AVR-Chat

Hi Folks,

I'm using the ATmega48V in a project and I need to monitor the battery
level (2 AAA cells) every so often and when low give an indication to
the user that it's time to change them.

I'm trying to minimize current consumption and I'm using the internal
1.1V reference so I'm using a voltage divider 1M + 3M to bring the
signal down to below the 1.1V reference.  I don't care how long the
conversion takes as I'm only making the measurement every 10 minutes
or so.

Do I need to use a buffer between the divider and the input to the
ADC?  If I do, what small, cheap and low power chip would you recommend?

Thanks,

Richard Cooke

I think you need to disconnect the voltage divider using an fet. The voltage divider will draw more juice than the avr in sleep mode! I assume you read the datasheet about the half dozen or so best ways to cut down on power consumption? (turn off a/d, turn off pullups, which sleep mode to use, etc)

-----Original Message-----
From: Richard Cooke <rcooke@digitalwavelength.com>
To: AVR-Chat@yahoogroups.com
Sent: Mon, 18 Jun 2007 12:30 pm
Subject: [AVR-Chat] Using the ADC to measure the battery supply?



Hi Folks,

I'm using the ATmega48V in a project and I need to monitor the battery
level (2 AAA cells) every so often and when low give an indication to
the user that it's time to change them.

I'm trying to minimize current consumption and I'm using the internal
1.1V reference so I'm using a voltage divider 1M + 3M to bring the
signal down to below the 1.1V reference.  I don't care how long the
conversion takes as I'm only making the measurement every 10 minutes
or so.

Do I need to use a buffer between the divider and the input to the
ADC?  If I do, what small, cheap and low power chip would you recommend?

Thanks,

Richard Cooke



 
Yahoo! Groups Links





________________________________________________________________________
AOL now offers free email to everyone.  Find out more about what's free from AOL at AOL.com.


[Non-text portions of this message have been removed]

This is excellent advice from Bob, using a fet to connect / disconnect
the power supply to the voltage divider also means you can use lower
value resistors for the divider, I have had some issues with resistors
> 1M OHM. Not sure what the input impedance of the A2D pin on the
micro is, but you want to pick voltage divider resistor values that
are much lower to prevent the A2D pin from loading the voltage divider.

What I have done in the past in a similar situation is actually used a
Output Port pin to drive the top of the voltage divider directly, this
eliminates using an external fet and the voltage diference between the
port pin output and supply rail is minimal.

Hope this helps.

Cheers
Hein B
Auckland, New Zealand.

--- In AVR-Chat@yahoogroups.com, BobGardner@... wrote:
>
> I think you need to disconnect the voltage divider using an fet. The
voltage divider will draw more juice than the avr in sleep mode! I
assume you read the datasheet about the half dozen or so best ways to
cut down on power consumption? (turn off a/d, turn off pullups, which
sleep mode to use, etc)
> 
> -----Original Message-----
> From: Richard Cooke <rcooke@...>
> To: AVR-Chat@yahoogroups.com
> Sent: Mon, 18 Jun 2007 12:30 pm
> Subject: [AVR-Chat] Using the ADC to measure the battery supply?
> 
> 
> 
> Hi Folks,
> 
> I'm using the ATmega48V in a project and I need to monitor the battery
> level (2 AAA cells) every so often and when low give an indication to
> the user that it's time to change them.
> 
> I'm trying to minimize current consumption and I'm using the internal
> 1.1V reference so I'm using a voltage divider 1M + 3M to bring the
> signal down to below the 1.1V reference.  I don't care how long the
> conversion takes as I'm only making the measurement every 10 minutes
> or so.
> 
> Do I need to use a buffer between the divider and the input to the
> ADC?  If I do, what small, cheap and low power chip would you recommend?
> 
> Thanks,
> 
> Richard Cooke
> 
> 
> 
>  
> Yahoo! Groups Links
> 
> 
> 
> 
> 
> ________________________________________________________________________
> AOL now offers free email to everyone.  Find out more about what's
free from AOL at AOL.com.

> 
> 
> [Non-text portions of this message have been removed]
>

Thanks folks for the quick replies.

