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2 DACS / One Arduino Nano / Two Independent voltages

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robert_south1978:
Hi All,

I want to build a voltage referance test stand. I want to generate 20 diffeferant voltages (0.0 to 3.3 vdc). I have had success with 2 MCP4725 DACs on one Arduino Nano.
( I want to try 4 DACS on one NANO. ) I want to be able to connect the DAC Outputs plus to minus as if they are independent supplies ( or batteries shall we say.) As they have common ground,
connecting plus to minus will cause a short. Do I Need something like a virtual ground for each DAC Output ? Using Op amps?
Thanks for your help !

Robert

Ian.M:
You'd need an isolated reference supply for each DAC and you'd need to isolate their I2C interfaces, so their Vout and Vss terminals are fully floating.   The alternative idea of summing the voltages using OPAMPs would degrade accuracy even if you use 0.01% precision resistors.

However, as the MCP4725 only has a single address pin and you have to order different part numbers to get different high bits for the address, (MCP4725A0 for 00x through to  MCP4725A3 for 11x), and its still  somewhat more difficult to isolate an I2C bus cheaply compared to a SPI bus, it may well be preferable to switch to a SPI interface DAC, possibly with an internal reference, as a SPI bus can be cheaply optoisolated and can support large numbers of identical chips as long as you provide  individual slave select signals and their SS (/CS) pin tristates their MISO pin when not asserted.

robert_south1978:
 a: You'd need an isolated reference supply for each DAC  /     I suppose I could use a cheap buck Booster Power supply for each DAC ?

b: you'd need to isolate their I2C Interfaces / Again another Buck Booster PS for the NANO ( or other uP ) ?

c: As far as I2C addressing, I have found differant mfg of the MCP4725 use differant adresses. So I could use two from AdaFruit, two from other mfg ( thats four* )

What do you think ?

Thanks for your response  !!

* Only Limit here is NANO Memory space

Ian.M:
No.

A: Cheap buck-boosts typically aren't isolated.    You'd need regulated output isolated DC-DC converters, one per DAC,  extra filtering on the output of each DC-DC converter to reduce ripple at their switching frequency, and a calibrated precision reference for each DAC.

B: You'd need an individual I2C isolator chip (not a simple bus switch) for each DAC.   You don't need to isolate the Nano - it doesn't even matter if it shares a ground with your PC via a USB cable as long as the DACs (and any other 'front panel' inputs and outputs on the device) are fully isolated.

C: The high bits of the I2C address are encoded in the part number.  It doesn't matter who you order them from: If you order a MCP4725A0 it will have the same two possible addresses.   Of course some suppliers may have ....A1 ....A2 or ....A3 parts in stock which use different addresses.  Some suppliers may be too dumb to care and which ....An variant you get will be a lottery.  Such dumb suppliers are best avoided.

robert_south1978:
Hello Mr. Ian M.

A: Of course you are right.

I have decided to use 20 NANOs each with one DAC . Each NANO/DAC powered by  12v/5v 200ma isolated DC/DC converters. 
 ( TracoPower TME 1205S DC/DC-Wandler, Print 12 V/DC 5 V/DC 200 mA 1 W Anzahl )

This way I can power all 20 by one 12v DIN Rail Supply.
Plus I will have no worries about I2C Signals.

You are right about no Name suppliers, I have have many Problems with no Name. Never a Problem with Adafruit !

P.S. we are testing fuel cells hence the 20 voltages, to be used during dummy tests.

I keep you up to date if you want.

Thanks for your help!!

Robert

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