Author Topic: decent but affordable 16 bit DAC  (Read 6224 times)

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Offline nemail2Topic starter

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decent but affordable 16 bit DAC
« on: June 07, 2019, 11:52:03 pm »
Hi

currently I'm using a MCP4922 in my Lab PSU design. However as I have a gain of 6 currently for my voltage set circuit (DAC goes to opamp, opamp has a gain of 6 and has a darlington in it's feedback loop) and I want to output more than 12.228V (which is VREF = 2048mV * gain of 6) by still being able to control the voltage with 1mV steps, I thought I'd need a higher res DAC.
The MCP4922 does have an internal programmable gain of 1 and 2 but with my current setup, at an internal gain of 2 and an external gain of 6 at a VREF of 2048mV I'd be setting my voltage in 6mV steps instead of 1mV.

So I found this one: http://www.ti.com/lit/ds/symlink/dac8563t.pdf and was wondering if it was even possible to feed it with 5V (which would give me 5V full scale output) but use 3.3V (that's the MCU's voltage) for the SPI interface. According to the datasheet, the DAC recognizes voltage above 2.1V as logic input HIGH. According to the datasheet it should be possible even without the need of any voltage level shifters.

Am I missing something or should this work?

Does anybody know better suited/cheaper DACs with 16 bits or even a completely different/better idea how to solve this?

Thanks!
« Last Edit: June 07, 2019, 11:53:56 pm by nemail2 »
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Offline thm_w

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Re: decent but affordable 16 bit DAC
« Reply #1 on: June 08, 2019, 12:23:41 am »
So I found this one: http://www.ti.com/lit/ds/symlink/dac8563t.pdf and was wondering if it was even possible to feed it with 5V (which would give me 5V full scale output) but use 3.3V (that's the MCU's voltage) for the SPI interface. According to the datasheet, the DAC recognizes voltage above 2.1V as logic input HIGH. According to the datasheet it should be possible even without the need of any voltage level shifters.

Yeah looks like you are right, no outputs so no level shifting needed. You may need to do some calibration if the accuracy isn't good enough for you, but the typical values look good. Also depends on your external reference.
$8/ea https://www.ti.com/product/DAC8563T
There are some DAC8562SDGSR on LCSC for under $5: https://lcsc.com/products/Digital-To-Analog-Converters-DACs_615.html

I tried to see what DAC is used in Rigol/Siglent PSU but couldn't find it.
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Offline mariush

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Re: decent but affordable 16 bit DAC
« Reply #2 on: June 08, 2019, 12:35:49 am »
Have you looked at something like LTC2631H or LTC2640H  (12bit, 4.096v internal reference) ... not sure you'd get mV precision, but 2-3 mV should be doable.
May seem expensive for a 12bit DAC but if you include the built in reference...

or maybe LTC2612 (14bit , needs external reference)
« Last Edit: June 08, 2019, 12:47:56 am by mariush »
 
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Offline MasterT

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Re: decent but affordable 16 bit DAC
« Reply #3 on: June 08, 2019, 01:55:37 am »
MCP4912 has INL +-12 LSB, so basically its' 8-bits DAC and you never get 1mV anyway, even in 0-2.048 range. It's easier /cheaper to use a sigma-delta adc (mcp3421, mcp 3551) and  digital pots to lock a loop via micro. Accuracy 18 -3 bits (resistor divider to scale output back) =15-bits or so.
If still prefer your design, than look at max5717A - 16 bits, INL +-1
 
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Offline David Hess

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Re: decent but affordable 16 bit DAC
« Reply #4 on: June 08, 2019, 03:00:39 am »
MCP4912 has INL +-12 LSB, so basically its' 8-bits DAC and you never get 1mV anyway, even in 0-2.048 range.

I was going to say this first but you beat me to it.  The MCP4922 only has the absolute accuracy of an 8 bit DAC.  Higher resolution than accuracy is common in servo applications where the only requirement is monotonic operation.

Quote
It's easier /cheaper to use a sigma-delta adc (mcp3421, mcp 3551) and  digital pots to lock a loop via micro. Accuracy 18 -3 bits (resistor divider to scale output back) =15-bits or so.
If still prefer your design, than look at max5717A - 16 bits, INL +-1

Or there are 12 bit DACs which have 12 bit accuracy if that is required.

