Post by thread:[PIC]: 16F877 ADC Probs (What will it measure)

Sorry, can someone answer my other question? >Hi I'm having some problems with the ADC on the 16F877. Am I correct in >thinking that I can connect the sampling pin straight to a voltage (is this >limited to 0 -> 5 Volts). Is some sort of protection recomended. > Thanks Graham _________________________________________________________________________ Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

You can connect it to a voltage between 0 and Vdd plus or minus whatever the datasheet says. No series rsistance. The datasheet also says the source impedance should be 10K Ohms or less. Dale On Mon, 23 Apr 2001, Graham North wrote: {Quote hidden}> Sorry, can someone answer my other question? > > > >Hi I'm having some problems with the ADC on the 16F877. Am I correct in > >thinking that I can connect the sampling pin straight to a voltage (is this > >limited to 0 -> 5 Volts). Is some sort of protection recomended. > > > > Thanks > > Graham > > _________________________________________________________________________ > Get Your Private, Free E-mail from MSN Hotmail at http://www.hotmail.com. > > -- > http://www.piclist.com hint: The list server can filter out subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > --- The most exciting phrase to hear in science, the one that heralds new discoveries, is not "Eureka!" (I found it!) but "That's funny ..." -- Isaac Asimov -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

At 05:10 PM 4/23/01 -0500, you wrote: >You can connect it to a voltage between 0 and Vdd plus or minus whatever >the datasheet says. No series rsistance. The datasheet also says the >source impedance should be 10K Ohms or less. > >Dale Note that this limitation (or a maximum input current limitation) applies even if Vdd = 0. IOW, if you apply an input voltage to the analog input with the PIC not powered, the voltage must be within the range -300mV < Vin < 300mV (or whatever the data sheet says), OR the current must be limited to the maximum current allowable on the data sheet (with some margin, maybe 10mA). Otherwise Bad Things can happen, especially at high temperatures, involving parasitic thyristors on the chip. Best regards, =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany --"it's the network..." "The Journey is the reward" spam_OUTspeffTakeThisOuTinterlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= -- http://www.piclist.com hint: The list server can filter out subtopics (like ads or off topics) for you. See http://www.piclist.com/#topics

Graham North wrote: > > Sorry, can someone answer my other question? > > >Hi I'm having some problems with the ADC on the 16F877. Am I correct in > >thinking that I can connect the sampling pin straight to a voltage (is this > >limited to 0 -> 5 Volts). Is some sort of protection recomended. Hi Graham, first, your code sample showed you were setting ADCON1 to "all pins digital", this is a good possibility why your ADC is not working. Re the ADC input pin, I suggest using the largest resistor you can, this gives greater safety to the PIC and reduces strain on it's protect diodes if you mess up. With a signal that changes very fast, you will need to keep the series resistor smaller to reduce RC effects on the input pin, but 1k should be fine for most things. For most slowly changing analogue signals I use a 10k resistor. If you need to measure a higher voltage, like a 12v battery for instance, just use two resistors in a voltage divider. say a 4.7k and 2.2k gives about 3:1 and will give you 0v to 15v measuring. :o) -Roman -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

{Quote hidden}> -----Original Message----- > From: Roman Black [SMTP:.....fastvidKILLspam@spam@EZY.NET.AU] > Sent: Tuesday, April 24, 2001 8:41 AM > To: PICLISTKILLspamMITVMA.MIT.EDU > Subject: Re: [PIC]: 16F877 ADC Probs (What will it measure) > > Graham North wrote: > > > > Sorry, can someone answer my other question? > > > > >Hi I'm having some problems with the ADC on the 16F877. Am I correct in > > >thinking that I can connect the sampling pin straight to a voltage (is > this > > >limited to 0 -> 5 Volts). Is some sort of protection recomended. > > > Hi Graham, first, your code sample showed you > were setting ADCON1 to "all pins digital", this > is a good possibility why your ADC is not working. > > Re the ADC input pin, I suggest using the largest > resistor you can, this gives greater safety to the > PIC and reduces strain on it's protect diodes if > you mess up. With a signal that changes very fast, > you will need to keep the series resistor smaller > to reduce RC effects on the input pin, but 1k > should be fine for most things. For most slowly > changing analogue signals I use a 10k resistor. > > If you need to measure a higher voltage, like a > 12v battery for instance, just use two resistors > in a voltage divider. say a 4.7k and 2.2k gives > about 3:1 and will give you 0v to 15v measuring. > :o) > -Roman > 10K is the maximum recommended impedance of your measurement source (actually, my data book says 10 kW !!!). It's a good idea to make this value as low as possible for the most accurate conversions. Due to the way the internal ADC works, the pin can be loaded by varying amounts during conversion (according to an FAE). A high source impedance will give a greater voltage change during conversion as well as increasing acquisistion time. Ideally use an op-amp to buffer and optionaly filter your analogue signal. Regards Mike -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.