On Wed, 17 Jun 2009 01:52:59 -0700, Robert Baer
<robertbaer@localnet.com> wrote:
>Ebrahim wrote:
>> Robert Baer wrote:
>>
>>> * "Equivalent schematics" AFAIK have never shown protection diodes,
>>> and in many cases will imply possible operation in regions that just
>>> cannot be achieved with a real device.
>>> Almost all analog and digital devices have a reverse polarity
>>> protection diode "built in" under the bonding pad and many have the
>>> same at the outputs; this appears to be standard practice and is
>>> ASS-u-ME-ed known and therefore not alluded to by most people in the
>>> industry.
>>> In a number of cases, an additional diode may be added to the supply
>>> rail and/or the doping of the pad-diode may be adjusted for a (say)
>>> zener breakdown of 5-8V.
>>> Cannot say about that as i have not tried to measure this
>>> presumption - and again industry is fairly quiet..
>> ***
>> Robert:
>> They do look at Op27
>> datasheet(http://www.analog.com/en/other/militaryaerospace/op27/products/product.html)
>> it also has shown equivalent schematics. I can show you many BJT opamp
>> that have shown protection diodes and also can show you many ultra low
>> bias current opamps without protection.
>>
>> If it is assumed to be known there should be some references to show
>> that can prove this. I couldn't have find one. I'm not expecting you to
>> show me a reference but I expect people that accuse me of "going beyond
>> competence" just because I posted something that I assume is true to
>> provide references to back their claims. I have provided my references:
>> 2 datasheets op27 and opa627(I can post many more).
>>
>> Regards,
>> Ebrahim
>>
> The protection diodes shown are between inputs.
> Related to inputs, note those to the substrate (V-) and if
>implemented, those to the supply are most definitely not shown.
> For both digital and analog devices, note protection diodes to the
>substrate are not shown.
> And diodes to the supply (if added) seem to be almost state secrets..
There's a good OP27 model on my website... one I did in the early
'90's.
...Jim Thompson
--
| James E.Thompson, P.E. | 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 |
Obama... Recklessness Cloaked in Righteousness
Reply by Robert Baer●June 17, 20092009-06-17
Ebrahim wrote:
> Robert Baer wrote:
>
>> * "Equivalent schematics" AFAIK have never shown protection diodes,
>> and in many cases will imply possible operation in regions that just
>> cannot be achieved with a real device.
>> Almost all analog and digital devices have a reverse polarity
>> protection diode "built in" under the bonding pad and many have the
>> same at the outputs; this appears to be standard practice and is
>> ASS-u-ME-ed known and therefore not alluded to by most people in the
>> industry.
>> In a number of cases, an additional diode may be added to the supply
>> rail and/or the doping of the pad-diode may be adjusted for a (say)
>> zener breakdown of 5-8V.
>> Cannot say about that as i have not tried to measure this
>> presumption - and again industry is fairly quiet..
> ***
> Robert:
> They do look at Op27
> datasheet(http://www.analog.com/en/other/militaryaerospace/op27/products/product.html)
> it also has shown equivalent schematics. I can show you many BJT opamp
> that have shown protection diodes and also can show you many ultra low
> bias current opamps without protection.
>
> If it is assumed to be known there should be some references to show
> that can prove this. I couldn't have find one. I'm not expecting you to
> show me a reference but I expect people that accuse me of "going beyond
> competence" just because I posted something that I assume is true to
> provide references to back their claims. I have provided my references:
> 2 datasheets op27 and opa627(I can post many more).
>
> Regards,
> Ebrahim
>
The protection diodes shown are between inputs.
Related to inputs, note those to the substrate (V-) and if
implemented, those to the supply are most definitely not shown.
For both digital and analog devices, note protection diodes to the
substrate are not shown.
And diodes to the supply (if added) seem to be almost state secrets..
Reply by Phil Hobbs●June 15, 20092009-06-15
miso@sushi.com wrote:
> On Jun 14, 7:32 pm, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>> m...@sushi.com wrote:
>>> On Jun 14, 6:51 am, Phil Hobbs
>>> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>>>> Ebrahim wrote:
>>>>> Winfield Hill wrote:
>>>>>> Ahem. Well, in 32 posts, has anyone answered Ebrahim's
>>>>>> question? Which was, does a C-B junction make a good
>>>>>> low-leakage high-voltage diode, and my answer is yes.