 
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Offline Kleinstein

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Re: decent but affordable 16 bit DAC
« Reply #5 on: June 08, 2019, 08:32:13 am »
As in a lab supply there is no need to change the settings very fast, one could use PWM and a filter.  With some care it can be reasonably linear and it essentially always is monotonic. There is still some effort for a good filter, that does not add much drift and it needs care with the layout and reference buffering.
 
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Offline Arjunan M R

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Re: decent but affordable 16 bit DAC
« Reply #6 on: June 08, 2019, 08:38:43 am »
I am also  designing a lab PSU using MCP4922 ;D.
To measure current and voltage I am using MCP3421 ADC.
« Last Edit: June 08, 2019, 08:42:51 am by Arjunan M R »
 

Offline nemail2Topic starter

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Re: decent but affordable 16 bit DAC
« Reply #7 on: June 08, 2019, 01:25:54 pm »
Also depends on your external reference.
I'd like to use the internal reference of the DAC. The ADC I'm using (ADS1115) as well has an internal reference so I'd get away without any external references at all (which saves board space).

There are some DAC8562SDGSR on LCSC for under $5: https://lcsc.com/products/Digital-To-Analog-Converters-DACs_615.html
Thanks! Didn't think of LCSC being an option! I'm kinda buying everything at Mouser which sometimes costs me more money than necessary. Have to work on that :-)

Have you looked at something like LTC2631H or LTC2640H  (12bit, 4.096v internal reference) ... not sure you'd get mV precision, but 2-3 mV should be doable.
May seem expensive for a 12bit DAC but if you include the built in reference...

or maybe LTC2612 (14bit , needs external reference)

To get the desired resolution and output voltage, I'd need 14 bit at least. Unfortunately the LTC2612 is a bit on the expensive side, being even without an internal VREF...

  • I could go with 4.096V VREF @ 12 Bit DAC and Gain of 5 though, that'd give me 20.480V max. output which is good but I'd have to go with 5mV steps (not too shabby and better than 3mV in terms of nice steps). i guess I'd find a decent, rather cheap 12 Bit DAC which accepts 3.3V and below als logic input high while running @ 5V. So I guess that'd be a rather cheap but not as precise solution. And it would be a shame, using resolution here, as I just beefed up the ADCs for being able to measure in 1mV steps.
  • 2.500V VREF @ 16 Bit DAC and Gain of 5 would give me 16.500V max output (with internal DAC Gain of 2) at 0.03814697265625mV steps (internal DAC gain of 1) or 0.0762939453125mV steps (internal DAC gain of 2). not beautiful but I could "repair" some of that in software. this seems to be the easiest and cheapest solution in terms of hardware.
  • 4.096V VREF @ 14 Bit DAC and Gain of 4 would be the nicest option in my eyes. 0-16.384V in 1mV steps that would be.
    4.096V VREF / 16384 (14 bits) = 0,25mV per step
    Gain of 4 makes that 1mV per step
    Gain of 4 makes 16.384V max output out of the 4.096V VREF. VDD would be 5V for the DAC and it would have to accept low-ish voltages like 3.3V (MCU voltage) as logic input high. Also, there should not be any outputs cause they'd drive at 5V which would fry my MCU (and I don't want to use a voltage level shifter). That seems to be the easiest solution in terms of software and traceability due to the simplicity of the calculations.
  • while writing, it came to me that 16 Bit DAC and gain of 4 @ 4.096V VREF would work good as well, same as 14 Bit DAC and Gain of 8 @ 2.048V VREF should work. 16 Bit DAC and Gain of 8 @ 2.048 VREF should work too.

As far as I can tell by parametric search results, there is no such 14 Bit DAC with integrated 4.096mV VREF so I guess it'd be something like the LTC2612 or DAC8162T + external 4.096V VREF if I want to go the 14 Bit way @ Gain of 4.

My head is smoking. Is gain of 8 a smart thing to do? That'd amplify the minimum output of the DAC (which in my case with the MCP4922 currently seems to be about 1-2mV) by 8 so it raises the minimum output voltage...

