LT3080 - PWM/DAC+Opamps VS digipot

[Tooms]

Hi

I  am an beginner in electronics and i playing with the LT3080 and can see the SET pin only has around 10 uA.

So why are you using PWM+opapms or DAC+opapms to drive the SET pin, will it not just be better to use some digipots to control it ?

Some thing like 4 cheap 8bit digipots in serias and that will give you 24bit (4*8bit) control

Thanks
Thomas

[sacherjj]

Some thing like 4 cheap 8bit digipots in serias and that will give you 24bit (4*8bit) control

It isn't quite that simple.  The "wiper" resistance of digipots is factor that will compound with more in series.  This will give you an overall offset of something other than zero. 

DAC wins in resolution and accuracy.  You would have to add up all the error in a digipot, to get your worse case error.  This would involve a shift of values, due to missing the zero point, plus worse case error of 4x the single error of an 8-bit pot.

[Bored@Work]

Digital pots are the band aids of inexperienced designers. In most cases I have seen them used it was a bodged job.

You can't get digital pots with really high resolution. 1024 is the most many manufacturers offer, if at all. 256 is common.

Digital pots have horrible tolerances, think 20%.

Digital pots can't be adjusted down to zero.

Digital pot wiper resistance depends somewhat on power supply voltage.

[Tooms]

ahh yes offcource the tolerances will get very bad when stacking up the digipots.

I was just thinking that it was cheaper and easyer to get an very high bit sample with the stacking of the digipots then it was to get the PWM/DAC with high bit samples



Thanks
Thomas

[T4P]

A DAC is easier to use 
No nasty tolerance differences and it's not easy to parallel 20% tolerance things , it can end up quite inaccurate