important thing is to make sure that the inputs to the digital potentiometers never exceed VCC (5V in you case) or go lower than ground potential. Either condition can damage the chip. Therefor you will need to attenuate the 12VPk output from the amplifier.
The buffer on the output is a good choice because drawing too much current from the output can also damage the chip.
Use a star ground for the circuits to keep the potentiometerssafe from ground loop transients.
Good luck.
 

Thread Starter

szabikka

Joined Sep 3, 2014
94
The most important thing is to make sure that the inputs to the digital potentiometers never exceed VCC (5V in you case) or go lower than ground potential. Either condition can damage the chip. Therefor you will need to attenuate the 12VPk output from the amplifier.
The buffer on the output is a good choice because drawing too much current from the output can also damage the chip.
Use a star ground for the circuits to keep the potentiometerssafe from ground loop transients.
Good luck.
Thanks Keith! Yeah, I thought about the 12V peak from the amp. But a simple capacitive coupling between the digital pot and the amplifier input is enough to tackle that problem... I think. I always include a star ground in audio circuits, otherwise my designs are just noise generators... yeah, I'm no expert, just a hobbyist.
 

KeithWalker

Joined Jul 10, 2017
1,919
Thanks Keith! Yeah, I thought about the 12V peak from the amp. But a simple capacitive coupling between the digital pot and the amplifier input is enough to tackle that problem... I think. I always include a star ground in audio circuits, otherwise my designs are just noise generators... yeah, I'm no expert, just a hobbyist.
Don't use capacitive coupling from the amplifier to the potentiometer. That will not reduce the overall amplitude and the signal will go below ground potential. Use direct coupling through a resistive divider to maintain the correct DC levels. The output from the amplifier will go from ground to +12V max. The directly coupled attenuator should reduce the signal to go from ground to +5V max.
 

Ian0

Joined Aug 7, 2020
3,280
Provided you don't exceed the supply voltages (as mentioned by @KeithWalker), they behave much like an ordinary pot. The only slight difference is that there is a resistance (400Ω in the case of the DS1801) in series with the wiper terminal. Most of the time that won't make the slightest bit of difference.
So you can design around them much the same way as you would any other potentiometer.
 
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