I'm not going to worry too much about the current consumption while
the unit is turned on since the RF chip will be using 12 mA so 1 or 2
uA more won't kill me.   I'm just worried about the input impedance
affecting the ADC reading.

Thanks,

Richard Cooke

--- In AVR-Chat@yahoogroups.com, "kernels_nz" <kernels@...> wrote:
>
> This is excellent advice from Bob, using a fet to connect / disconnect
> the power supply to the voltage divider also means you can use lower
> value resistors for the divider, I have had some issues with resistors
> > 1M OHM. Not sure what the input impedance of the A2D pin on the
> micro is, but you want to pick voltage divider resistor values that
> are much lower to prevent the A2D pin from loading the voltage divider.
> 
> What I have done in the past in a similar situation is actually used a
> Output Port pin to drive the top of the voltage divider directly, this
> eliminates using an external fet and the voltage diference between the
> port pin output and supply rail is minimal.
> 
> Hope this helps.
> 
> Cheers
> Hein B
> Auckland, New Zealand.
> 
> --- In AVR-Chat@yahoogroups.com, BobGardner@ wrote:
> >
> > I think you need to disconnect the voltage divider using an fet. The
> voltage divider will draw more juice than the avr in sleep mode! I
> assume you read the datasheet about the half dozen or so best ways to
> cut down on power consumption? (turn off a/d, turn off pullups, which
> sleep mode to use, etc)
> > 
> > -----Original Message-----
> > From: Richard Cooke <rcooke@>
> > To: AVR-Chat@yahoogroups.com
> > Sent: Mon, 18 Jun 2007 12:30 pm
> > Subject: [AVR-Chat] Using the ADC to measure the battery supply?
> > 
> > 
> > 
> > Hi Folks,
> > 
> > I'm using the ATmega48V in a project and I need to monitor the battery
> > level (2 AAA cells) every so often and when low give an indication to
> > the user that it's time to change them.
> > 
> > I'm trying to minimize current consumption and I'm using the internal
> > 1.1V reference so I'm using a voltage divider 1M + 3M to bring the
> > signal down to below the 1.1V reference.  I don't care how long the
> > conversion takes as I'm only making the measurement every 10 minutes
> > or so.
> > 
> > Do I need to use a buffer between the divider and the input to the
> > ADC?  If I do, what small, cheap and low power chip would you
recommend?
> > 
> > Thanks,
> > 
> > Richard Cooke
> > 
> > 
> > 
> >  
> > Yahoo! Groups Links
> > 
> > 
> > 
> > 
> > 
> >
________________________________________________________________________

> > AOL now offers free email to everyone.  Find out more about what's
> free from AOL at AOL.com.
> > 
> > 
> > [Non-text portions of this message have been removed]
> >
>

On Mon, 18 Jun 2007 13:02:17 -0400 you wrote:

>I think you need to disconnect the voltage divider using an fet. The voltage divider will draw more juice than the avr in sleep mode! I assume you read the datasheet about the half dozen or so best ways to cut down on power consumption? (turn off a/d, turn off pullups, which sleep mode to use, etc)
>

Forget the FET. 10M resistors are cheaper if the consumption is that critical.
The only buffer needed is a 0.1uF cap.

>-----Original Message-----
>From: Richard Cooke <rcooke@digitalwavelength.com>
>To: AVR-Chat@yahoogroups.com
>Sent: Mon, 18 Jun 2007 12:30 pm
>Subject: [AVR-Chat] Using the ADC to measure the battery supply?
>
>
>
>Hi Folks,
>
>I'm using the ATmega48V in a project and I need to monitor the battery
>level (2 AAA cells) every so often and when low give an indication to
>the user that it's time to change them.
>
>I'm trying to minimize current consumption and I'm using the internal
>1.1V reference so I'm using a voltage divider 1M + 3M to bring the
>signal down to below the 1.1V reference.  I don't care how long the
>conversion takes as I'm only making the measurement every 10 minutes
>or so.
>
>Do I need to use a buffer between the divider and the input to the
>ADC?  If I do, what small, cheap and low power chip would you recommend?
>
>Thanks,
>
>Richard Cooke

--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net

Most microcontroller's ADC will not like a 10M load, the charge time will
be very high and not very accurate.  I have used a low power OpAmp to run
very high Z dividers into and the output of the OpAmp is nicely low Z for
that ADC to be happy with.  Its a few more pins and passive parts than the
FET but the readings tend to be more repeatable.