>>>>>> Likely as low as 10pA, but I'd want to check specific parts.
>>>>>> It's the same question one asks before using a transistor at
>>>>>> extremely low collector currents. OK, not too many people
>>>>>> use BJT transistors at 10pA, for one thing they're very slow,
>>>>>> but it's surprising how well they work down in that territory.
>>>>>> Often when doing this trick, one wants very low capacitance,
>>>>>> and I've been disappointed with the high capacitance of the
>>>>>> usual candidates, e.g., 5pF. This is what steers people
>>>>>> back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA
>>>>>> When one is in this pA territory, PCB leakage can be a big
>>>>>> problem, and teflon posts and in-air wiring are often used.
>>>>> ***Hello Winfield :) :
>>>>> Thank you very much for your technical and non-political answer.
>>>>> So it makes sense to look for such a candidate among high frequency
>>>>> transistors that should have low capacitance.
>>>>> Thank you again.
>>>>> Best wishes for you,
>>>>> Ebrahim
>>>> The part of this that makes zero sense is the 100V part. Almost all op
>>>> amps, in fact AFAIK every single one ever sold, has PN junctions between
>>>> its inputs and its supply pins.(*) That's why the Absolute Maximum
>>>> Ratings section of datasheets specifies that you mustn't take their
>>>> inputs more than 0.3V outside the supplies.
>>>> Not everything on an IC maps well onto a schematic--e.g. schematics
>>>> don't show the parasitic SCR inherent in junction-isolated CMOS
>>>> processes. It's there, though, as you'll find if you try dumping 20 mA
>>>> into an input.
>>>> If you want the same sort of function provided by the input protection
>>>> diodes, without a big leakage problem, you can use something like this:
>>>> 0 VDD
>>>> |
>>>> _
>>>> A
>>>> |
>>>> *---Ri------*--------Rf-----------*
>>>> | | |
>>>> GGG _ |
>>>> A |
>>>> | |
>>>> | |\ |
>>>> 0-------------RRRRR---*---RRRRR---|+\ |
>>>> | \ |
>>>> | >---*-*---0
>>>> *---|- / |
>>>> | | / |
>>>> | |/ |
>>>> | |
>>>> *---Ri------*---Rf------*
>>>> |
>>>> GGG
>>>> (Noninverting shown because inverting is reasonably obvious, and
>>>> negative polarity left as an exercise for the reader.) You have to
>>>> scale the resistances to fit your problem. The idea is that the first
>>>> diode has nearly no voltage across it, and hence no leakage or
>>>> capacitive current. This isn't a complete solution, because it costs
>>>> you SNR eventually, but it's a lot better than nothing.
>>>> The diode needs almost no breakdown voltage at all--certainly not 100V..
>>>> Something like a BFT25 B-C junction (about 0.3 pF) will work great..
>>>> (WDNNS PAD-1!)
>>>> Cheers
>>>> Phil Hobbs
>>>> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead,
>>>> because their inputs are intended to be overdriven by volts. Also, some
>>>> rail-to-rail input op amps use charge pumps to allow them to run their
>>>> input stages on more than VDD-VSS.
>>>> --
>>>> Dr Philip C D Hobbs
>>>> Principal
>>>> ElectroOptical Innovations
>>>> 55 Orchard Rd
>>>> Briarcliff Manor NY 10510
>>>> 845-480-2058
>>>> hobbs at electrooptical dot nethttp://electrooptical.net
>>> I'm not so sure about zener input protection. Generally when you need
>>> to go above the rail, you just use a N mosfet with gate tied to source
>>> and the drain going to the pad. You depend on the breakdown of the fet
>>> to provide input protection. [If you are trying this in layout,
>>> antenna rules may apply.]
>>> Every time I investigated chip damage after ESD testing, it was always
>>> due to over-voltage versus over current. I'm not a fan of high fields
>>> (as found in reverse bias) in semiconductors. They seem to find the
>>> weak spot in the junction and zap it. Forward biased junction might
>>> have hot spots, i.e. current hogging, but in general they are pretty
>>> rugged.
