Author Topic: DAC or Level Shifter for power supply  (Read 3045 times)

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Offline filip_croTopic starter

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DAC or Level Shifter for power supply
« on: January 16, 2014, 06:21:39 pm »
I need to get 0-4.096V from ATmega88. I will most likely use ADR444BRZ reference (+/-1.6 mV, max current +10mA/-5mA).
http://www.farnell.com/datasheets/1048061.pdf

I found two options using I2C DAC or using PWM + Level shifter +Low pass filter.

a) using 10 bit I2C DAC

I think high end are expensive. They have bigger offset error and it will be harder to use. I will be using EU Farnell for components and for now I can't find anything suitable.

b) using Level shifter and LPF

I can't find good high performance Level shifter. PMW will most likely work on 16 or 20 MHZ. It need to have low resistance to GND and low resistance to V+. Also use less than 5mA given by Vref.

MCU will work on 5V. It can't be powered with Vref, it can use up to 9mA. Op amp will have +/- 2nA input current max. 

Any suggestion?


 

 

Offline sync

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Re: DAC or Level Shifter for power supply
« Reply #1 on: January 16, 2014, 07:25:53 pm »
I had good results with a 74HC14 as PWM driver. It was directly powered by a 5V reference IC (LT1021). For your case 5V input with 4V supply you need a resistor divider at the input or use 5V tolerant inputs, e.g. 74AHC.
 

Offline filip_croTopic starter

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Re: DAC or Level Shifter for power supply
« Reply #2 on: January 16, 2014, 07:40:04 pm »
In 74HC14 datasheet by NPX. If I out = 20uA, V drop can be 0.1V. To much for 10mV capable power supply.
 

Offline fcb

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Re: DAC or Level Shifter for power supply
« Reply #3 on: January 16, 2014, 07:56:09 pm »
I need to get 0-4.096V from ATmega88. I will most likely use ADR444BRZ reference (+/-1.6 mV, max current +10mA/-5mA).
http://www.farnell.com/datasheets/1048061.pdf

I found two options using I2C DAC or using PWM + Level shifter +Low pass filter.

a) using 10 bit I2C DAC

I think high end are expensive. They have bigger offset error and it will be harder to use. I will be using EU Farnell for components and for now I can't find anything suitable.

b) using Level shifter and LPF

I can't find good high performance Level shifter. PMW will most likely work on 16 or 20 MHZ. It need to have low resistance to GND and low resistance to V+. Also use less than 5mA given by Vref.

MCU will work on 5V. It can't be powered with Vref, it can use up to 9mA. Op amp will have +/- 2nA input current max. 

Any suggestion?

What speed do you need the 'DAC' to run at?
What are you doing with the output (controlling Vout on a bench PSU? synthesizing 'events'?)?

https://electron.plus Power Analysers, VI Signature Testers, Voltage References, Picoammeters, Curve Tracers.
 

Offline sync

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Re: DAC or Level Shifter for power supply
« Reply #4 on: January 16, 2014, 08:04:34 pm »
In 74HC14 datasheet by NPX. If I out = 20uA, V drop can be 0.1V. To much for 10mV capable power supply.
That's the max. value. I used 5 buffers parallel, 10bit PWM + 14 bit dithering for 24bit resolution. The lowest voltage was 70uV and that was limited by the buffer amp (LTC1050).

There were non-linearities at the low and high end. But I don't think that matters with plain 10bit PWM.
 

Offline filip_croTopic starter

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Re: DAC or Level Shifter for power supply
« Reply #5 on: January 16, 2014, 08:56:35 pm »
What speed do you need the 'DAC' to run at?
What are you doing with the output (controlling Vout on a bench PSU? synthesizing 'events'?)?
DAC is used to control V out and to set current limit. It doesn't need to run fast. But as it will be amplified 2.5 times it can't have big offset. (by using OP2177ARZ and 0.05% resistors)
 

Offline fcb

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Re: DAC or Level Shifter for power supply
« Reply #6 on: January 16, 2014, 09:42:49 pm »
You could use PWM for sure, although I'd probably do it using a serial DAC.

I've used MCP4921 (12bit) before (great part), there is also an interchangeable part MCP4911 (10bit version).  You might need to bump up the gain on the o/p (as they don't go to 4.1v I think).

If you do want to use PWM, then probably run a buffer from your REF.
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Offline filip_croTopic starter

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Re: DAC or Level Shifter for power supply
« Reply #7 on: January 16, 2014, 10:11:16 pm »
I need SPI for programming (and debugging) so, I think using I2C will be easier. Do you have some good explanations for ADC and DAC? (like Dave's Op Amp videos) I don't have experience with them and would like to learn more. 
 

Offline fcb

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Re: DAC or Level Shifter for power supply
« Reply #8 on: January 16, 2014, 10:17:15 pm »
Just bit-bang the DAC, if you haven't got a spare SPI module (sorry - I don't speak ATmega).
https://electron.plus Power Analysers, VI Signature Testers, Voltage References, Picoammeters, Curve Tracers.
 


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