Post by thread:[PIC]: AD877 with negative input voltage

I'm not agree, see my conclusion below. On Fri, 2 Mar 2001, Drew Vassallo wrote: > >I am attempting to do a A/D conversion to a signal ranging from -1 VDC to > >+5.5. VDC. I am using a PIC16F877. I understand to need to use the VREF, > >+VREF, and -VREF pins in order to do this. The problem is that I have not > >found a good description of how to use/apply these pins. > > This may be out of spec. The analog input minimum voltage spec for Vain and > Vref- is Vss-0.3 Volts. I don't know what the chip would do with > -1.0 V. > Well, I've dig out enough to afirm that 877 AD WORKS OK with negative input signals. I know many of guru's of this list will cry: IT'S OUT OF SPECIFICATION. Who care, if works perfectly ! Here are three important tested conclusion: 1. With 877 suplied from +5.00V and gnd ( single power supply ) -Vref to -0.5V, +Vref to +4.5V input range is between -0.5 to +4.5V all other ad specification respect data sheet information. Do not try to increase -vref level, will not work because at -0.6V...-0.7V the input protective diodes will go on and some important currents will flow on these diodes ( so the vss-0.3V is a joke, they have standard silicon protective input diodes, maybe schottky) 2. If you need a greater gain that AD has ( 0...5V to 0...1023 points; 4.8mv resolution ) you may use +vref and -vref but also to respect the following condition: a. +vref - |-vref| >= 1.7V where the |-vref| is the absolute value of -vref, b. |-vref| < +Vref c. |-vref| <= 2.5V, +Vref >= 2.5V but respect a. condition. Example: AD will work with 10bit resolution if -vref = 2V, +vref = 4V so, for a 0...2V input voltage we have 0...1023 points, 1.9mV resolution which is pretty good... AD will work with less than 10bit resolution if +vref - |-vref| < 1.7V AD will not working if +vref < |- vref| 3.AD could measure input signals between -2.5V to +2.5V without any external active components ( operational amplifiers, etc...) But I want to hear first your solutions, gentlemans... Cheers, Vasile -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> Well, I've dig out enough to afirm that 877 AD WORKS OK with negative > input signals. I know many of guru's of this list will cry: IT'S OUT OF > SPECIFICATION. Who care, if works perfectly ! ****IT'S OUT OF SPECIFICATION**** Yeah, it works perfectly: on the one chip you tested at room temperature at Vdd = 5.00V etc. (btw: does responding to this bait make me a guru?) Bob Ammerman RAm Systems (contract development of high performance, high function, low-level software) {Quote hidden}> Here are three important tested conclusion: > 1. With 877 suplied from +5.00V and gnd ( single power supply ) > -Vref to -0.5V, +Vref to +4.5V input range is between -0.5 to +4.5V > all other ad specification respect data sheet information. > Do not try to increase -vref level, will not work because at > -0.6V...-0.7V the input protective diodes will go on and some important > currents will flow on these diodes > ( so the vss-0.3V is a joke, they have standard silicon protective > input diodes, maybe schottky) > 2. If you need a greater gain that AD has ( 0...5V to 0...1023 > points; 4.8mv resolution ) > you may use +vref and -vref but also to respect the following > condition: > a. +vref - |-vref| >= 1.7V where the |-vref| is the absolute > value of -vref, > b. |-vref| < +Vref > c. |-vref| <= 2.5V, +Vref >= 2.5V but respect a. condition. > > Example: AD will work with 10bit resolution if -vref = 2V, +vref = 4V > so, for a 0...2V input voltage we have 0...1023 points, 1.9mV > resolution which is pretty good... > AD will work with less than 10bit resolution if +vref - |-vref| < 1.7V > AD will not working if +vref < |- vref| > > 3.AD could measure input signals between -2.5V to +2.5V without any > external active components ( operational amplifiers, etc...) > But I want to hear first your solutions, gentlemans... > > Cheers, > Vasile > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

> Well, I've dig out enough to afirm that 877 AD WORKS OK with negative > input signals. I know many of guru's of this list will cry: IT'S OUT OF > SPECIFICATION. Who care, if works perfectly ! Whoever has to use it would. Just because it appears to work on one PIC at a particular temperature doesn't mean it will work on other PICs, or the same PIC at a different temperature. You also don't know if you are causing hidden damage that will catch you later. An obvious possibility is blowing the protection diodes. ***************************************************************** Olin Lathrop, embedded systems consultant in Devens Massachusetts (978) 772-3129, spam_OUTolinTakeThisOuTembedinc.com, http://www.embedinc.com -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

