Re: digital analog conversion

to

My gut feeling says that when you use a 1uF cap (MKT or MKS) and a mosfet opamp to follow, the voltage will have very little droop.

No, because you charge from a low impedance output of an opamp, via an electronic switch (mosfet). Try a google search for sample and hold amplifiers.

Meindert

Just to put my aim in perspective: I'm neither trying to fool you nor trying to get my homework solved (like a given individual presumed). Why I am talking about a basic condition with respect to the "one D/A converter for multiple output channels"-configuration is that this single D/A converter already exists in hardware. It is there, physical, for me to touch, already bought... And now I want to use this very D/A converter to feed several output channels. Of course I could buy a DAC for every channel but that's not my intention. The hardware setup does not allow to solder other devices on the board. So PLEASE just take it as it is! I want to solve the problem that way. So don't try to proselytize me like that jehovah's witnesses guys... ;-)

Hope you come up with more constructive suggestions!

Veronica

Yes, a sample-and-hold can be constructed with a switch, cap and op-amp. Be sure to use low current and leakage types. There are also specialized S&H devices.

But why is the requirement to hold the output for 5 minutes? Is it not possible to update them a lot faster? Just keep updating the outputs with the same value for 5 minutes, this lowers the requirements for the S&H.

I understood you hope to hold the outputs for 5 minutes. Though theoreticaly one could achieve this with a high value capacitor/analog switch/amp op as suggested by some, that wouldn't be the wiser approach due to capacitors and switches leakages and amp ops' input bias current, that increase with temperature, not to mention leakages due to contaminants at the printed circuit board. Even if you use a sample and hold circuit I don't thihk they guarantee 5 minutes hold. Nowadays the best approach hands down is to use one of these multi channel D/A that are rather inexpensive. I would consider a requirements change.

Regards.

Elder.

And that was a clever, saavy, high brow, smart reply I suppose?

Ian

It's simple math dV/dT = I/C Figure out how much droop you can stand in 5 minutes. Figure out how big a cap you can stand. That will tell you the TOTAL from all sources leakage you can stand. The current output of your D/A into the cap will give you the charging slew rate.

While it would be possible to do what you want, you won't like the price tag. You'd be much better off to "refresh" the cap more frequently. The above equation gives you the tradeoffs directly. There is no free lunch. mike

Uh, no, it was meant to be punitive, to the best of my feeble ability. Do you think it was uncalled for?

Does it look to you like they do think it was uncalled for?

-- Lady Chatterly

"She's a bot, you fool." -- Homerun Frogbutt

I do.

Ian