>> Transzorbs are pretty bulletproof--its job would be just to limit the
>> overvoltage across the series protection resistor long enough for the
>> polyfuse to switch. The resistor and the input diodes would protect the
>> input devices, at least if it were done right.
>>
>> Cheers
>>
>> Phil Hobbs
>>
>> --
>> Dr Philip C D Hobbs
>> Principal
>> ElectroOptical Innovations
>> 55 Orchard Rd
>> Briarcliff Manor NY 10510
>> 845-480-2058
>> hobbs at electrooptical dot nethttp://electrooptical.net
>
> I am referring to on-chip protection and failure analysis I've done.
> I've used transorbs, but only on power rails. In any event, putting
> zeners on chip isn't done to my knowledge, at least in MOS/BiCMOS.
>
> See the slide "ground gate NMOS"
> http://www.icims.csl.uiuc.edu/icap00/sjoshi_icap00.pdf
>
I'm sure it isn't done anymore, but check out the CD4049UB for an
example. http://www.fairchildsemi.com/ds/CD%2FCD4049UBC.pdf, page 2.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Reply by miso...@sushi.com●June 15, 20092009-06-15
On Jun 14, 7:32=A0pm, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> m...@sushi.com wrote:
> > On Jun 14, 6:51 am, Phil Hobbs
> > <pcdhSpamMeSensel...@electrooptical.net> wrote:
> >> Ebrahim wrote:
> >>> Winfield Hill wrote:
> >>>> =A0Ahem. =A0Well, in 32 posts, has anyone answered Ebrahim's
> >>>> =A0question? =A0Which was, does a C-B junction make a good
> >>>> =A0low-leakage high-voltage diode, and my answer is yes.
> >>>> =A0Likely as low as 10pA, but I'd want to check specific parts.
> >>>> =A0It's the same question one asks before using a transistor at
> >>>> =A0extremely low collector currents. =A0OK, not too many people
> >>>> =A0use BJT transistors at 10pA, for one thing they're very slow,
> >>>> =A0but it's surprising how well they work down in that territory.
> >>>> =A0Often when doing this trick, one wants very low capacitance,
> >>>> =A0and I've been disappointed with the high capacitance of the
> >>>> =A0usual candidates, e.g., 5pF. =A0This is what steers people
> >>>> =A0back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA
> >>>> =A0When one is in this pA territory, PCB leakage can be a big
> >>>> =A0problem, and teflon posts and in-air wiring are often used.
> >>> ***Hello Winfield :) :
> >>> Thank you very much for your technical and non-political answer.
> >>> So it makes sense to look for such a candidate among high frequency
> >>> transistors that should have low capacitance.
> >>> Thank you again.
> >>> Best wishes for you,
> >>> Ebrahim
> >> The part of this that makes zero sense is the 100V part. =A0Almost all=
op
> >> amps, in fact AFAIK every single one ever sold, has PN junctions betwe=
en
> >> its inputs and its supply pins.(*) =A0That's why the Absolute Maximum
> >> Ratings section of datasheets specifies that you mustn't take their
> >> inputs more than 0.3V outside the supplies.
>
> >> Not everything on an IC maps well onto a schematic--e.g. schematics
> >> don't show the parasitic SCR inherent in junction-isolated CMOS
> >> processes. =A0It's there, though, as you'll find if you try dumping 20=
mA
> >> into an input.
>
> >> If you want the same sort of function provided by the input protection
> >> diodes, without a big leakage problem, you can use something like this=
> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*---Rf------*
> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> >> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GGG
>
> >> (Noninverting shown because inverting is reasonably obvious, and
> >> negative polarity left as an exercise for the reader.) =A0You have to
> >> scale the resistances to fit your problem. =A0The idea is that the fir=
st
> >> diode has nearly no voltage across it, and hence no leakage or
> >> capacitive current. =A0This isn't a complete solution, because it cost=
s
> >> you SNR eventually, but it's a lot better than nothing.
>
> >> The diode needs almost no breakdown voltage at all--certainly not 100V=
.
> >> =A0 Something like a BFT25 B-C junction (about 0.3 pF) will work great=
.
> >> (WDNNS PAD-1!)
