DAC or Level Shifter for power supply - Page 1
Forum section: electronics / beginners

author: filip_cro
author_rank: Regular Contributor
timestamp: January 16, 2014, 06:21:39 pm
content: 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.pdfI found two options using I2C DAC or using PWM + Level shifter +Low pass filter.a) using 10 bit I2C DACI 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 LPFI 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?

author: sync
author_rank: Frequent Contributor
timestamp: January 16, 2014, 07:25:53 pm
content: 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.

author: filip_cro
author_rank: Regular Contributor
timestamp: January 16, 2014, 07:40:04 pm
content: In 74HC14 datasheet by NPX. If I out = 20uA, V drop can be 0.1V. To much for 10mV capable power supply.

author: fcb
author_rank: Super Contributor
timestamp: January 16, 2014, 07:56:09 pm
content: Quote from: filip_cro on January 16, 2014, 06:21:39 pmWhat speed do you need the 'DAC' to run at?What are you doing with the output (controlling Vout on a bench PSU? synthesizing 'events'?)?

author: sync
author_rank: Frequent Contributor
timestamp: January 16, 2014, 08:04:34 pm
content: Quote from: filip_cro on January 16, 2014, 07:40:04 pmThat'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.

author: filip_cro
author_rank: Regular Contributor
timestamp: January 16, 2014, 08:56:35 pm
content: Quote from: fcb on January 16, 2014, 07:56:09 pmDAC 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)

author: fcb
author_rank: Super Contributor
timestamp: January 16, 2014, 09:42:49 pm
content: 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.

author: filip_cro
author_rank: Regular Contributor
timestamp: January 16, 2014, 10:11:16 pm
content: 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.

author: fcb
author_rank: Super Contributor
timestamp: January 16, 2014, 10:17:15 pm
content: Just bit-bang the DAC, if you haven't got a spare SPI module (sorry - I don't speak ATmega).