Forums

DGND and AGND

Started by codemsp430 September 5, 2003
Hi,
    
    In the hardware design part i have a small query.I am trying to 
interface a analog circuit to the microcontroller.While i simulated 
earlier I used two seperate batteries one for analog circuit and 
other for the micro(both 3V).The analog signals are send to the ADC 
of the micro.as you know the ADC takes the battery of the micro as 
reference.Now in the PCB design i am trying to use only one battery 
for analog and as well as the micro. How to take care of that.I know 
i have to connect a diode across the battery but what should be the 
ideal characteristic of diode to be considered or is there any other 
way that i can overcome this .

Thanks,
Raghu


Beginning Microcontrollers with the MSP430

Why do you think you need to connect a diode across the battery? this is 
unnecessary, and undesirable. The simple way to do this is to use a star 
  connection for the graound and power signals. basically at the point 
on the PCB where the battery terminals are you run separate tracks or 
planes for digital and analog, and make sure that at no time does a 
digital circuit connect to the analog track, You separately decouple 
these tracks. Normally I build 4 layer boards with internal power and 
ground. these are digital power and ground. I run a separate track from 
the positive terminal and decouple it right at the terminal to digital 
ground. I then decouple it along the track length. Similarly If I am 
unable to to create a separate split plane for the analog ground, again 
making sure that the origin of both is a single point at the negative 
terminal, I run a comparatively thick track on the outer layers.

Al

codemsp430 wrote:
> Hi,
>     
>     In the hardware design part i have a small query.I am trying to 
> interface a analog circuit to the microcontroller.While i simulated 
> earlier I used two seperate batteries one for analog circuit and 
> other for the micro(both 3V).The analog signals are send to the ADC 
> of the micro.as you know the ADC takes the battery of the micro as 
> reference.Now in the PCB design i am trying to use only one battery 
> for analog and as well as the micro. How to take care of that.I know 
> i have to connect a diode across the battery but what should be the 
> ideal characteristic of diode to be considered or is there any other 
> way that i can overcome this .
> 
> Thanks,
> Raghu
> 
> 
> 
> .
> 
>  
> 
> ">http://docs.yahoo.com/info/terms/ 
> 
> 
> 


Depending on the application, one might have to put a
diode in series of the battery to prevent damage to
the circuit if the battery is connected in reverse.
(There might be a voltage regulator which achieves the
same but it will add few more cents to the cost)
Use schottky diode. The diode will burn out 0.2 to 0.3
volts. Use internal reference of the uC for ADC that
way your reference voltage does not change with
battery discharge.

-Sumukh

--- onestone <onestone@ones...> wrote:
> Why do you think you need to connect a diode
across
> the battery? this is 
> unnecessary, and undesirable. The simple way to do
> this is to use a star 
>   connection for the graound and power signals.
> basically at the point 
> on the PCB where the battery terminals are you run
> separate tracks or 
> planes for digital and analog, and make sure that at
> no time does a 
> digital circuit connect to the analog track, You
> separately decouple 
> these tracks. Normally I build 4 layer boards with
> internal power and 
> ground. these are digital power and ground. I run a
> separate track from 
> the positive terminal and decouple it right at the
> terminal to digital 
> ground. I then decouple it along the track length.
> Similarly If I am 
> unable to to create a separate split plane for the
> analog ground, again 
> making sure that the origin of both is a single
> point at the negative 
> terminal, I run a comparatively thick track on the
> outer layers.
> 
> Al
> 
> codemsp430 wrote:
> > Hi,
> >     
> >     In the hardware design part i have a small
> query.I am trying to 
> > interface a analog circuit to the
> microcontroller.While i simulated 
> > earlier I used two seperate batteries one for
> analog circuit and 
> > other for the micro(both 3V).The analog signals
> are send to the ADC 
> > of the micro.as you know the ADC takes the battery
> of the micro as 
> > reference.Now in the PCB design i am trying to use
> only one battery 
> > for analog and as well as the micro. How to take
> care of that.I know 
> > i have to connect a diode across the battery but
> what should be the 
> > ideal characteristic of diode to be considered or
> is there any other 
> > way that i can overcome this .
> > 
> > Thanks,
> > Raghu
> > 
> > 
> > 
> > .
> > 
> >  
> > 
> > ">http://docs.yahoo.com/info/terms/ 
> > 
> > 
> > 
> 
> 


__________________________________


Hi thank you for your reply,but my basic concern is in getting the 
circuit working bcos the ADC takes the reference point as battery 
and ofcourse the analog circuit also consumes power leading to a 
decrease in power of single battery faster and how will i have to 
change the software for it accordingly for making sure that the ADC 
is working ok.

