super cheap DAC idea #2 (better?)

[Hank]

Ok, I just realized I might need 4 total DAC outputs and my uC only has two 16
bit pwm outputs and one 8 bit.

:-/

Well...what about buying a single channel 10 to 12 bit medium speed DAC (about
$3 or so) and a $0.25 eight channel analog multiplexer/demultiplexer like a
74HC4051 and four 0.1uF caps to hold the voltage values. If I choose a DAC with
a 10uSec settling time (common), I can "refresh" each cap at about a 25khz rate
and buffer each with an op-amp. I would then have a 4 channel DAC for about the
cost of a single channel device.

I think the pwm scheme would be too slow to multiplex out in this fashion, so I
would have to splurge and buy the one channel DAC. Still, it is a design
savings of $9 or so, which is more than the uC costs.

opinions?

 

[Joerg]

Hi Hank,

$3 ain't exactly cheap. There should be cheaper DACs. Check out the
audio market. Just how fast does this DAC arrangement have to be?

A real crude solution for very slow outputs would be to simply take a
port or part of it, then bang out sequences into slow RC filters. The
sequences would be changed according to the desired voltages.

Regards, Joerg

 

[John Larkin]

Ok, I just realized I might need 4 total DAC outputs and my uC only has two 16
bit pwm outputs and one 8 bit.

:-/

Well...what about buying a single channel 10 to 12 bit medium speed DAC (about
$3 or so) and a $0.25 eight channel analog multiplexer/demultiplexer like a
74HC4051 and four 0.1uF caps to hold the voltage values. If I choose a DAC with
a 10uSec settling time (common), I can "refresh" each cap at about a 25khz rate
and buffer each with an op-amp. I would then have a 4 channel DAC for about the
cost of a single channel device.

I think the pwm scheme would be too slow to multiplex out in this fashion, so I
would have to splurge and buy the one channel DAC. Still, it is a design
savings of $9 or so, which is more than the uC costs.

opinions?

Quad 10-bit serial DACs are very cheap these days... $3 range. The
demux thing is hardly worth the trouble any more.

John

 

[Jan Panteltje]

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Ok, I just realized I might need 4 total DAC outputs and my uC only has two 16
bit pwm outputs and one 8 bit.

:-/

Well...what about buying a single channel 10 to 12 bit medium speed DAC (about
$3 or so) and a $0.25 eight channel analog multiplexer/demultiplexer like a
74HC4051 and four 0.1uF caps to hold the voltage values. If I choose a DAC with
a 10uSec settling time (common), I can "refresh" each cap at about a 25khz rate
and buffer each with an op-amp. I would then have a 4 channel DAC for about the
cost of a single channel device.

I think the pwm scheme would be too slow to multiplex out in this fashion, so I
would have to splurge and buy the one channel DAC. Still, it is a design
savings of $9 or so, which is more than the uC costs.

opinions?

Makes sense.
JP

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[Spehro Pefhany]

Ok, I just realized I might need 4 total DAC outputs and my uC only has two 16
bit pwm outputs and one 8 bit.

:-/

Well...what about buying a single channel 10 to 12 bit medium speed DAC (about
$3 or so) and a $0.25 eight channel analog multiplexer/demultiplexer like a
74HC4051 and four 0.1uF caps to hold the voltage values. If I choose a DAC with
a 10uSec settling time (common), I can "refresh" each cap at about a 25khz rate
and buffer each with an op-amp. I would then have a 4 channel DAC for about the
cost of a single channel device.

I think the pwm scheme would be too slow to multiplex out in this fashion, so I
would have to splurge and buy the one channel DAC. Still, it is a design
savings of $9 or so, which is more than the uC costs.

opinions?

The PWM subsytems in some micros might allow you to use an external
2-bit counter addressing the demultiplexer to synchronously switch the
PWM so you end up with f/4 at each output. If they are double buffered
and can generate an interrupt on loading.. Still two chips (plus
output capacitors and whatever buffers you need), but maybe 30 cents
total.

Best regards,
Spehro Pefhany

 

[Ken Smith]

Ok, I just realized I might need 4 total DAC outputs and my uC only has two 16
bit pwm outputs and one 8 bit.

Use two micros?

 

[Joerg]

Hi Ken,

Use two micros?

Considering that some of the uCs are under a Dollar now that is quite a
good idea indeed. But a few people would probably say that while it may
be the solution that makes the most economical sense it isn't elegant
enough .

Regards, Joerg

 

[Ken Smith]

Hi Ken,

Considering that some of the uCs are under a Dollar now that is quite a
good idea indeed. But a few people would probably say that while it may
be the solution that makes the most economical sense it isn't elegant
enough .

There is a product that has 7 micros in it. Each thing that needed doing
got an micro to do it.

If you have an extra micro you can find some cool thing to do with it like
watching over the first to make sure its working right.

 

[Terry Given]

Hi Ken,

Considering that some of the uCs are under a Dollar now that is quite a
good idea indeed. But a few people would probably say that while it may
be the solution that makes the most economical sense it isn't elegant
enough .

Regards, Joerg

I guess it depends on your definition of elegant. Personally, I do whatever
is cheapest (yet allows me to meet ALL operating requirements, especially
the tricky ones)

cheers
Terry

 

[Joerg]

Hi Terry,

I guess it depends on your definition of elegant. Personally, I do whatever
is cheapest (yet allows me to meet ALL operating requirements, especially
the tricky ones)

Same here. But I remember when I replaced a programmable phase shifter
($40 a pop or so, single sourced and all that) by a FET array, a DAC08
and some discretes. This dropped the cost to under $3 a pop. When it was
my turn during the design review the unanimous reaction was "Yecch...".
That's plain ugly, most said. But it was implemented anyway. The
financial guy really liked it...

Regards, Joerg