>
> >> Cheers
>
> >> Phil Hobbs
>
> >> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead,
> >> because their inputs are intended to be overdriven by volts. =A0Also, =
some
> >> rail-to-rail input op amps use charge pumps to allow them to run their
> >> input stages on more than VDD-VSS.
>
> >> --
> >> Dr Philip C D Hobbs
> >> Principal
> >> ElectroOptical Innovations
> >> 55 Orchard Rd
> >> Briarcliff Manor NY 10510
> >> 845-480-2058
> >> hobbs at electrooptical dot nethttp://electrooptical.net
>
> > I'm not so sure about zener input protection. Generally when you need
> > to go above the rail, you just use a N mosfet with gate tied to source
> > and the drain going to the pad. You depend on the breakdown of the fet
> > to provide input protection. [If you are trying this in layout,
> > antenna rules may apply.]
>
> > Every time I investigated chip damage after ESD testing, it was always
> > due to over-voltage versus over current. I'm not a fan of high fields
> > (as found in reverse bias) in semiconductors. They seem to find the
> > weak spot in the junction and zap it. Forward biased junction might
> > have hot spots, i.e. current hogging, but in general they are pretty
> > rugged.
>
> Transzorbs are pretty bulletproof--its job would be just to limit the
> overvoltage across the series protection resistor long enough for the
> polyfuse to switch. =A0The resistor and the input diodes would protect th=
e
> input devices, at least if it were done right.
>
> Cheers
>
> Phil Hobbs
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
> hobbs at electrooptical dot nethttp://electrooptical.net
I am referring to on-chip protection and failure analysis I've done.
I've used transorbs, but only on power rails. In any event, putting
zeners on chip isn't done to my knowledge, at least in MOS/BiCMOS.
See the slide "ground gate NMOS"
http://www.icims.csl.uiuc.edu/icap00/sjoshi_icap00.pdf
Reply by Phil Hobbs●June 14, 20092009-06-14
miso@sushi.com wrote:
> On Jun 14, 6:51 am, Phil Hobbs
> <pcdhSpamMeSensel...@electrooptical.net> wrote:
>> Ebrahim wrote:
>>> Winfield Hill wrote:
>>>> Ahem. Well, in 32 posts, has anyone answered Ebrahim's
>>>> question? Which was, does a C-B junction make a good
>>>> low-leakage high-voltage diode, and my answer is yes.
>>>> Likely as low as 10pA, but I'd want to check specific parts.
>>>> It's the same question one asks before using a transistor at
>>>> extremely low collector currents. OK, not too many people
>>>> use BJT transistors at 10pA, for one thing they're very slow,
>>>> but it's surprising how well they work down in that territory.
>>>> Often when doing this trick, one wants very low capacitance,
>>>> and I've been disappointed with the high capacitance of the
>>>> usual candidates, e.g., 5pF. This is what steers people
>>>> back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA
>>>> When one is in this pA territory, PCB leakage can be a big
>>>> problem, and teflon posts and in-air wiring are often used.
>>> ***Hello Winfield :) :
>>> Thank you very much for your technical and non-political answer.
>>> So it makes sense to look for such a candidate among high frequency
>>> transistors that should have low capacitance.
>>> Thank you again.
>>> Best wishes for you,
>>> Ebrahim
>> The part of this that makes zero sense is the 100V part. Almost all op
>> amps, in fact AFAIK every single one ever sold, has PN junctions between
>> its inputs and its supply pins.(*) That's why the Absolute Maximum
>> Ratings section of datasheets specifies that you mustn't take their
>> inputs more than 0.3V outside the supplies.
>>
>> Not everything on an IC maps well onto a schematic--e.g. schematics
>> don't show the parasitic SCR inherent in junction-isolated CMOS
>> processes. It's there, though, as you'll find if you try dumping 20 mA
>> into an input.
