Forums

ADC limit

Started by john Mcdonald September 9, 2003
 
Hello all,
 
Please help.  My question is in regard to ADC on MSP430F149 please.  If AVCC=3.0
Vref=1.5 what could my maximum input voltage to A/D be?  If I select Vref=2.5
what is my maximum input signal, please.
 
Thanks,
John



---------------------------------
Do you Yahoo!?
Yahoo! SiteBuilder - Free, easy-to-use web site design software




Beginning Microcontrollers with the MSP430

> Please help.  My question is in regard to ADC on MSP430F149 
please.  If AVCC=3.0 Vref=1.5 what could my maximum input voltage to 
A/D be?  If I select Vref=2.5 what is my maximum input signal, please.

Hi John,

You get 0x0FFF as ADC result when the input signal reaches Vref. So 
the maximum USEFUL input voltage is Vref.
However, you won't damage the part with an input signal of up to 
VCC+0.3V (see absolute maximum ratings).

Hope this helped,
Wolfgang




As I see it your limited to the +Vcc voltage that your applying to the
MCU.
Thus if your supplying 3.3v then the ADC is limited to 3.3v also.
You need to use a resistor divider circuit to limit the voltage to the
ADC.
For example you could feed a 5v max signal into something like a 1k
resistor going to a 2k resistor going to Vss. You would tap off between
the two resistors to the ADC.
12v would need higher values to keep it so that the max the ADC sees
doesn't exceed 3.3v.
  You can increase the resistor network values to reduce current
consumption too.
 
 
-----Original Message-----
From: john Mcdonald [mailto:eeaj2002@eeaj...] 
Sent: Tuesday, September 09, 2003 10:46 AM
To: msp430@msp4...
Subject: [msp430] ADC limit
 

Hello all,

Please help.  My question is in regard to ADC on MSP430F149 please.  If
AVCC=3.0 Vref=1.5 what could my maximum input voltage to A/D be?  If I
select Vref=2.5 what is my maximum input signal, please.

Thanks,
John



---------------------------------

005378:HM/A54452/R=0/SIGtjeu2hk/*http:/www.netflix.com/Default?mqs
o`178324&partid856299> click here

 
<http://us.adserver.yahoo.com/l?M%1812.3856299.5103760.1261774/D=egrou
pmail/S=:HM/A54452/randi8785804> 

.



">http://docs.yahoo.com/info/terms/>  Terms of Service. 






Just a word of caution: 
If you use an resistive divider at the input of the
ADC make sure that RC (C being the input capacitance
of the pin/internal capacitance) is much smaller than
samping duration of the Sample and Hold ckt of the
ADC.

-Sumukh




--- Earl Bollinger <earlwbollinger@earl...> wrote:
> As I see it your limited to the +Vcc voltage
that
> your applying to the
> MCU.
> Thus if your supplying 3.3v then the ADC is limited
> to 3.3v also.
> You need to use a resistor divider circuit to limit
> the voltage to the
> ADC.
> For example you could feed a 5v max signal into
> something like a 1k
> resistor going to a 2k resistor going to Vss. You
> would tap off between
> the two resistors to the ADC.
> 12v would need higher values to keep it so that the
> max the ADC sees
> doesn't exceed 3.3v.
>   You can increase the resistor network values to
> reduce current
> consumption too.
>  
>  
> -----Original Message-----
> From: john Mcdonald [mailto:eeaj2002@eeaj...] 
> Sent: Tuesday, September 09, 2003 10:46 AM
> To: msp430@msp4...
> Subject: [msp430] ADC limit
>  
> 
> Hello all,
> 
> Please help.  My question is in regard to ADC on
> MSP430F149 please.  If
> AVCC=3.0 Vref=1.5 what could my maximum input
> voltage to A/D be?  If I
> select Vref=2.5 what is my maximum input signal,
> please.
> 
> Thanks,
> John
> 
> 
> 
> ---------------------------------
> 
>
005378:HM/A54452/R=0/SIGtjeu2hk/*http:/www.netflix.com/Default?mqs
> o`178324&partid856299> click here
> 
>  
>
<http://us.adserver.yahoo.com/l?M%1812.3856299.5103760.1261774/D=egrou
> pmail/S=:HM/A54452/randi8785804> 
> 
> .
> 
> 
> 
> ">http://docs.yahoo.com/info/terms/>  Terms of
> Service. 
> 
> 
> [Non-text portions of this message have been
> removed]
> 
> 
> 


__________________________________


 
 
I am not sure but I think your max A/D singnal is based on your Vref. 
Therefore, it does not mater what your AVCC is.  If you select Vref=1.5v
therefore when your input signal to A/D is equal to Vref that is your max.
 