DLC

> On Mon, 18 Jun 2007 13:02:17 -0400 you wrote:
>
>>I think you need to disconnect the voltage divider using an fet. The
>> voltage divider will draw more juice than the avr in sleep mode! I assume
>> you read the datasheet about the half dozen or so best ways to cut down
>> on power consumption? (turn off a/d, turn off pullups, which sleep mode
>> to use, etc)
>>
>
> Forget the FET. 10M resistors are cheaper if the consumption is that
> critical.
> The only buffer needed is a 0.1uF cap.
>
>>-----Original Message-----
>>From: Richard Cooke <rcooke@digitalwavelength.com>
>>To: AVR-Chat@yahoogroups.com
>>Sent: Mon, 18 Jun 2007 12:30 pm
>>Subject: [AVR-Chat] Using the ADC to measure the battery supply?
>>
>>
>>
>>Hi Folks,
>>
>>I'm using the ATmega48V in a project and I need to monitor the battery
>>level (2 AAA cells) every so often and when low give an indication to
>>the user that it's time to change them.
>>
>>I'm trying to minimize current consumption and I'm using the internal
>>1.1V reference so I'm using a voltage divider 1M + 3M to bring the
>>signal down to below the 1.1V reference.  I don't care how long the
>>conversion takes as I'm only making the measurement every 10 minutes
>>or so.
>>
>>Do I need to use a buffer between the divider and the input to the
>>ADC?  If I do, what small, cheap and low power chip would you recommend?
>>
>>Thanks,
>>
>>Richard Cooke
>
> --
> Ralph Hilton
> http://www.ralphhilton.org
> C-Meter: http://www.cmeter.org
> FZAOINT http://www.fzaoint.net
>
>
>
> Yahoo! Groups Links
>
>
>
>

On 6/18/07, kernels_nz <kernels@slingshot.co.nz> wrote:
> This is excellent advice from Bob, using a fet to connect / disconnect
> the power supply to the voltage divider also means you can use lower
> value resistors for the divider, I have had some issues with resistors
> > 1M OHM.

This is documented, the ADC inputs are not perfect infinite impedance
inputs with no leakage current.. You're best to feed them with a real
low Z source, and disconnect when not looking.

On Mon, 18 Jun 2007 15:44:07 -0600 (MDT) you wrote:

>Most microcontroller's ADC will not like a 10M load, the charge time will
>be very high and not very accurate.  I have used a low power OpAmp to run
>very high Z dividers into and the output of the OpAmp is nicely low Z for
>that ADC to be happy with.  Its a few more pins and passive parts than the
>FET but the readings tend to be more repeatable.
>
>DLC

It isn't 10M if a cap is across it! 

>> On Mon, 18 Jun 2007 13:02:17 -0400 you wrote:
>>
>>>I think you need to disconnect the voltage divider using an fet. The
>>> voltage divider will draw more juice than the avr in sleep mode! I assume
>>> you read the datasheet about the half dozen or so best ways to cut down
>>> on power consumption? (turn off a/d, turn off pullups, which sleep mode
>>> to use, etc)
>>>
>>
>> Forget the FET. 10M resistors are cheaper if the consumption is that
>> critical.
>> The only buffer needed is a 0.1uF cap.
>>

--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net

If the cap is across it then the ADC might not get the correct voltage 
either.  The ADC relies upon charging a cap inside the module.  That 
relies upon a certain target RC charge rate, that charge rate also must 
not exceed the current capacity of the pin.  You are playing a finicky 
game by using a cap to boost the inrush current to offset the RC time 
constant.  It might work under some circumstances but since the ADC 
modules specifically states that you should only be using slowly 
changing signals on the circuit with low-pass filters that would tend to 
contra indicate using a bypass cap I think.  Have you used such a 
circuit successfully before?