Thanks,
raghu

--- In msp430@msp4..., onestone <onestone@b...> wrote:
> Why do you think you need to connect a diode
across the battery? 
this is 
> unnecessary, and undesirable. The simple way to do
this is to use 
a star 
>   connection for the graound and power signals.
basically at the 
point 
> on the PCB where the battery terminals are you run
separate tracks 
or 
> planes for digital and analog, and make sure that
at no time does 
a 
> digital circuit connect to the analog track, You
separately 
decouple 
> these tracks. Normally I build 4 layer boards with
internal power 
and 
> ground. these are digital power and ground. I run
a separate track 
from 
> the positive terminal and decouple it right at the
terminal to 
digital 
> ground. I then decouple it along the track length.
Similarly If I 
am 
> unable to to create a separate split plane for the
analog ground, 
again 
> making sure that the origin of both is a single
point at the 
negative 
> terminal, I run a comparatively thick track on the
outer layers.
> 
> Al
> 
> codemsp430 wrote:
> > Hi,
> >     
> >     In the hardware design part i have a small query.I am trying 
to 
> > interface a analog circuit to the
microcontroller.While i 
simulated 
> > earlier I used two seperate batteries one for
analog circuit and 
> > other for the micro(both 3V).The analog signals are send to the 
ADC 
> > of the micro.as you know the ADC takes the
battery of the micro 
as 
> > reference.Now in the PCB design i am trying
to use only one 
battery 
> > for analog and as well as the micro. How to
take care of that.I 
know 
> > i have to connect a diode across the battery
but what should be 
the 
> > ideal characteristic of diode to be
considered or is there any 
other 
> > way that i can overcome this .
> > 
> > Thanks,
> > Raghu
> > 
> > 
> > 
> > .
> > 
> >  
> > 
> > ">http://docs.yahoo.com/info/terms/ 
> > 
> > 
> >


On Sat, Sep 06, 2003 at 03:23:54AM +0930, onestone wrote:
> Why do you think you need to connect a diode
across the battery? this is 
> unnecessary, and undesirable. 

I think the diode across the bat is to protect the MSP for reverse battery
voltage. Advantage is no voltage drop, disadvantage a burned battery,
however I have never seen a small Li battery getting hot because of short
circuit.

        Matthias

OK, your first post specified "across the battery". This is in
parallel, 
not in series. Personally I would avoid this anyway on battery powered 
equipment, since you assume the battery holder is designed to work one 
way. The voltage drop is too much in a low power circuit.

Al

Sumukh Pathare wrote:
> Depending on the application, one might have to put a
> diode in series of the battery to prevent damage to
> the circuit if the battery is connected in reverse.
> (There might be a voltage regulator which achieves the
> same but it will add few more cents to the cost)
> Use schottky diode. The diode will burn out 0.2 to 0.3
> volts. Use internal reference of the uC for ADC that
> way your reference voltage does not change with
> battery discharge.
> 
> -Sumukh
> 
> --- onestone <onestone@ones...> wrote:
> 
>>Why do you think you need to connect a diode across
>>the battery? this is 
>>unnecessary, and undesirable. The simple way to do
>>this is to use a star 
>>  connection for the graound and power signals.
>>basically at the point 
>>on the PCB where the battery terminals are you run
>>separate tracks or 
>>planes for digital and analog, and make sure that at
>>no time does a 
>>digital circuit connect to the analog track, You
>>separately decouple 
>>these tracks. Normally I build 4 layer boards with
>>internal power and 
>>ground. these are digital power and ground. I run a
>>separate track from 
>>the positive terminal and decouple it right at the
>>terminal to digital 
>>ground. I then decouple it along the track length.
>>Similarly If I am 
>>unable to to create a separate split plane for the
>>analog ground, again 
>>making sure that the origin of both is a single
>>point at the negative 
>>terminal, I run a comparatively thick track on the
>>outer layers.
>>
>>Al
>>
>>codemsp430 wrote:
>>
>>>Hi,
>>>    
>>>    In the hardware design part i have a small
>>
>>query.I am trying to 
>>
>>>interface a analog circuit to the
>>
>>microcontroller.While i simulated 
>>
>>>earlier I used two seperate batteries one for
>>
>>analog circuit and 
>>
>>>other for the micro(both 3V).The analog signals
>>
>>are send to the ADC 
>>
>>>of the micro.as you know the ADC takes the battery
>>
>>of the micro as 
>>
>>>reference.Now in the PCB design i am trying to use
>>
>>only one battery 
>>
>>>for analog and as well as the micro. How to take
>>
>>care of that.I know 
>>
>>>i have to connect a diode across the battery but
>>
>>what should be the 
>>
>>>ideal characteristic of diode to be considered or
>>
>>is there any other 
>>
>>>way that i can overcome this .
>>>
>>>Thanks,
>>>Raghu
>>>
>>>
>>>
>>>.
>>>
>>> 
>>>
>>>">http://docs.yahoo.com/info/terms/ 
>>
>>>
>>>
>>
> 
> 
> __________________________________
> 
> 
> 
> .
> 
>  
> 
> ">http://docs.yahoo.com/info/terms/ 
> 
> 
> 


