EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: filip_cro 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 (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?
-
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.
-
In 74HC14 datasheet by NPX. If I out = 20uA, V drop can be 0.1V. To much for 10mV capable power supply.
-
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 (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'?)?
-
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.
-
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)
-
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.
-
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.
-
Just bit-bang the DAC, if you haven't got a spare SPI module (sorry - I don't speak ATmega).