>>
>> If you want the same sort of function provided by the input protection
>> diodes, without a big leakage problem, you can use something like this:
>>
>> 0 VDD
>> |
>> _
>> A
>> |
>> *---Ri------*--------Rf-----------*
>> | | |
>> GGG _ |
>> A |
>> | |
>> | |\ |
>> 0-------------RRRRR---*---RRRRR---|+\ |
>> | \ |
>> | >---*-*---0
>> *---|- / |
>> | | / |
>> | |/ |
>> | |
>> *---Ri------*---Rf------*
>> |
>> GGG
>>
>> (Noninverting shown because inverting is reasonably obvious, and
>> negative polarity left as an exercise for the reader.) You have to
>> scale the resistances to fit your problem. The idea is that the first
>> diode has nearly no voltage across it, and hence no leakage or
>> capacitive current. This isn't a complete solution, because it costs
>> you SNR eventually, but it's a lot better than nothing.
>>
>> The diode needs almost no breakdown voltage at all--certainly not 100V.
>> Something like a BFT25 B-C junction (about 0.3 pF) will work great.
>> (WDNNS PAD-1!)
>>
>> Cheers
>>
>> Phil Hobbs
>>
>> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead,
>> because their inputs are intended to be overdriven by volts. Also, some
>> rail-to-rail input op amps use charge pumps to allow them to run their
>> input stages on more than VDD-VSS.
>>
>> --
>> Dr Philip C D Hobbs
>> Principal
>> ElectroOptical Innovations
>> 55 Orchard Rd
>> Briarcliff Manor NY 10510
>> 845-480-2058
>> hobbs at electrooptical dot nethttp://electrooptical.net
>
> I'm not so sure about zener input protection. Generally when you need
> to go above the rail, you just use a N mosfet with gate tied to source
> and the drain going to the pad. You depend on the breakdown of the fet
> to provide input protection. [If you are trying this in layout,
> antenna rules may apply.]
>
> Every time I investigated chip damage after ESD testing, it was always
> due to over-voltage versus over current. I'm not a fan of high fields
> (as found in reverse bias) in semiconductors. They seem to find the
> weak spot in the junction and zap it. Forward biased junction might
> have hot spots, i.e. current hogging, but in general they are pretty
> rugged.
>
Transzorbs are pretty bulletproof--its job would be just to limit the
overvoltage across the series protection resistor long enough for the
polyfuse to switch. The resistor and the input diodes would protect the
input devices, at least if it were done right.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
Reply by George Herold●June 14, 20092009-06-14
On Jun 14, 9:51=A0am, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> Ebrahim wrote:
> > Winfield Hill wrote:
> >> =A0Ahem. =A0Well, in 32 posts, has anyone answered Ebrahim's
> >> =A0question? =A0Which was, does a C-B junction make a good
> >> =A0low-leakage high-voltage diode, and my answer is yes.
> >> =A0Likely as low as 10pA, but I'd want to check specific parts.
>
> >> =A0It's the same question one asks before using a transistor at
> >> =A0extremely low collector currents. =A0OK, not too many people
> >> =A0use BJT transistors at 10pA, for one thing they're very slow,
> >> =A0but it's surprising how well they work down in that territory.
>
> >> =A0Often when doing this trick, one wants very low capacitance,
> >> =A0and I've been disappointed with the high capacitance of the
> >> =A0usual candidates, e.g., 5pF. =A0This is what steers people
> >> =A0back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA
>
> >> =A0When one is in this pA territory, PCB leakage can be a big
> >> =A0problem, and teflon posts and in-air wiring are often used.
>
> > ***Hello Winfield :) :
> > Thank you very much for your technical and non-political answer.
> > So it makes sense to look for such a candidate among high frequency
> > transistors that should have low capacitance.
>
> > Thank you again.
> > Best wishes for you,
> > Ebrahim
>
> The part of this that makes zero sense is the 100V part. =A0Almost all op
> amps, in fact AFAIK every single one ever sold, has PN junctions between
> its inputs and its supply pins.(*) =A0That's why the Absolute Maximum
> Ratings section of datasheets specifies that you mustn't take their
> inputs more than 0.3V outside the supplies.
>
> Not everything on an IC maps well onto a schematic--e.g. schematics
> don't show the parasitic SCR inherent in junction-isolated CMOS
> processes. =A0It's there, though, as you'll find if you try dumping 20 mA
> into an input.
