Jim Thompson wrote:> On Sat, 02 Jun 2012 16:50:10 -0400, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: > > >>Jim Thompson wrote: >> >>>On Thu, 24 May 2012 18:14:18 -0700, John Larkin >>><jlarkin@highlandtechnology.com> wrote: >>> >>> >>> >>>>On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden >>>><bperryb@bowdenshobbycircuits.info> wrote: >>>> >>>> >>>> >>>>>On May 24, 7:29 am, John Larkin >>>>><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>> >>>>> >>>>>>On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden >>>>>> >>>>>><bper...@bowdenshobbycircuits.info> wrote: >>>>>> >>>>>> >>>>>>>The LM339 spec sheet indicates the maximum negative input voltage >>>>>>>should not be more than -0.3 volts. The schematic shows the input >>>>>>>connected to the base of a PNP transistor with the collector at the >>>>>>>negative supply terminal. So, it looks like if the input or base of >>>>>>>the transistor goes more negative than about 0.7 volts relative to the >>>>>>>negative supply point, the transistor will conduct like a diode from >>>>>>>base to collector. Is this situation acceptable, and will the >>>>>>>comparator work with one input equal to the negative supply (0 volts) >>>>>>>while the other is -0.7 volts below the negative supply point? >>>>>> >>>>>>>-Bill >>>>>> >>>>>>The 339, like the LM324, has some sneak paths that squirt current all >>>>>>around the chip if you pull any input much below ground. One such >>>>>>pulled-low input, even a tiny current, can freak out all four channels >>>>>>in weird ways. It's better to not allow any input to go negative. The >>>>>>severity seems to vary among vendors, with the classic National part >>>>>>being really, really bad. >>>>>> >>>>> >>>>>How about using a diode across the input to limit the negative >>>>>voltage? The comparator input reads about 1000 using a DMM to check >>>>>diode drops. A silicon diode reads about 800, so it seems a diode >>>>>across the comparator input to ground would absorb all the reverse >>>>>current? >>>>> >>>>>-Bill >>>> >>>>As I recall, a silicon PN diode isn't good enough. The chip screws up >>>>at lower voltages than you'd expect from the ESD diode drop. Both the >>>>Fairchild and National data sheets rate the abs max input voltage >>>>range as -0.3 to +36. That -0.3 number seems to be serious. >>>> >>>>A power schottky diode might work, if it keeps the negative swing >>>>below -0.3. >>> >>> >>>The "quality" way to do it, avoiding leaky Schottky's or finding a >>>Germanium diode, is an active clamp, as I demonstrated several years >>>ago in a 4-20mA circuit for Spehro... >>> >>> http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf >>> >>>The 324/339 input stage architecture is a Darlington differential pair >>>made up of lateral PNP's, with an NPN mirror load on the inner pair of >>>PNP's. >>> >>>This "screws up" if driven below ground by a Vbe, because a lateral >>>device doesn't know its emitter from its collector, never mind a hole >>>in the ground. >>> >>>Thus a positive input pulled below ground causes a phase reversal and >>>the output rises instead of continuing to fall. >>> >>>Driven hard, this could inject current into the substrate and upset >>>things, but I've never seen that... rational designers limit current >>>;-) >>> >>>Another effect is what I think Larkin confuses with the front-end >>>below ground... take inputs too high and lateral PNP mirrors saturate. >>>Cheapy approaches, in certain brands, used split collector >>>arrangements rather than separate-well devices, so one mirror >>>saturating screws all sections of the device. >>> >>> ...Jim Thompson >> >> VCC >> | >> - >> ^ high speed diode >> + >> lowest R possible | >> ___ | >> +------+|___|+--+---------------+ To Comp input >> Unknown signal |+ >> | >> + >> - >> ^ high speed diode >> | >> === >> GND >>(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) >> >> >>I am just throwing shit again the wall ! >> >>Jamie > > > It's not a speed issue... an LM339 is a micro-second range slug-speed > device. To avoid the problem, you need a clamp that holds at a > voltage less than a silicon Vbe forward. > > AND: Your diode to VCC is useless, you will have already exceeded the > allowed common-mode range (about 1.5V BELOW VCC) before it conducts. > > ...Jim ThompsonHow so? the reference isn't passing through 2 diodes, it is only doing one diode, depending on if it's a low or high signal coming in? The idea is to have the diodes only conduct when the input exceeds either way by a margin of 0.7 volts.? You being the semi engineer, I most likely am over looking something very obvious to you. I have used that before and it has worked for me. Maybe I got lucky ? Jamie
LM339 Comparator input limits?