curious,
DLC



Ralph Hilton wrote:
> On Mon, 18 Jun 2007 15:44:07 -0600 (MDT) you wrote:
> 
> 
>>Most microcontroller's ADC will not like a 10M load, the charge time will
>>be very high and not very accurate.  I have used a low power OpAmp to run
>>very high Z dividers into and the output of the OpAmp is nicely low Z for
>>that ADC to be happy with.  Its a few more pins and passive parts than the
>>FET but the readings tend to be more repeatable.
>>
>>DLC
> 
> 
> It isn't 10M if a cap is across it! 
> 
> 
>>>On Mon, 18 Jun 2007 13:02:17 -0400 you wrote:
>>>
>>>
>>>>I think you need to disconnect the voltage divider using an fet. The
>>>>voltage divider will draw more juice than the avr in sleep mode! I assume
>>>>you read the datasheet about the half dozen or so best ways to cut down
>>>>on power consumption? (turn off a/d, turn off pullups, which sleep mode
>>>>to use, etc)
>>>>
>>>
>>>Forget the FET. 10M resistors are cheaper if the consumption is that
>>>critical.
>>>The only buffer needed is a 0.1uF cap.
>>>
> 
> 
> --
> Ralph Hilton
> http://www.ralphhilton.org
> C-Meter: http://www.cmeter.org
> FZAOINT http://www.fzaoint.net

-- 
-------------------------------------------------
Dennis Clark          TTT Enterprises
www.techtoystoday.com
-------------------------------------------------

I'm also curious, but a cap IS a low pass filter.

Cat
----- Original Message ----- 

From: "dlc" <dlc@frii.com>
To: <AVR-Chat@yahoogroups.com>
Sent: Monday, June 18, 2007 11:00 PM
Subject: Re: [AVR-Chat] Using the ADC to measure the battery supply?


> If the cap is across it then the ADC might not get the correct voltage 
> either.  The ADC relies upon charging a cap inside the module.  That 
> relies upon a certain target RC charge rate, that charge rate also must 
> not exceed the current capacity of the pin.  You are playing a finicky 
> game by using a cap to boost the inrush current to offset the RC time 
> constant.  It might work under some circumstances but since the ADC 
> modules specifically states that you should only be using slowly 
> changing signals on the circuit with low-pass filters that would tend to 
> contra indicate using a bypass cap I think.  Have you used such a 
> circuit successfully before?
> 
> curious,
> DLC
> 
>

1) The concept here is that the 0.1uF cap is way larger than the input cap
in the A/D and will present a low impedance source to the input.

2) What do you mean by "a certain RC charge rate?" AFAIK, the slope has
nothing to do with the A/D measurement, other than to say that you want a
steady input voltage during the actual measurement.

3) How in the world could adding a cap across the A/D input cause the input
current to exceed the capacity of the pin? We're not talking about a voltage
doubler - just a simple low pass filter.

4) I have used such a circuit very successfully before.

Phillip
________________________________

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of dlc
Sent: Tuesday, June 19, 2007 1:01 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Using the ADC to measure the battery supply?



If the cap is across it then the ADC might not get the correct voltage 
either. The ADC relies upon charging a cap inside the module. That 
relies upon a certain target RC charge rate, that charge rate also must 
not exceed the current capacity of the pin. You are playing a finicky 
game by using a cap to boost the inrush current to offset the RC time 
constant. It might work under some circumstances but since the ADC 
modules specifically states that you should only be using slowly 
changing signals on the circuit with low-pass filters that would tend to 
contra indicate using a bypass cap I think. Have you used such a 
circuit successfully before?

curious,
DLC

> I'm also curious, but a cap IS a low pass filter.

Depends, series or shunt?  Big cap or small?

