Hi,
I need to make a volume control for a stereo balanced signal. Since 4-gang pots are difficult to get, I have been considering using a digital potentiometer IC.
There are lots about; Wolfson WM8816, Maxim DS1844, National LM1972 etc..
My question is whether these can really be thought of as passive resistive elements or do they introduce distortion of their own (say though transistor switching of the resistor ladder)? The WM1816 datasheet gives a THD+N figure of -100dB but maybe this is really down to the op-amp buffer?
I need to make a volume control for a stereo balanced signal. Since 4-gang pots are difficult to get, I have been considering using a digital potentiometer IC.
There are lots about; Wolfson WM8816, Maxim DS1844, National LM1972 etc..
My question is whether these can really be thought of as passive resistive elements or do they introduce distortion of their own (say though transistor switching of the resistor ladder)? The WM1816 datasheet gives a THD+N figure of -100dB but maybe this is really down to the op-amp buffer?
The switches are the most non-linear part in my understanding - CMOS switches have a non-linear voltage vs resistance curve. This should mean the distortion reduces as the signal level goes down. Whilst the output opamp won't show much THD, chances are the audio performance is limited by low level IMD.
You can get passive-type digipots, but they tend to have quite high distortion.
You're better to go with the chips intended as volume controls such as the Wolfson you named or the TI PGA23xx series or Cirrus equivalent. I think the TI chips actually have the best paper performance, but these things are not set in stone.
The volume controls are not purely passive, they generally offer gain as well as cut, this is provided by internal opamps. This is not a good reason to reject them as the levels of distortion they achieve are almost certainly sub-audible. Some are only intended to work with maximum 5V levels, but some will work with +/-15V.
A device that shows low THD is very unlikely to have high IMD.
Chances are that the IMD is insignificant.
You're better to go with the chips intended as volume controls such as the Wolfson you named or the TI PGA23xx series or Cirrus equivalent. I think the TI chips actually have the best paper performance, but these things are not set in stone.
The volume controls are not purely passive, they generally offer gain as well as cut, this is provided by internal opamps. This is not a good reason to reject them as the levels of distortion they achieve are almost certainly sub-audible. Some are only intended to work with maximum 5V levels, but some will work with +/-15V.
A device that shows low THD is very unlikely to have high IMD.
Chances are that the IMD is insignificant.
Thanks for the replies chaps. Looks like the LM1972 is the best from the spec sheet. However I know nothing about programming micro-controllers! I think that is something to spend a bit of time reading up on 
On the other hand, if I got a order of quad linear pots (Vishay P9) would people be interested in buying some off me? Obviously linear pots can be make log with the addition of a law faking resistor. I guess I'd go for either 1K or 5K.
On the other hand, if I got a order of quad linear pots (Vishay P9) would people be interested in buying some off me? Obviously linear pots can be make log with the addition of a law faking resistor. I guess I'd go for either 1K or 5K.
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