First of all the ADC can take eight different combinations of reference. 
Vr+ can be internal 1.5V, internal 2.5V, AVcc or an externally supplied 
reference voltage. If you follow my previous posting to its obvious 
conclusion AVcc is derived from the same battery as DVcc, but contains 
no digital noise, as it is isolated at source. You should follow the 
usual decoupling recommendations in SLAU049C. otherwise the setup is 
absolutely no different to the one you would have used with 2 batteries. 
If you use AVcc  as the reference before there is no need to change 
this. Forget the diode.

Al

codemsp430 wrote:

> Hi thank you for your reply,but my basic concern
is in getting the 
> circuit working bcos the ADC takes the reference point as battery 
> and ofcourse the analog circuit also consumes power leading to a 
> decrease in power of single battery faster and how will i have to 
> change the software for it accordingly for making sure that the ADC 
> is working ok.
> 
> Thanks,
> raghu
> 
> --- In msp430@msp4..., onestone <onestone@b...> wrote:
> 
>>Why do you think you need to connect a diode across the battery? 
> 
> this is 
> 
>>unnecessary, and undesirable. The simple way to do this is to use 
> 
> a star 
> 
>>  connection for the graound and power signals. basically at the 
> 
> point 
> 
>>on the PCB where the battery terminals are you run separate tracks 
> 
> or 
> 
>>planes for digital and analog, and make sure that at no time does 
> 
> a 
> 
>>digital circuit connect to the analog track, You separately 
> 
> decouple 
> 
>>these tracks. Normally I build 4 layer boards with internal power 
> 
> and 
> 
>>ground. these are digital power and ground. I run a separate track 
> 
> from 
> 
>>the positive terminal and decouple it right at the terminal to 
> 
> digital 
> 
>>ground. I then decouple it along the track length. Similarly If I 
> 
> am 
> 
>>unable to to create a separate split plane for the analog ground, 
> 
> again 
> 
>>making sure that the origin of both is a single point at the 
> 
> negative 
> 
>>terminal, I run a comparatively thick track on the outer layers.
>>
>>Al
>>
>>codemsp430 wrote:
>>
>>>Hi,
>>>    
>>>    In the hardware design part i have a small query.I am trying 
> 
> to 
> 
>>>interface a analog circuit to the microcontroller.While i 
> 
> simulated 
> 
>>>earlier I used two seperate batteries one for analog circuit and 
>>>other for the micro(both 3V).The analog signals are send to the 
> 
> ADC 
> 
>>>of the micro.as you know the ADC takes the battery of the micro 
> 
> as 
> 
>>>reference.Now in the PCB design i am trying to use only one 
> 
> battery 
> 
>>>for analog and as well as the micro. How to take care of that.I 
> 
> know 
> 
>>>i have to connect a diode across the battery but what should be 
> 
> the 
> 
>>>ideal characteristic of diode to be considered or is there any 
> 
> other 
> 
>>>way that i can overcome this .
>>>
>>>Thanks,
>>>Raghu
>>>
>>>
>>>
>>>.
>>>
>>> 
>>>
>>>">http://docs.yahoo.com/info/terms/ 
> 
>>>
>>>
> 
> 
> 
> .
> 
>  
> 
> ">http://docs.yahoo.com/info/terms/ 
> 
> 
> 


I've seen batteries blow spectacularly using this over simplistic 
protection. The simple and easy answer is a battery clip that is polarised.

Al

Matthias Weingart wrote:

> On Sat, Sep 06, 2003 at 03:23:54AM +0930,
onestone wrote:
> 
>>Why do you think you need to connect a diode across the battery? this is

>>unnecessary, and undesirable. 
> 
> 
> I think the diode across the bat is to protect the MSP for reverse battery
> voltage. Advantage is no voltage drop, disadvantage a burned battery,
> however I have never seen a small Li battery getting hot because of short
> circuit.
> 
>         Matthias
> 
> 
> .
> 
>  
> 
> ">http://docs.yahoo.com/info/terms/ 
> 
> 
>