DLC

> Cat
> ----- Original Message -----
> From: "dlc" <dlc@frii.com>
> To: <AVR-Chat@yahoogroups.com>
> Sent: Monday, June 18, 2007 11:00 PM
> Subject: Re: [AVR-Chat] Using the ADC to measure the battery supply?
>
>
>> If the cap is across it then the ADC might not get the correct voltage
>> either.  The ADC relies upon charging a cap inside the module.  That
>> relies upon a certain target RC charge rate, that charge rate also must
>> not exceed the current capacity of the pin.  You are playing a finicky
>> game by using a cap to boost the inrush current to offset the RC time
>> constant.  It might work under some circumstances but since the ADC
>> modules specifically states that you should only be using slowly
>> changing signals on the circuit with low-pass filters that would tend to
>> contra indicate using a bypass cap I think.  Have you used such a
>> circuit successfully before?
>>
>> curious,
>> DLC
>>
>>
>
>
>
> Yahoo! Groups Links
>
>
>
>

The word "across" in your message I quoted means something to me.
Big or Small doesn't make a difference (still LPF, better or worse).

----- Original Message ----- 

From: <dlc@frii.com>
To: <AVR-Chat@yahoogroups.com>
Sent: Tuesday, June 19, 2007 9:16 AM
Subject: Re: [AVR-Chat] Using the ADC to measure the battery supply?


>> I'm also curious, but a cap IS a low pass filter.
>
> Depends, series or shunt?  Big cap or small?
>
> DLC
>
>> Cat
>> ----- Original Message -----
>> From: "dlc" <dlc@frii.com>
>> To: <AVR-Chat@yahoogroups.com>
>> Sent: Monday, June 18, 2007 11:00 PM
>> Subject: Re: [AVR-Chat] Using the ADC to measure the battery supply?
>>
>>
>>> If the cap is across it then the ADC might not get the correct voltage
>>> either.  The ADC relies upon charging a cap inside the module.  That
>>> relies upon a certain target RC charge rate, that charge rate also must
>>> not exceed the current capacity of the pin.  You are playing a finicky
>>> game by using a cap to boost the inrush current to offset the RC time
>>> constant.  It might work under some circumstances but since the ADC
>>> modules specifically states that you should only be using slowly
>>> changing signals on the circuit with low-pass filters that would tend to
>>> contra indicate using a bypass cap I think.  Have you used such a
>>> circuit successfully before?
>>>
>>> curious,
>>> DLC
>>>
>>>
>>
>>
>>
>> Yahoo! Groups Links
>>
>>
>>
>>
>
>
>
>
>
> Yahoo! Groups Links
>
>
>
>

On Mon, 18 Jun 2007 23:00:54 -0600 you wrote:

We are talking about a battery voltage indicator needing 3 bits resolution max.

Lets get a bit more real here.

Keep it simple.

The original requirement was:

"I'm using the ATmega48V in a project and I need to monitor the battery
level (2 AAA cells) every so often and when low give an indication to
the user that it's time to change them."

Some of the points mentioned are possibly relevant for much higher resolution
and accuracy but the requirement is a simple indicator to tell the user to
change the battery !!!

>If the cap is across it then the ADC might not get the correct voltage 
>either.  The ADC relies upon charging a cap inside the module.  That 
>relies upon a certain target RC charge rate, that charge rate also must 
>not exceed the current capacity of the pin.  You are playing a finicky 
>game by using a cap to boost the inrush current to offset the RC time 
>constant.  It might work under some circumstances but since the ADC 
>modules specifically states that you should only be using slowly 
>changing signals on the circuit with low-pass filters that would tend to 
>contra indicate using a bypass cap I think.  Have you used such a 
>circuit successfully before?
>
>curious,
>DLC
>
>
>
>Ralph Hilton wrote:
>> On Mon, 18 Jun 2007 15:44:07 -0600 (MDT) you wrote:
>> 
>> 
>>>Most microcontroller's ADC will not like a 10M load, the charge time will
>>>be very high and not very accurate.  I have used a low power OpAmp to run
>>>very high Z dividers into and the output of the OpAmp is nicely low Z for
>>>that ADC to be happy with.  Its a few more pins and passive parts than the
>>>FET but the readings tend to be more repeatable.
>>>
>>>DLC
>> 
>> 
>> It isn't 10M if a cap is across it! 
>> 
>> 
>>>>On Mon, 18 Jun 2007 13:02:17 -0400 you wrote:
>>>>
>>>>
>>>>>I think you need to disconnect the voltage divider using an fet. The
>>>>>voltage divider will draw more juice than the avr in sleep mode! I assume
>>>>>you read the datasheet about the half dozen or so best ways to cut down
>>>>>on power consumption? (turn off a/d, turn off pullups, which sleep mode
>>>>>to use, etc)
>>>>>
>>>>
>>>>Forget the FET. 10M resistors are cheaper if the consumption is that
>>>>critical.
>>>>The only buffer needed is a 0.1uF cap.
>>>>