Started by ●May 24, 2012
Reply by ●June 2, 20122012-06-02
Reply by ●June 2, 20122012-06-02
On Sat, 02 Jun 2012 18:04:34 -0400, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:>Jim Thompson wrote: > >> On Sat, 02 Jun 2012 16:50:10 -0400, Jamie >> <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: >> >> >>>Jim Thompson wrote: >>> >>>>On Thu, 24 May 2012 18:14:18 -0700, John Larkin >>>><jlarkin@highlandtechnology.com> wrote: >>>> >>>> >>>> >>>>>On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden >>>>><bperryb@bowdenshobbycircuits.info> wrote: >>>>> >>>>> >>>>> >>>>>>On May 24, 7:29 am, John Larkin >>>>>><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >>>>>> >>>>>> >>>>>>>On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden >>>>>>> >>>>>>><bper...@bowdenshobbycircuits.info> wrote: >>>>>>> >>>>>>> >>>>>>>>The LM339 spec sheet indicates the maximum negative input voltage >>>>>>>>should not be more than -0.3 volts. The schematic shows the input >>>>>>>>connected to the base of a PNP transistor with the collector at the >>>>>>>>negative supply terminal. So, it looks like if the input or base of >>>>>>>>the transistor goes more negative than about 0.7 volts relative to the >>>>>>>>negative supply point, the transistor will conduct like a diode from >>>>>>>>base to collector. Is this situation acceptable, and will the >>>>>>>>comparator work with one input equal to the negative supply (0 volts) >>>>>>>>while the other is -0.7 volts below the negative supply point? >>>>>>> >>>>>>>>-Bill >>>>>>> >>>>>>>The 339, like the LM324, has some sneak paths that squirt current all >>>>>>>around the chip if you pull any input much below ground. One such >>>>>>>pulled-low input, even a tiny current, can freak out all four channels >>>>>>>in weird ways. It's better to not allow any input to go negative. The >>>>>>>severity seems to vary among vendors, with the classic National part >>>>>>>being really, really bad. >>>>>>> >>>>>> >>>>>>How about using a diode across the input to limit the negative >>>>>>voltage? The comparator input reads about 1000 using a DMM to check >>>>>>diode drops. A silicon diode reads about 800, so it seems a diode >>>>>>across the comparator input to ground would absorb all the reverse >>>>>>current? >>>>>> >>>>>>-Bill >>>>> >>>>>As I recall, a silicon PN diode isn't good enough. The chip screws up >>>>>at lower voltages than you'd expect from the ESD diode drop. Both the >>>>>Fairchild and National data sheets rate the abs max input voltage >>>>>range as -0.3 to +36. That -0.3 number seems to be serious. >>>>> >>>>>A power schottky diode might work, if it keeps the negative swing >>>>>below -0.3. >>>> >>>> >>>>The "quality" way to do it, avoiding leaky Schottky's or finding a >>>>Germanium diode, is an active clamp, as I demonstrated several years >>>>ago in a 4-20mA circuit for Spehro... >>>> >>>> http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf >>>> >>>>The 324/339 input stage architecture is a Darlington differential pair >>>>made up of lateral PNP's, with an NPN mirror load on the inner pair of >>>>PNP's. >>>> >>>>This "screws up" if driven below ground by a Vbe, because a lateral >>>>device doesn't know its emitter from its collector, never mind a hole >>>>in the ground. >>>> >>>>Thus a positive input pulled below ground causes a phase reversal and >>>>the output rises instead of continuing to fall. >>>> >>>>Driven hard, this could inject current into the substrate and upset >>>>things, but I've never seen that... rational designers limit current >>>>;-) >>>> >>>>Another effect is what I think Larkin confuses with the front-end >>>>below ground... take inputs too high and lateral PNP mirrors saturate. >>>>Cheapy approaches, in certain brands, used split collector >>>>arrangements rather than separate-well devices, so one mirror >>>>saturating screws all sections of the device. >>>> >>>> ...Jim Thompson >>> >>> VCC >>> | >>> - >>> ^ high speed diode >>> + >>> lowest R possible | >>> ___ | >>> +------+|___|+--+---------------+ To Comp input >>> Unknown signal |+ >>> | >>> + >>> - >>> ^ high speed diode >>> | >>> === >>> GND >>>(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) >>> >>> >>>I am just throwing shit again the wall ! >>> >>>Jamie >> >> >> It's not a speed issue... an LM339 is a micro-second range slug-speed >> device. To avoid the problem, you need a clamp that holds at a >> voltage less than a silicon Vbe forward. >> >> AND: Your diode to VCC is useless, you will have already exceeded the >> allowed common-mode range (about 1.5V BELOW VCC) before it conducts. >> >> ...Jim Thompson > >How so? the reference isn't passing through 2 diodes, it is only doing >one diode, depending on if it's a low or high signal coming in?After I pressed "Send" I realized that an LM339 will be OK CM-wise if the reference is 1.5V below rail. Not so for the LM324...presuming you have feedback ;-)> > The idea is to have the diodes only conduct when the input exceeds >either way by a margin of 0.7 volts.?You don't need a diode in the positive direction unless you talking like +30V. The positive direction applies _reverse_ potentials to the lateral junctions... they breakdown at the same voltage as the NPN BVcbo, 30-35V> > You being the semi engineer, I most likely am over looking something >very obvious to you. I have used that before and it has worked for me. > Maybe I got lucky ? > > >JamieAs I pointed out above, the positive-direction diode isn't needed. In the negative direction, most discrete diode Vbe's are smaller than that of the 339 silicon, so silicon diodes are OK... most of the time. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●June 4, 20122012-06-04