Regards,
John.

Earl Bollinger <earlwbollinger@earl...> wrote:
As I see it your limited to the +Vcc voltage that your applying to the
MCU.
Thus if your supplying 3.3v then the ADC is limited to 3.3v also.
You need to use a resistor divider circuit to limit the voltage to the
ADC.
For example you could feed a 5v max signal into something like a 1k
resistor going to a 2k resistor going to Vss. You would tap off between
the two resistors to the ADC.
12v would need higher values to keep it so that the max the ADC sees
doesn't exceed 3.3v.
  You can increase the resistor network values to reduce current
consumption too.


-----Original Message-----
From: john Mcdonald [mailto:eeaj2002@eeaj...] 
Sent: Tuesday, September 09, 2003 10:46 AM
To: msp430@msp4...
Subject: [msp430] ADC limit


Hello all,

Please help.  My question is in regard to ADC on MSP430F149 please.  If
AVCC=3.0 Vref=1.5 what could my maximum input voltage to A/D be?  If I
select Vref=2.5 what is my maximum input signal, please.

Thanks,
John



---------------------------------

005378:HM/A54452/R=0/SIGtjeu2hk/*http:/www.netflix.com/Default?mqs
o`178324&partid856299> click here


<http://us.adserver.yahoo.com/l?M%1812.3856299.5103760.1261774/D=egrou
pmail/S=:HM/A54452/randi8785804> 

.



">http://docs.yahoo.com/info/terms/>  Terms of Service. 








.



 


---------------------------------
Do you Yahoo!?
Yahoo! SiteBuilder - Free, easy-to-use web site design software




You're right of course. For some reason I didn't see the original post. 
You can input any voltage up to the maximum allowed on an anlog input, 
but all voltages higher than Vref will return 0x0FFF.

Al

john Mcdonald wrote:

>  
>  
> I am not sure but I think your max A/D singnal is based on your Vref. 
Therefore, it does not mater what your AVCC is.  If you select Vref=1.5v
therefore when your input signal to A/D is equal to Vref that is your max.
>  
> Regards,
> John.
> 
> Earl Bollinger <earlwbollinger@earl...> wrote:
> As I see it your limited to the +Vcc voltage that your applying to the
> MCU.
> Thus if your supplying 3.3v then the ADC is limited to 3.3v also.
> You need to use a resistor divider circuit to limit the voltage to the
> ADC.
> For example you could feed a 5v max signal into something like a 1k
> resistor going to a 2k resistor going to Vss. You would tap off between
> the two resistors to the ADC.
> 12v would need higher values to keep it so that the max the ADC sees
> doesn't exceed 3.3v.
>   You can increase the resistor network values to reduce current
> consumption too.
> 
> 
> -----Original Message-----
> From: john Mcdonald [mailto:eeaj2002@eeaj...] 
> Sent: Tuesday, September 09, 2003 10:46 AM
> To: msp430@msp4...
> Subject: [msp430] ADC limit
> 
> 
> Hello all,
> 
> Please help.  My question is in regard to ADC on MSP430F149 please.  If
> AVCC=3.0 Vref=1.5 what could my maximum input voltage to A/D be?  If I
> select Vref=2.5 what is my maximum input signal, please.
> 
> Thanks,
> John
> 
> 
> 
> ---------------------------------
> 
> 005378:HM/A54452/R=0/SIGtjeu2hk/*http:/www.netflix.com/Default?mqs
> o`178324&partid856299> click here
> 
> 
> <http://us.adserver.yahoo.com/l?M%1812.3856299.5103760.1261774/D=egrou
> pmail/S=:HM/A54452/randi8785804> 
> 
> .
> 
> 
> 
> ">http://docs.yahoo.com/info/terms/>  Terms of Service. 
> 
> 
> 
> 
> 
> 
> 
> 
> .
> 
> 
> 
>  
> 
> 
> ---------------------------------
>  
> 
> 
>