multiple power supplies for AVR project

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My AVR is connected to 3 pressure sensors on the Analog inputs. the sensors report 0 - 5 volts. The AVR is powered by a LDO 5v regulator and the sensors are powered by their own 5 volt regulator and both regulators are supplied by the same 12 volt supply.

For best accuracy then I Connect the AREF pin to the same 5v the regulators are connected to.

What else can I do to reduce or eliminate any voltage drift between the 2 regulatos?

Don't Let the smoke out!

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Use just one regulator?
One 317 or a 337 if you need a lot of current.

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Short circuits eliminate drift pretty well. :)

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Quote:
For best accuracy then I Connect the AREF pin to the same 5v the regulators are connected to.
DO NOT DO THIS. AREF should be left floating with just a 100nF cap to ground.

If you need to switch to a different AREF source like an internal one you are likely to kill the chip. ie internal AREF (2.56V??) shorts out to VCC.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
Quote:
For best accuracy then I Connect the AREF pin to the same 5v the regulators are connected to.
DO NOT DO THIS. AREF should be left floating with just a 100nF cap to ground.

If you need to switch to a different AREF source like an internal one you are likely to kill the chip. ie internal AREF (2.56V??) shorts out to VCC.

So... what is the AREF for if not used to referance a voltage? I'm confused.

And there is a typo in my statemnt, I ment to say the same 5v the SENSORS are connected to.

I can use the same 5v supply for both, but the sensors are off board. So I guess I will just need to run aditional wires.

Don't Let the smoke out!

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Quote:

what is the AREF for if not used to referance a voltage?

Which AVR model(s) are you concerned with?

Most will have an option to internally use AVcc as the reference voltage. the cap on the AREF pin allows for some filtering, and allows a choice of [internal] reference voltages to be selected.

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.

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theusch wrote:
Quote:

what is the AREF for if not used to referance a voltage?

Which AVR model(s) are you concerned with?

Most will have an option to internally use AVcc as the reference voltage. the cap on the AREF pin allows for some filtering, and allows a choice of [internal] reference voltages to be selected.

I working with the MEGA1280,
Normally I don't connect to the AREF pin other than a cap as suggested - But because I have a situation where the sensors were powered by a different 5v regulator I thought it best to referance that voltage instead of it's supply voltage - but I guess not.

Things seem to work ok without it, but I will probably re-design it so both the 1280 and the sensors use the same v-regulator.

Don't Let the smoke out!

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Quote:

I working with the MEGA1280,

You certainly can use a specific voltage if you care to. Why would one 5V be better than another?

Note that totally split supplies can cause mysteries in operation. consider a short power-off interval, and a lightly loaded AVcc. Vcc will drop below the brown-out level and the AVR will go into reset. But the brown-out condition will not unlatch until AVcc drops below the brown-out level. With no load, that may take a long time to happen. So it the appears that the AVR fails to start.

Consider also, given the mention of accuracy, if your "good" voltage is 5.00V and the "common" voltage used at AVcc is 4.95V. Are you going to get those top two bits correctly?

Table 26-3. Voltage Reference Selections for ADC
REFS1 REFS0 Voltage Reference Selection(1)
0 0 AREF, Internal VREF turned off
0 1 AVCC with external capacitor at AREF pin
1 0 Internal 1.1V Voltage Reference with external capacitor at AREF pin
1 1 Internal 2.56V Voltage Reference with external capacitor at AREF pin

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.

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You could measure the sensor 5V via a resistor divider on yet another analogue input; if the sensors are sensitive to their supply voltage. If the sensors have their own accurate reference voltage there is not much point.

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Quote:
So... what is the AREF for if not used to referance a voltage?
As long as you understand that selecting anything other than AREF you will be causing an internal short circuit between the selected internal reference (1.1V, 2.56V for the 1280) and the AREF pin.

So if you want to connect AREF to another VCC make sure you put a suitable resistor on series with that to avoid problems.

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Quote:

So if you want to connect AREF to another VCC make sure you put a suitable resistor on series with that to avoid problems.

I guess I didn't mention that, but yah I am aware of it, but is that the only pitfall?

Don't Let the smoke out!

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I would still put a series resistor on AREF as the 2 supplies may not turn on or off at the same time, may not have any problems but then you could or it may happen in a few months from now or not at all. :)

ie "Don't let the smoke out"

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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js wrote:
I would still put a series resistor on AREF as the 2 supplies may not turn on or off at the same time, may not have any problems but then you could or it may happen in a few months from now or not at all. :)

ie "Don't let the smoke out"

Gotcha ya, thanks!

Don't Let the smoke out!