Jim Thompson wrote:> > On Sat, 02 Jun 2012 16:50:10 -0400, Jamie > <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote: > > >Jim Thompson wrote: > >> On Thu, 24 May 2012 18:14:18 -0700, John Larkin > >> <jlarkin@highlandtechnology.com> wrote: > >> > >> > >>>On Thu, 24 May 2012 18:00:20 -0700 (PDT), Bill Bowden > >>><bperryb@bowdenshobbycircuits.info> wrote: > >>> > >>> > >>>>On May 24, 7:29 am, John Larkin > >>>><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: > >>>> > >>>>>On Wed, 23 May 2012 20:58:31 -0700 (PDT), Bill Bowden > >>>>> > >>>>><bper...@bowdenshobbycircuits.info> wrote: > >>>>> > >>>>>>The LM339 spec sheet indicates the maximum negative input voltage > >>>>>>should not be more than -0.3 volts. The schematic shows the input > >>>>>>connected to the base of a PNP transistor with the collector at the > >>>>>>negative supply terminal. So, it looks like if the input or base of > >>>>>>the transistor goes more negative than about 0.7 volts relative to the > >>>>>>negative supply point, the transistor will conduct like a diode from > >>>>>>base to collector. Is this situation acceptable, and will the > >>>>>>comparator work with one input equal to the negative supply (0 volts) > >>>>>>while the other is -0.7 volts below the negative supply point? > >>>>> > >>>>>>-Bill > >>>>> > >>>>>The 339, like the LM324, has some sneak paths that squirt current all > >>>>>around the chip if you pull any input much below ground. One such > >>>>>pulled-low input, even a tiny current, can freak out all four channels > >>>>>in weird ways. It's better to not allow any input to go negative. The > >>>>>severity seems to vary among vendors, with the classic National part > >>>>>being really, really bad. > >>>>> > >>>> > >>>>How about using a diode across the input to limit the negative > >>>>voltage? The comparator input reads about 1000 using a DMM to check > >>>>diode drops. A silicon diode reads about 800, so it seems a diode > >>>>across the comparator input to ground would absorb all the reverse > >>>>current? > >>>> > >>>>-Bill > >>> > >>>As I recall, a silicon PN diode isn't good enough. The chip screws up > >>>at lower voltages than you'd expect from the ESD diode drop. Both the > >>>Fairchild and National data sheets rate the abs max input voltage > >>>range as -0.3 to +36. That -0.3 number seems to be serious. > >>> > >>>A power schottky diode might work, if it keeps the negative swing > >>>below -0.3. > >> > >> > >> The "quality" way to do it, avoiding leaky Schottky's or finding a > >> Germanium diode, is an active clamp, as I demonstrated several years > >> ago in a 4-20mA circuit for Spehro... > >> > >> http://www.analog-innovations.com/SED/SpehroLoopProtect2.pdf > >> > >> The 324/339 input stage architecture is a Darlington differential pair > >> made up of lateral PNP's, with an NPN mirror load on the inner pair of > >> PNP's. > >> > >> This "screws up" if driven below ground by a Vbe, because a lateral > >> device doesn't know its emitter from its collector, never mind a hole > >> in the ground. > >> > >> Thus a positive input pulled below ground causes a phase reversal and > >> the output rises instead of continuing to fall. > >> > >> Driven hard, this could inject current into the substrate and upset > >> things, but I've never seen that... rational designers limit current > >> ;-) > >> > >> Another effect is what I think Larkin confuses with the front-end > >> below ground... take inputs too high and lateral PNP mirrors saturate. > >> Cheapy approaches, in certain brands, used split collector > >> arrangements rather than separate-well devices, so one mirror > >> saturating screws all sections of the device. > >> > >> ...Jim Thompson > > > > VCC > > | > > - > > ^ high speed diode > > + > > lowest R possible | > > ___ | > > +------+|___|+--+---------------+ To Comp input > > Unknown signal |+ > > | > > + > > - > > ^ high speed diode > > | > > === > > GND > >(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de) > > > > > >I am just throwing shit again the wall ! > > > >Jamie > > It's not a speed issue... an LM339 is a micro-second range slug-speed > device. To avoid the problem, you need a clamp that holds at a > voltage less than a silicon Vbe forward. > > AND: Your diode to VCC is useless, you will have already exceeded the > allowed common-mode range (about 1.5V BELOW VCC) before it conducts.His "lowest R possible" would be zero. -- You can't have a sense of humor, if you have no sense.
