generating analog voltage using microcontroller

[aruna1]

Hi

I need to generate an analog voltage using a microcontroller (16F877A)

I have two options -


1.)Use R-2R ladder and make a 10bit DAC

2.)generate 1kHz PWM signal--> send it through a 6 pole lowpass filter--> get analog voltage relevant to duty cycle.



Since I couldn't find any DAC ics, I'm stuck with above two options (they are the only methods I know of, are there any other methods?)

I have to order some DAC ICs but it will take about 3 weeks to reach me, so untill then I have to use some method to generate voltage.

What method do you think most reliable? that is more accurate and steady?

Thank you

 

[ronsimpson]

The PWM uses less parts! (pins)
Both DACs require your power supply to be stable.
PWM = 0 to 5V
R-2R = 0 to 2.5V
You probably will need to buffer both.
A software PWM will eat up CPU time!

 

[aruna1]

The PWM uses less parts! (pins)
Both DACs require your power supply to be stable.
PWM = 0 to 5V
R-2R = 0 to 2.5V
You probably will need to buffer both.
A software PWM will eat up CPU time!

PWM is genearted using hardware module inside the Microcontroller


why R-2R provide only 0-2.5V ?

 

[ronsimpson]

You are right I was thinking of a different DAC. It will output 0 to 1 bit short of supply. 0 to 4.995
The R-2R needs very good resistors if you want 10 bits.
I think the 16F877A does not have a hardware PWM.

 

[aruna1]

16F877A has 2 CCP modules. CCP uses timer2 to generate pwm

 

[ronsimpson]

OK
Run the PWM as fast as possable to reduce the size of the filter.

 

[crutschow]

Definitely use PWM. An R-2R DAC is too difficult to build from discrete parts.

If you can use a PWM frequency higher than 1kHz, you will reduce your filter requirements.

 

[aruna1]

Definitely use PWM. An R-2R DAC is too difficult to build from discrete parts.

If you can use a PWM frequency higher than 1kHz, you will reduce your filter requirements.

hi what about the stability and accuracy of PWM method

 

[ronsimpson]

very good.
If you supply is good.
The duty cycle is set by the clock and the clock is good.

 

[aruna1]

very good.
If you supply is good.
The duty cycle is set by the clock and the clock is good.

I see, thank you

 

[MrAl]

Hi,

I dont know if you want to add another chip, but you can get a serial DA converter and that way you can get stable high resolution DC voltages.

 

[aruna1]

Hi,

I dont know if you want to add another chip, but you can get a serial DA converter and that way you can get stable high resolution DC voltages.

ya but problem is finding one. I have to order one from ebay, but will take 3 weeks to get here

 

[ronsimpson]

Looking for parts....digikey.com clames they have resorces in Sri Lanka. They do 1.5 billion/year. I have ordered / shipped parts world wide with out problems.

 

[aruna1]

I checked every local store I know but couldn't find one. but didn't try digikey. I haven't brought any thing from digikey yet. I will check

 

[MrAl]

Hi,

How fast do you need it? I think digikey is pretty quick on shipping.
I used to know someone else from Sri Lanka too until their business got wiped by a flood from the tsunami.

 

[carbonzit]

How fast do you need it? I think digikey is pretty quick on shipping.

Digikey = good.

Just received my first order from them in a long time. Shipped USPS, took 4-5 days. Not bad.

Plus their parametric search is so much better than, say, Jameco's. Plus better prices and a lot more stuff.

Haven't dealt w/Mouser yet; wonder how they compare?

 

[aruna1]

digikey is too expensive.
for an example in here MC34063 is about $0.4 where in digikey its $1.4 and shipping is $32.

 

[MrAl]

Hi,

Wow, sorry to hear that. Anywhere else closer to you maybe?

 

[aruna1]

nope, but i was able to order some DAC samples from TI.com. hopefully the will arrive next week

 

[3v0]

Mouser ships the same day if orders are in by 3PM. Prices are better then Digikey. I expect way higher then most places in Sri Lanka.