>
> If you want the same sort of function provided by the input protection
> diodes, without a big leakage problem, you can use something like this:
>
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =A0VDD
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0_
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*--------Rf-----------*
> =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*---Rf------*
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GGG
>
> (Noninverting shown because inverting is reasonably obvious, and
> negative polarity left as an exercise for the reader.) =A0You have to
> scale the resistances to fit your problem. =A0The idea is that the first
> diode has nearly no voltage across it, and hence no leakage or
> capacitive current. =A0This isn't a complete solution, because it costs
> you SNR eventually, but it's a lot better than nothing.
>
> The diode needs almost no breakdown voltage at all--certainly not 100V.
> =A0 Something like a BFT25 B-C junction (about 0.3 pF) will work great.
> (WDNNS PAD-1!)
>
> Cheers
>
> Phil Hobbs
>
> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead,
> because their inputs are intended to be overdriven by volts. =A0Also, som=
e
> rail-to-rail input op amps use charge pumps to allow them to run their
> input stages on more than VDD-VSS.
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
> hobbs at electrooptical dot nethttp://electrooptical.net- Hide quoted tex=
t -
>
> - Show quoted text -
Wow, Thanks Phil, I've scribbled the circuit down in my notebook and
I'll analyze it if I ever need low leakage protection.
George H.
Reply by George Herold●June 14, 20092009-06-14
On Jun 13, 1:54=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Sat, 13 Jun 2009 21:48:27 +0430, Ebrahim <Engineer....@gmail.com>
> wrote:
>
>
>
>
>
> >Phil Hobbs wrote:
> >> Ebrahim wrote:
>
> >> What op amp have you ever seen that didn't, and how do you know? =A0Su=
ch a
> >> device might be quite interesting in applications, but I don't know of=
one.
> > > Even femtoamp leakage devices (e.g. the venerable LMC660) have
> > > protection diodes. =A0You can use them for resetting integrators, by
> > > briefly inverting the power rails (with suitable current limits). =A0=
Win
> > > Hill posted a circuit for that in this very group a few years back.
>
> >***
> >Hello Phil
>
> >Look at data sheet of OPA627, It has shown equivalent schematic. I do
> >not see any protection diodes in inputs.
>
> An ohmmeter will tell. If it has them, all you need to do is limit the
> current.
>
> If not, use the c-b junctions of a BFT25.
>
> John- Hide quoted text -
>
> - Show quoted text -
Cool, Thanks John I'm going to have to probe the inputs of some
opamps. I've always assumed they had protection diodes on the front
end. I should be able to find the series resistance if I'm careful.
George H.
Reply by miso...@sushi.com●June 14, 20092009-06-14
On Jun 13, 9:13=A0pm, Ebrahim <Engineer....@gmail.com> wrote:
> m...@sushi.com wrote:
> > Getting back to the basic question, breakdown voltages are only
> > guaranteed at a minimum. Suppose you get a device that has exceedingly
> > good breakdown?
>
> I did not understand what is your mean. will you explain more please ?
>
> Regards,
> Ebrahim
A manufacturer sells a device that passes a minimum breakdown. It is a
one sided specification. That is, a minimum voltage is specified, but
not a maximum.
Maybe the breakdown voltage will be much higher than where you want it
to be.
I see all sorts of ideas in this forum on how to cheat and make a part
do something that it is not intended to do. Fine, but you are on your
own. I've seen specifications of parts change wildly over the years.
Case in point was a SC filter which had a pretty good THD. The THD was
not a guaranteed specification, but there was an internal spec to make
sure things didn't go wild. Well somebody changed epi vendors, and the
THD went about the internal limit. Since the parts didn't have a THD
spec, they went out into the public, though the old epi vendor was
used again.
If you want a bullet proof design, take every line item without limits
and blot them out with a sharpie, then go design.
Reply by miso...@sushi.com●June 14, 20092009-06-14
On Jun 14, 6:51=A0am, Phil Hobbs
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> Ebrahim wrote:
> > Winfield Hill wrote:
> >> =A0Ahem. =A0Well, in 32 posts, has anyone answered Ebrahim's
> >> =A0question? =A0Which was, does a C-B junction make a good
> >> =A0low-leakage high-voltage diode, and my answer is yes.
> >> =A0Likely as low as 10pA, but I'd want to check specific parts.
>
> >> =A0It's the same question one asks before using a transistor at
> >> =A0extremely low collector currents. =A0OK, not too many people
> >> =A0use BJT transistors at 10pA, for one thing they're very slow,
> >> =A0but it's surprising how well they work down in that territory.
