R-2R Ladder DAC with Potential Divider

[Lord Grezington]

Hi

This is more of an electronic questions rather than a flowcode one, but it is something I need to get right first time and I will be using flowcode within the application.

I am running short of time on a project so unfortunately I have not much time for testing, so I was hoping some of you could help...

I need to produce a number of different voltages from 0 - 1.8Vdc

I am a bit unsure with this project if I can put the potential divider circuit before or after the Opamp, I am a bit wary whether or not it is going to affect the R-2R Ladder.

Any Feedback would be appreciated - I cant use high cost components.

Thanks

[Enamul]

Hi,

I need to produce a number of different voltages from 0 - 1.8Vdc. I am a bit unsure with this project if I can put the potential divider circuit before or after the Opamp, I am a bit wary whether or not it is going to affect the R-2R Ladder.

From the above question and your schematic, I feel you want to switch D0-D3 by counter or PIC which ultimately change Vin with D0-D3 combinations...Now the important bit is if you use TTL logic for D0-D3 in that you should use higher voltage (like 9V) for op-amp supply otherwise op-amp output will saturate..Op-amp is here buffer amplifier so it's obviously better to use potential divider after buffer to change the range 0 to 1.8V. But using potential divider you will loose current drive facility..it will be just voltage with low current. You can use another buffer after potential divider which will help to provide current...if you need
You can have a look in the following link which uses similar idea..
http://users.ntua.gr/dpiperid/MyWebPage ... rdacEN.htm

[Lord Grezington]

Hi Enamul

Yes, I will be switched D0-D3 from the pic.

I chose to use the MCP6401RT as it is apparently a rail to rail op-amp so I did not need to use a higher voltage. I was hoping that I could get around the higher voltage problem this way (I could be wrong). I know that it may not be very accurate near the top and bottom rails although the application does really need this.

Thanks
Graham

[Enamul]

Hi,
You can try to calculate output voltage with B0-B3 all high using superposition theorem. Or can be simply in breadboard with the R-2R ladder connected and B0-B3 connected to Vcc=+5V. If Vout equal or close to 5V in that case even using MCP6401RT you need to have voltage supply greater than 5V as all these devices has an internal drop which they can't avoid.

[petesmart]

Hi,

I might be on the wrong track here, however, could you use a PWM output and a filter on the input of the op amp to generate your voltages?... It would be a much simpler cct and provides granularity of control via software.

Just a thought.

Best

Pete

[Enamul]

Hi Pete,
No you are not in wrong track. He could easily use that option with buffer amplifier..I thought that option but didn't mention as he was thinking in this way..he might build his circuit. It's nice that you posted which will help to go for second thought. Thanks Pete.

[Steve001]

Hello,

Just a suggestion, have you looked at digital potentiometers . .

I have used the intersil one in one of my projects and was happy with it

http://ww1.microchip.com/downloads/en/A ... /00219.pdf

http://uk.farnell.com/intersil/x9c103pz ... t=120-8129

http://uk.farnell.com/microchip/mcp4141 ... t=183-4962

you could use one with an adjustable voltage regulator or a buffered voltage referance

Steve

[petesmart]

Hi

Yes Steve, they work well... I have used the microchip versions on I2C interface. I think I may have posted something on this a while ago...

try searching on digital potentiometer

Best

Pete

[Steve001]

have a look here

http://www.matrixmultimedia.com/mmforum ... 000#p39000


steve

[Lord Grezington]

Thank you everyone for all you comments...

- I was reluctant to use a PWM through a buffer as while doing my research I have found that there will near enough always be an unavoidable ripple. Secondly I am trying to save my PWM outputs for other control interfaces.

- I have looked into using DAC's, I have been reluctant to use these due to cost (of good ones) and also the I2C or SPI interfaces. I have never used them before although there must be a first time at some point.

- The digital POT looks interesting. I have not had too long to look into your flc but at a quick glance it loos as though you bit banged the SPI? or have you found a different digital pot which can take digital inputs? Is it possible for you to post a schematic (including the components used) which go with this FLC?

Thank you all for your input

Graham

[Steve001]

- The digital POT looks interesting. I have not had too long to look into your flc but at a quick glance it loos as though you bit banged the SPI? or have you found a different digital pot which can take digital inputs? Is it possible for you to post a schematic (including the components used) which go with this FLC?

Graham

No the digital pot i used has 3 digital inputs depending how you select them depends upon raise , lower, standby etc

will try and make a drawing tonight.

if you look on "panel" in my flowchart (may be hidden on auto hide ) there is a diagram on how the chip select works

steve

[petesmart]

Hi Graham,

Check out this thread... Towards the bottom you will find a flow chart that drives a dual pot....I did this a little while ago, it's quite straight forward to get it going. It's also appeared to be repeatidly accurate.

http://www.matrixmultimedia.com/mmforum ... 46&t=10869

See how you go with this

Best

Pete

[Lord Grezington]

Hi all

Thanks for you help - I am going to go for the digital pot idea. I have found a really good one AD5116BCPZ10-500R7 and is well priced and very easy to use. It is also non-volatile which is a good addition.

Does anyone see any issues with me adding a potential divider circuit on the output of this?

Graham

[petesmart]

Hi Graham,

I may have mis understood, can you use the potentiometer as a potential divider between the two rails. The wiper is connected to the input of the opamp configure as a 1:1 buffer amp.

Perhaps put a series resistor in between the wiper and iut of the op amp for protection..

Best

Pete

[Lord Grezington]

Hi Pete

I was hoping to use the Pot (AD5116BCPZ10-500R7) as a potential divider to give me 64 positions between 0-5V (rail to rail). I was then going to add a 100K and 56K resistor as a potential divider so that I can get 64 positions between 0-1.8V.

I was hoping by doing this I wont need to use the buffer Op-Amp.

Graham