Voltage divider for VREF --- what value resistors? - Electronic Circuits Projects Diagrams Free

futz · 18th May 2008, 10:23 PM

I want to use two equal value resistors for a voltage divider to get a 2.5V VREF+ from my 5V VDD. What value resistors are recommended for this?

blueroomelectronics · 18th May 2008, 10:25 PM

Use a TL431, else it needs to be about 250ohms impedance. So a pair of 247ohm resistors would do the trick.
I've done it on the Junebug with VR2 and a 10uf cap in CON4

futz · 18th May 2008, 10:29 PM

Quote:

Originally Posted by blueroomelectronics

Use a TL431, else it needs to be about 250ohms impedance. So a pair of 247ohm resistors would do the trick.

Cool. I don't have a TL431. Should order some in my next order because I keep running into this problem. I'll do it with resistors for now. Nearest resistors I have to 247

hm: is 330

hm:. Close enough? Probably fine, right? There's virtually no current draw on it, I would think.

blueroomelectronics · 18th May 2008, 10:34 PM

It's in the 18F1320 datasheet, the A/D actually sinks and sources the VREF+ pin. A cap should stop that. The TL431 is pretty handy too.

futz · 18th May 2008, 10:44 PM

Quote:

Originally Posted by blueroomelectronics

It's in the 18F1320 datasheet, the A/D actually sinks and sources the VREF+ pin. A cap should stop that.

OK, here's where my electronics ignorance shows.

What does that mean?

The datasheet recommends 250

hm: max impedance for VREF+ source. (whatever that means

).

futz · 18th May 2008, 10:48 PM

Oh ya, Bill, your blog here won't allow me to post a comment. I was going to say (in re: your Lab-Easy comment) that DipMicro is liquidating Lab-Easy products. Poor sales? Promo? Guess I should buy some while it's cheap...

blueroomelectronics · 19th May 2008, 12:57 AM

Quote:

Originally Posted by futz

Oh ya, Bill, your blog here won't allow me to post a comment. I was going to say (in re: your Lab-Easy comment) that DipMicro is liquidating Lab-Easy products. Poor sales? Promo? Guess I should buy some while it's cheap...

Yes get some, they are cheaper than cost.

Dunno why you can't post a comment, Pommie did.

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18th May 2008, 10:23 PM	(permalink)
futz	Voltage divider for VREF --- what value resistors? I want to use two equal value resistors for a voltage divider to get a 2.5V VREF+ from my 5V VDD. What value resistors are recommended for this? __________________ ========================= Futz's Microcontrollers & Robotics ========================= Last edited by futz; 18th May 2008 at 10:26 PM.

18th May 2008, 10:25 PM	(permalink)
blueroomelectronics	Use a TL431, else it needs to be about 250ohms impedance. So a pair of 247ohm resistors would do the trick. I've done it on the Junebug with VR2 and a 10uf cap in CON4 __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

18th May 2008, 10:34 PM	(permalink)
blueroomelectronics	It's in the 18F1320 datasheet, the A/D actually sinks and sources the VREF+ pin. A cap should stop that. The TL431 is pretty handy too. __________________ Bill Smart Kits build Smart People http://www.blueroomelectronics.com

18th May 2008, 10:48 PM	(permalink)
futz	Oh ya, Bill, your blog here won't allow me to post a comment. I was going to say (in re: your Lab-Easy comment) that DipMicro is liquidating Lab-Easy products. Poor sales? Promo? Guess I should buy some while it's cheap... __________________ ========================= Futz's Microcontrollers & Robotics =========================