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decent but affordable 16 bit DAC

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OM222O:
As others pointed out, INL and DNL are your major constraints here and you have previously mentioned that you are not able to calibrate them out  :-/O which is a bit painful. the LTC2601 is a reasonable replacement, but still not fantastic. at worst case scenario it has 64 bits INL  :palm: making it a 10 bit ADC ... so cross your fingers and hope you get a golden sample ... or order a few and test them individually (I once had to do that with op amps since the more expensive stuff would have costed about more than 10 of the cheaper ones  :-DD I got lucky and found a golden sample in them).

what op amp are you using for the gain stage? I suggest a rail to rail chopper amplifier since you need low input offset voltage for any level accuracy. maximum allowed offset would be the tolerance, divided by gain, so 1/6 mV in your case.

David Hess:

--- Quote from: mariush on June 08, 2019, 02:23:48 pm ---Here's a thought .... could you compromise in some way and configure the gain of your opamp dynamically depending on the output voltage, so you'd get 1mV steps below 5v, 2mV below 10v, 5mV below 20v or something like that?
--- End quote ---

That is not an uncommon way to handle it.  Often you have to dig into the specifications to find if this was done.

nemail2:

--- Quote from: mariush on June 08, 2019, 02:23:48 pm ---Here's a thought .... could you compromise in some way and configure the gain of your opamp dynamically depending on the output voltage, so you'd get 1mV steps below 5v, 2mV below 10v, 5mV below 20v or something like that?

Maybe it's something as simple as shorting resistors on the gain of your opamp to take them out of circuit?

--- End quote ---


--- Quote from: David Hess on June 08, 2019, 06:29:40 pm ---That is not an uncommon way to handle it.  Often you have to dig into the specifications to find if this was done.

--- End quote ---

nah, wouldn't be an issue for me. I mean, who needs 12.000V and couldn't get away with 12.010V (if that'd be the only settable voltage due to resolution)? I know, bad example - but for me, that wouldn't matter.

How'd I do that, with MOSFETs? Are they the way to go? Or is it another "it depends" like always in engineering?  :scared: :-DD


--- Quote from: chris_leyson on June 08, 2019, 03:07:15 pm ---"DAC 2 Click" from MikroElektronica uses the 16-bit LTC2601 which has 8 LSB INL, and the "ADC 8 Click" uses the ADS1115.

--- End quote ---


--- Quote from: OM222O on June 08, 2019, 05:13:28 pm ---at worst case scenario it has 64 bits INL
--- End quote ---

^
|
This... that's a horrible number... The ADS1115 is pretty decent though, and I'm going to use that as it performed well on my breadboard tests with my Lab PSU circuit on a PCB already (bodged in).


--- Quote from: OM222O on June 08, 2019, 05:13:28 pm ---so cross your fingers and hope you get a golden sample ... or order a few and test them individually (I once had to do that with op amps since the more expensive stuff would have costed about more than 10 of the cheaper ones  :-DD I got lucky and found a golden sample in them).

--- End quote ---
:-DD nah, not gonna do that. I'll be better off, just ordering a DAC which doesn't have those bad figures in the first place.


--- Quote from: OM222O on June 08, 2019, 05:13:28 pm ---what op amp are you using for the gain stage? I suggest a rail to rail chopper amplifier since you need low input offset voltage for any level accuracy. maximum allowed offset would be the tolerance, divided by gain, so 1/6 mV in your case.

--- End quote ---
OPA2197, they seem to work pretty good, in general I get really good voltage setting results with my current setup (MCP4922 + OPA2197) with measured voltage being in general within 0.1% of the set value (measuring with my multimeter + the ADC, both measurements match).


--- Quote from: OM222O on June 08, 2019, 05:13:28 pm ---and you have previously mentioned that you are not able to calibrate them out  :-/O which is a bit painful.

--- End quote ---
yeah, that's simply beyond my knowledge, I wouldn't even know where to start. and even if, I'd need somebody to explain it to my like to a braindead monkey before i'd get it... unfortunately I sometimes do learn really fast and sometimes things take ages.


--- Quote from: David Hess on June 08, 2019, 06:29:40 pm ---That is not an uncommon way to handle it.  Often you have to dig into the specifications to find if this was done.

--- End quote ---

I guess I kinda like the idea, however I'd be happy if I'd get away without having to implement that as it'd add more complexity to the circuit and I'm running out of GPIOs on my MCU already. Sure one could do the switching with schmitt triggers and stuff but yeah.. complexity and board space  :palm: :-DD

nemail2:
How about this puppy? https://www.mouser.at/datasheet/2/609/AD5641-1501608.pdf

I could power it through the (to be chosen) 4.096V voltage reference and all'd be fine.

One thing comes to mind, however: will this thing really output 4.096V at max (rail to rail output)? I wasn't able to find anything(!) in the datasheet which declines that. Everywhere they are talking about 0 to VDD output.

or, what about this one, even better values as far as i can tell: http://www.ti.com/lit/ds/symlink/dac8311.pdf

nemail2:
ok so I came to the following conclusion, which should work for my design:


* use DAC8311 14 Bit DAC @ 4.096V VDD=VREF and Gain of 4 at my Opamp output stage
* use a second DAC8311 14 Bit DAC @ 4.096V VDD=VREF for my CC mode circuit (instead of currently 12 Bit @ 2.048V VREF so I'll be able to set the current in 0,5mA steps or just jump two steps at a time if i want to keep 1mA resolution)
* use REF5040 (standard grade) as a voltage reference and to power both DACs
DAC8311 datasheet: http://www.ti.com/lit/ds/symlink/dac8311.pdf
REF5040 datasheet: http://www.ti.com/lit/ds/symlink/ref5040.pdf

As I'm currently using the REF3120 (standard grade) as VREF for my DAC, the new VREF should be an improvement as well.

My only concern is the output power needed to drive both DACs driving the opamps, especially in regards of the input voltage I'd apply to the REF5040.
The only voltage which is usable and near the REF5040 is 15V so there'd be some dissipation and warmup in the reference, if the DACs will draw the max. specified 10mA. Question is, if they will. I don't think so but I'm not 100% sure, as I didn't really have to worry about it until now.

Please have a look at my schematics, first page, upper right part: https://github.com/mamama1/LabPSU_Darlington/blob/master/schematics.pdf
Sorry for the mess, if I'd redraw it, I'd have to reroute everything as well :-(

Thanks!

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