MCP4912 has INL +-12 LSB, so basically its' 8-bits DAC and you never get 1mV anyway, even in 0-2.048 range. It's easier /cheaper to use a sigma-delta adc (mcp3421, mcp 3551) and  digital pots to lock a loop via micro. Accuracy 18 -3 bits (resistor divider to scale output back) =15-bits or so.
If still prefer your design, than look at max5717A - 16 bits, INL +-1
But I'm using the MCP4922, not the MCP4912 currently and it works quite good (3mV steps work). Only thing I want to replace it is because of the 3mV steps, I'd like to have 1mV steps... that digipot design sounds interesting, never thought of doing it like that. However for now, I'd like to stick with my design...

As in a lab supply there is no need to change the settings very fast, one could use PWM and a filter.  With some care it can be reasonably linear and it essentially always is monotonic. There is still some effort for a good filter, that does not add much drift and it needs care with the layout and reference buffering.
I'm not good at designing filters (essentially, I do not know what I'm doing) and I'd need a 14 or 16 bit PWM then, at least. Wouldn't I?

I am also  designing a lab PSU using MCP4922 ;D.
To measure current and voltage I am using MCP3421 ADC.

That might be Dave's fault, using the MCP4922 in his µSupply video series years ago :D
For the ADC, I went with the ADS1115.
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Offline mariush

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Re: decent but affordable 16 bit DAC
« Reply #8 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?
 
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Offline chris_leyson

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Re: decent but affordable 16 bit DAC
« Reply #9 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.
 
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Offline OM222O

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Re: decent but affordable 16 bit DAC
« Reply #10 on: June 08, 2019, 05:13:28 pm »
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.
« Last Edit: June 08, 2019, 05:15:15 pm by OM222O »
 
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Offline David Hess

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Re: decent but affordable 16 bit DAC
« Reply #11 on: June 08, 2019, 06:29:40 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?

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

 
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Offline nemail2Topic starter

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Re: decent but affordable 16 bit DAC
« Reply #12 on: June 09, 2019, 11:56:50 am »
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?

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

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

"DAC 2 Click" from MikroElektronica uses the 16-bit LTC2601 which has 8 LSB INL, and the "ADC 8 Click" uses the ADS1115.

at worst case scenario it has 64 bits INL

^
|
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).

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).
:-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.

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.
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).

and you have previously mentioned that you are not able to calibrate them out  :-/O which is a bit painful.
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.

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

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
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Offline nemail2Topic starter

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Re: decent but affordable 16 bit DAC
« Reply #13 on: June 09, 2019, 12:20:26 pm »
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
« Last Edit: June 09, 2019, 12:26:15 pm by nemail2 »
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Offline nemail2Topic starter

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Re: decent but affordable 16 bit DAC
« Reply #14 on: June 09, 2019, 03:47:43 pm »
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!
« Last Edit: June 10, 2019, 11:31:57 am by nemail2 »
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Online SiliconWizard

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Re: decent but affordable 16 bit DAC
« Reply #15 on: June 09, 2019, 05:00:29 pm »
So I found this one: http://www.ti.com/lit/ds/symlink/dac8563t.pdf and was wondering if it was even possible to feed it with 5V (which would give me 5V full scale output) but use 3.3V (that's the MCU's voltage) for the SPI interface. According to the datasheet, the DAC recognizes voltage above 2.1V as logic input HIGH. According to the datasheet it should be possible even without the need of any voltage level shifters.

Datasheet states that logic inputs high level is 2.1V to AVDD, so you can safely use 3.3V to 5V logic if you're using a 5V AVDD.
Obviously don't forget to connect the 5V supply to the VREF pin as well, otherwise it will use the internal reference (2.5V). Note that the internal reference is pretty accurate so if a 2.5V full-scale output could be adequate, I'd suggest using it instead of a generic 5V the accuracy of which may not be all that good. Of course you can always use a 5V voltage reference both for VREF and AVDD: this DAC draws little current.


 
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Offline nemail2Topic starter

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Re: decent but affordable 16 bit DAC
« Reply #16 on: June 10, 2019, 12:41:35 am »
kinda decided to go with the DAC8311, unless something bad comes up... (see my previous post).

But thanks for your response anyway.
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Offline David Hess

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Re: decent but affordable 16 bit DAC
« Reply #17 on: June 10, 2019, 03:28:48 am »
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

It comes down to either switching a gain stage after the DAC or switching the gain of the control loop which has the complication of affecting the frequency compensation.  It could be done with MOSFETs but there are lots of ways.
 

