ATmega48V ARef pin current

Go To Last Post
10 posts / 0 new
Author
Message
#1
  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Hi there,

I would like to know which is the minimum input current for the ARef pin for the ATmega48V. Reading the datasheet 2545M–AVR–09/07 at page 312, I found that the RREF Reference Input Resistance = 32 kΩ. This is not help me.

Could somebody help or give me an explanation?

Thanks for your time.

Michael

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:
the RREF Reference Input Resistance = 32 kΩ. This is not help me.

Even this old bit jockey knows V=IR. If V is 3.2V & R is 32000, then I is 100uA right?

Since conversions and other uses take "sips" from this, it is still a good idea to have a small capacitor there IME.

Lee

You can put lipstick on a pig, but it is still a pig.

I've never met a pig I didn't like, as long as you have some salt and pepper.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thanks,

Michael

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Mind if I ask what difference does it make? If you are going to use a zener diode/resistor combo, the zener is going to use more current to reverse bias anyway. The datasheet for the M48 states that it is a high impedance input and only high impedance meters should be connected to it.

I can agree with this as I just finished a project and I used the AVCC configuration for Vref. I put my meter probe on vref just to see what would happen and the readings were totally off. The datasheet states that only a capacitor should be put on the pin if it is not being used as a vref input.

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Jim,

Actually I would like to have a 2,5V Aref voltage while the Vcc is 5V. My problem is that if I use an external voltage divider, let's say 10K and 10K, there will be a permanent current flow about 250 microamps. This is very bad because the microcontroller's functionallity is supported of a 12V rechargeable battery and there must be very low current flow after the battery cut off.
I think that the best way is to use the internal Aref 1.1V with an external cap connected to the Aref pin.

Thanks for your time.

Michael

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Quote:
I think that the best way is to use the internal Aref 1.1V with an external cap connected to the Aref pin.

Consider the tolerance of the internal voltage reference 1.0 to 1.2V (datasheet, 27.8. electrical characteristics - adc characteristics).

/Martin.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

So what else I could do????

Michael

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Possible hardware-solutions:
Do you need the reference permanent or just temporarely? In the second case you could connect VAREF via a resistor to +5V and via a second resistor and one transistor to ground.
Otherwise you could use an external voltage reference.
Possible software solution:
Connect VAREF with external cap as you suggested.
Switch VAREF to VCC, VBG to an input of the ADC and make a convertion. This way you will get an exact value of VBG.
Switch VAREF to VBG, make the conversion and use the previous measured value to compensate the tolerance of VBG.
If temperature does not change to fast, you could perform the measurement of VBG an periods of maybe minutes or even longer.

/Martin.

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

You could also use AVcc as well. If AVcc is +5v, then seet the proper bits to use AVcc as your reference

Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

  • 1
  • 2
  • 3
  • 4
  • 5
Total votes: 0

Thank you all,

Michael.

Michael.

User of:
IAR Embedded Workbench C/C++ Compiler
Altium Designer