Adjustable voltages - What to use? Digipots or DAC?

Thread Starter

atferrari

Joined Jan 6, 2004
4,195
I intend to implement four adjustable voltage signals to control curent sources of different types. Current sources are tested and work OK.


Control is basically up/down with a fixed minimum value at turn on (eventually I could retain the last in use at turn off).


After much reading I focused on two quad options by Microchip: DAC MCP4728 or digipot MCP4361. Datasheets attached.


Besides the complexity of controlling software with a small micro (not a problem for me), what is your opinion about which one to choose?


Comments, suggestions appreciated. No previous experience on this myself.
 

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crutschow

Joined Mar 14, 2008
26,351
Three differences come to mind.
The DAC has a lower output impedance with a buffered output.
The DAC has a built-in voltage reference.
The DAC has 12-bits of resolution and the digipot has eight or nine.

So, unless you need the 3-wire potentiometer function, I see no reason to use the digipot for your application.
 

Joël Huser

Joined Jun 30, 2019
42
As crutschow said, DAC has a built-in voltage reference, that's why the 4 buffered outputs are very precise and supposed to be stable in time and in temperature. I am not sure that the digipot has as good parameters as the DAC..... And is this case, indeed, the DAC is basically more precise with its 12 bits resolution.

So, I would say that you should use a DAC rather than a Digipot.

But both components are made for both different situations. You can face situations where you cannot use a DAC, because a DAC provides you a variable voltage signal referenced to a GND. In contrary to the digipot which is supposed to be a passive element free of potential (in the range of the digipot's power supply I suppose)..... So the digipot is useful, when you has to vary a resistor included and nested in an analog circuit, where you righly cannot use DAC !

Joël
 

Thread Starter

atferrari

Joined Jan 6, 2004
4,195
Three differences come to mind.
The DAC has a lower output impedance with a buffered output.
The DAC has a built-in voltage reference.
The DAC has 12-bits of resolution and the digipot has eight or nine.

So, unless you need the 3-wire potentiometer function, I see no reason to use the digipot for your application.
So evident, Carl. :) Thanks a lot.
 

joeyd999

Joined Jun 6, 2011
4,477
Here is a digitally controlled adjustable current source that I developed for a product about 15 years ago using a MCP41050 50K digital pot. It works well.

The current is computed as 1.22V/(R205 + the resistance from PW to PA) and is injected into the SEN+ pin. A buffered (and filtered) copy of the resulting voltage between SEN+ and SEN- (circuit ground) is produced at Vo.

Caveat emptor: the current is dependent upon the tempco of the digital pot.

Selection_039.png
 

Joël Huser

Joined Jun 30, 2019
42
Beautiful joeyd999 ! The operational amplifier mounted as a follower make things happen so that its output is at the same potential as its positive input. So it's like if U204 was directly in parallel of (R205 + R of U202), fixing the current through R206, R205 and SEN+/-. Very interesting circuit. Joël
 

danadak

Joined Mar 10, 2018
4,057
One possibility. This is a dithered 12 bit from 8 bit DAC, there are 4 onboard. If
you used its internal standard DAC its 8 bits each, no cap needed.

Vref is onchip as all the rest, OpAmps, DAC, except caps. Lots of resources left,
right hand window shows used/left.

If you needed more channels and can live with settling time and/or need higher
res use PWMs. This chip you could probably do >> 10 DACs at 16 bits that way,
but each needs an RC filter and you would need a buffer for each, although 4 could
be buffered with the 4 OpAmps on chip.

Stuff unused like DSP section, ARM M3 core, COM, LUT, Logic elements, counters,
A/D DelSig 20 bit and 12 bit SAR..... Component list attached. A component is an
onchip resource.

upload_2019-7-2_15-27-27.png

$ 10 board (with debug) - https://www.cypress.com/documentati...oc-5lp-prototyping-kit-onboard-programmer-and




Regards, Dana.
 

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