OK , it works. But be very careful. Diode forward voltage drops with temperature and schotty diode leakage increases dramatically with temperature. If the application is going to have to work at anything over 20C I'd strongly recommend a level shift of some sort and possibly a voltage divider to ensure the input voltage stays within limits. For a one-off try - great. For production -sorry! Richard P I'm not agree, see my conclusion below. On Fri, 2 Mar 2001, Drew Vassallo wrote: > >I am attempting to do a A/D conversion to a signal ranging from -1 VDC to > >+5.5. VDC. I am using a PIC16F877. I understand to need to use the VREF, > >+VREF, and -VREF pins in order to do this. The problem is that I have not > >found a good description of how to use/apply these pins. > > This may be out of spec. The analog input minimum voltage spec for Vain and > Vref- is Vss-0.3 Volts. I don't know what the chip would do with > -1.0 V. > Well, I've dig out enough to afirm that 877 AD WORKS OK with negative input signals. I know many of guru's of this list will cry: IT'S OUT OF SPECIFICATION. Who care, if works perfectly ! Here are three important tested conclusion: 1. With 877 suplied from +5.00V and gnd ( single power supply ) -Vref to -0.5V, +Vref to +4.5V input range is between -0.5 to +4.5V all other ad specification respect data sheet information. Do not try to increase -vref level, will not work because at -0.6V...-0.7V the input protective diodes will go on and some important currents will flow on these diodes ( so the vss-0.3V is a joke, they have standard silicon protective input diodes, maybe schottky) 2. If you need a greater gain that AD has ( 0...5V to 0...1023 points; 4.8mv resolution ) you may use +vref and -vref but also to respect the following condition: a. +vref - |-vref| >= 1.7V where the |-vref| is the absolute value of -vref, b. |-vref| < +Vref c. |-vref| <= 2.5V, +Vref >= 2.5V but respect a. condition. Example: AD will work with 10bit resolution if -vref = 2V, +vref = 4V so, for a 0...2V input voltage we have 0...1023 points, 1.9mV resolution which is pretty good... AD will work with less than 10bit resolution if +vref - |-vref| < 1.7V AD will not working if +vref < |- vref| 3.AD could measure input signals between -2.5V to +2.5V without any external active components ( operational amplifiers, etc...) But I want to hear first your solutions, gentlemans... Cheers, Vasile -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Mon, 5 Mar 2001, Bob Ammerman wrote: > > Well, I've dig out enough to afirm that 877 AD WORKS OK with negative > > input signals. I know many of guru's of this list will cry: IT'S OUT OF > > SPECIFICATION. Who care, if works perfectly ! > > ****IT'S OUT OF SPECIFICATION**** > > Yeah, it works perfectly: > > on the one chip you tested > at room temperature > at Vdd = 5.00V > etc. > > (btw: does responding to this bait make me a guru?) > > > 3.AD could measure input signals between -2.5V to +2.5V without any > > external active components ( operational amplifiers, etc...) > > But I want to hear first your solutions, gentlemans... > > Sorry, but when I do an affirmation it means I have made some test: on more than one chip ( tested on three ) at different temperature than room one ( do you haven't there climatic chambers ? ) but not at +50C, neither at -20C where any operational amplifiers or AD needs to pray to God...( and this ir REAL and not taken from specifications ) in standard TTL range power supply ( 4.75...5.25V ) But I hear no comments about my last affirmation (+/-2.5V ) which *is in specification* ,of course, with a trick. by, Vasile -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email .....listservKILLspam@spam@mitvma.mit.edu with SET PICList DIGEST in the body

On Mon, 5 Mar 2001, Olin Lathrop wrote: > > Well, I've dig out enough to afirm that 877 AD WORKS OK with negative > > input signals. I know many of guru's of this list will cry: IT'S OUT OF > > SPECIFICATION. Who care, if works perfectly ! > > Whoever has to use it would. Just because it appears to work on one PIC at > a particular temperature doesn't mean it will work on other PICs, or the > same PIC at a different temperature. You also don't know if you are causing > hidden damage that will catch you later. An obvious possibility is blowing > the protection diodes. > Blowing protection diodes is impossible since I have use input protection resistors. And I've tested on different "human" temperatures (0 to 30C ) Of course is possible to don't work on different pics or different pics from same family. Vasile -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body