Offline OM222O

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Re: decent but affordable 16 bit DAC
« Reply #18 on: June 10, 2019, 04:55:48 am »
Look up "universal shunt" or "ayrton shunt". Same idea applies here. You have a resistor in the feedback path of the op amp. If the shunt is disconnected, it only acts as a dummy resistor and you have unity gain. Then depending on the value of the universal shunt, you will have some gain which is 1+(feedback resistor/shunt resistor). If you want a specific impelementation for the universal shunt, I can provide a schematic using N channel fets.
Also your op amps seem good for a gain of 6 (100uV max offset) but keep the OPAx188 (OPA2188 in your case) as a drop in replacement upgrade  ;)
« Last Edit: June 10, 2019, 05:15:31 am by OM222O »
 

Offline OM222O

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Re: decent but affordable 16 bit DAC
« Reply #19 on: June 10, 2019, 05:14:01 am »
Actually I already have a universal shunt schematic with kelvin connections! See the attached picture.
You can Ignore Q4-6 since they are for the sense connections and you don't need them here. The DMN3018SSD should be good enough, but if you need lower capacitance, consider the FDS6930B (again, drop in replacement). This should give you 6 orders of magnitude resistance which is more than good enough for my applications, but needless to say, you have to tune the values and the ranges of the resistors for the gain values you want. Good luck  :-+
 

Offline David Hess

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Re: decent but affordable 16 bit DAC
« Reply #20 on: June 10, 2019, 05:20:55 am »
The tricky implementations of gain switching compensate for or are invariant of the series resistance of the switch.

https://www.analog.com/media/en/training-seminars/tutorials/MT-072.pdf
 

Offline Kleinstein

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Re: decent but affordable 16 bit DAC
« Reply #21 on: June 10, 2019, 06:48:40 am »
.....
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!

The DAC8311 should not nit much more than 160 µA + the output current. The only point I see a 10 mA limit is for the output current it can deliver at some point. So the supply current should not be a problem. If heating of the reference is a problem one could consider a series resistor to drop some of the voltage. This can also work as a filter.
 

Offline OM222O

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Re: decent but affordable 16 bit DAC
« Reply #22 on: June 10, 2019, 08:46:57 pm »
The tricky implementations of gain switching compensate for or are invariant of the series resistance of the switch.

https://www.analog.com/media/en/training-seminars/tutorials/MT-072.pdf

That is why I recommended different fets. Higher Rdson means lower capacitance and better transients, etc. but can introduce some errors. Higher capacitance has the opposite effect, therefore it's a tradeoff. Anyhow if the resistors are chosen to be large enough, often the  Rds is orders of magnitude better than the resistor tolerance itself, making it a non issue  ;D
 

Offline nemail2Topic starter

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Re: decent but affordable 16 bit DAC
« Reply #23 on: June 10, 2019, 10:50:19 pm »
Ok, so this is what I've come up with now.

i hope the digital traffic on the SPI and I2C lines won't spread to much into the analog circuitry, I've put ferrite beads in between every ADC and DAC supply line (L5, L6, L8) and I have followed datasheet instruction on bypassing and filter caps as well as I tried to follow layout recommendations.

I don't like the wasted space above the ENC pin header but I'd have to rearrange like EVERYTHING to get rid of that. Alternative would be to move the whole left part (from IC5 to the two DACs down but that would increase the trace length for current measurement (the shunt is a bit above IC13).

So basically I'm planning to go with the 4.096V voltage reference (REF5040) and with 2x DAC8311 at a fixed gain of 4 (which is even less than I have now: 6!) which will give me (nearly) zero to 16.384V output in 1mV steps.

Any objections or hints from anyone? :)
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Offline OM222O

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Re: decent but affordable 16 bit DAC
« Reply #24 on: June 11, 2019, 04:53:03 am »
The parts choices seem reasonable  :-+
But that layout though   :-DD no offense but it seems very willy nilly without any rhyme or reason. Usually the layout follows the schematic, i.e. you have sections that do specific things, rather than just running traces all over the shop. What software are you using for the pcb design? If it's easy eda I can help you clean it up. Otherwise I can upload some pictures as hints. It would be beet to seperate your analog and digital supplies btw. LDOs are so cheap these days that there isn't an excuse for not doing so. You also won't need the ferrite beeds if you have a clean supply to start with  :-/O
 


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