--
Ralph Hilton
http://www.ralphhilton.org
C-Meter: http://www.cmeter.org
FZAOINT http://www.fzaoint.net

Finally, someone read the actual task, and come back to reality.
It's great to see discussion on such topics, but sadly, us techies can get
so tied up in the design intricacies at times, we really need to keep
focused on the end goal and stay in perspective. Albeit, its fun at times to
get sidetracked ;)


  _____  

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of Ralph Hilton
Sent: Wednesday, 20 June 2007 10:14 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Using the ADC to measure the battery supply?



On Mon, 18 Jun 2007 23:00:54 -0600 you wrote:

We are talking about a battery voltage indicator needing 3 bits resolution
max.

Lets get a bit more real here.

Keep it simple.

The original requirement was:

"I'm using the ATmega48V in a project and I need to monitor the battery
level (2 AAA cells) every so often and when low give an indication to
the user that it's time to change them."

Some of the points mentioned are possibly relevant for much higher
resolution
and accuracy but the requirement is a simple indicator to tell the user to
change the battery !!!

>If the cap is across it then the ADC might not get the correct voltage 
>either. The ADC relies upon charging a cap inside the module. That 
>relies upon a certain target RC charge rate, that charge rate also must 
>not exceed the current capacity of the pin. You are playing a finicky 
>game by using a cap to boost the inrush current to offset the RC time 
>constant. It might work under some circumstances but since the ADC 
>modules specifically states that you should only be using slowly 
>changing signals on the circuit with low-pass filters that would tend to 
>contra indicate using a bypass cap I think. Have you used such a 
>circuit successfully before?
>
>curious,
>DLC
>
>
>
>Ralph Hilton wrote:
>> On Mon, 18 Jun 2007 15:44:07 -0600 (MDT) you wrote:
>> 
>> 
>>>Most microcontroller's ADC will not like a 10M load, the charge time will
>>>be very high and not very accurate. I have used a low power OpAmp to run
>>>very high Z dividers into and the output of the OpAmp is nicely low Z for
>>>that ADC to be happy with. Its a few more pins and passive parts than the
>>>FET but the readings tend to be more repeatable.
>>>
>>>DLC
>> 
>> 
>> It isn't 10M if a cap is across it! 
>> 
>> 
>>>>On Mon, 18 Jun 2007 13:02:17 -0400 you wrote:
>>>>
>>>>
>>>>>I think you need to disconnect the voltage divider using an fet. The
>>>>>voltage divider will draw more juice than the avr in sleep mode! I
assume
>>>>>you read the datasheet about the half dozen or so best ways to cut down
>>>>>on power consumption? (turn off a/d, turn off pullups, which sleep mode
>>>>>to use, etc)
>>>>>
>>>>
>>>>Forget the FET. 10M resistors are cheaper if the consumption is that
>>>>critical.
>>>>The only buffer needed is a 0.1uF cap.
>>>>

--
Ralph Hilton
http://www.ralphhil <http://www.ralphhilton.org> ton.org
C-Meter: http://www.cmeter. <http://www.cmeter.org> org
FZAOINT http://www.fzaoint. <http://www.fzaoint.net> net


 



[Non-text portions of this message have been removed]

On 6/19/07, Darren Worley <darren@worley.org.au> wrote:
> Finally, someone read the actual task, and come back to reality.
> It's great to see discussion on such topics, but sadly, us techies can get
> so tied up in the design intricacies at times, we really need to keep
> focused on the end goal and stay in perspective. Albeit, its fun at times to
> get sidetracked ;)

Before you think that the offset current is a sidetrack, I suggest you
look at the magnitude of that current, against the megohm-range values
of resistors that were suggested, and the real active range of a
battery measurement done that way, (in bits)then tell me that it's
irrelevant.