>
> >> =A0Often when doing this trick, one wants very low capacitance,
> >> =A0and I've been disappointed with the high capacitance of the
> >> =A0usual candidates, e.g., 5pF. =A0This is what steers people
> >> =A0back to the old PAD-1 beauties, spec'd at 0.5pF and 0.3pA
>
> >> =A0When one is in this pA territory, PCB leakage can be a big
> >> =A0problem, and teflon posts and in-air wiring are often used.
>
> > ***Hello Winfield :) :
> > Thank you very much for your technical and non-political answer.
> > So it makes sense to look for such a candidate among high frequency
> > transistors that should have low capacitance.
>
> > Thank you again.
> > Best wishes for you,
> > Ebrahim
>
> The part of this that makes zero sense is the 100V part. =A0Almost all op
> amps, in fact AFAIK every single one ever sold, has PN junctions between
> its inputs and its supply pins.(*) =A0That's why the Absolute Maximum
> Ratings section of datasheets specifies that you mustn't take their
> inputs more than 0.3V outside the supplies.
>
> Not everything on an IC maps well onto a schematic--e.g. schematics
> don't show the parasitic SCR inherent in junction-isolated CMOS
> processes. =A0It's there, though, as you'll find if you try dumping 20 mA
> into an input.
>
> If you want the same sort of function provided by the input protection
> diodes, without a big leakage problem, you can use something like this:
>
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A00 =A0VDD
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0_
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0A
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*--------Rf-----------*
> =A0 =A0 =A0 =A0 =A0 =A0| =A0 =A0 =A0 =A0 =A0 | =A0 =A0 =A0 =A0 =A0 =A0 =
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0*---Ri------*---Rf------*
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0|
> =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 =A0 GGG
>
> (Noninverting shown because inverting is reasonably obvious, and
> negative polarity left as an exercise for the reader.) =A0You have to
> scale the resistances to fit your problem. =A0The idea is that the first
> diode has nearly no voltage across it, and hence no leakage or
> capacitive current. =A0This isn't a complete solution, because it costs
> you SNR eventually, but it's a lot better than nothing.
>
> The diode needs almost no breakdown voltage at all--certainly not 100V.
> =A0 Something like a BFT25 B-C junction (about 0.3 pF) will work great.
> (WDNNS PAD-1!)
>
> Cheers
>
> Phil Hobbs
>
> (*) Some chips, e.g. old CMOS->TTL converters use Zeners instead,
> because their inputs are intended to be overdriven by volts. =A0Also, som=
e
> rail-to-rail input op amps use charge pumps to allow them to run their
> input stages on more than VDD-VSS.
>
> --
> Dr Philip C D Hobbs
> Principal
> ElectroOptical Innovations
> 55 Orchard Rd
> Briarcliff Manor NY 10510
> 845-480-2058
> hobbs at electrooptical dot nethttp://electrooptical.net
I'm not so sure about zener input protection. Generally when you need
to go above the rail, you just use a N mosfet with gate tied to source
and the drain going to the pad. You depend on the breakdown of the fet
to provide input protection. [If you are trying this in layout,
antenna rules may apply.]
Every time I investigated chip damage after ESD testing, it was always
due to over-voltage versus over current. I'm not a fan of high fields
(as found in reverse bias) in semiconductors. They seem to find the
weak spot in the junction and zap it. Forward biased junction might
have hot spots, i.e. current hogging, but in general they are pretty
rugged.
Reply by Phil Hobbs●June 14, 20092009-06-14
Ebrahim wrote:
> Phil Hobbs wrote:
<snip>
>> The diode needs almost no breakdown voltage at all--certainly not
>> 100V. Something like a BFT25 B-C junction (about 0.3 pF) will work
>> great. (WDNNS PAD-1!)
>>
> ***
> That's an interesting idea, you actually bootstrapped the first diode.
> Are you sure that any diode can do this? I mean I think I have done
> some measurements with 1N4007 and even in ~0 volt, its leakage was high.
> I don't remember the exact numbers, though I must find my papers.
Rectifiers intended for 60 Hz applications are very slow, both slow to
turn off and (interestingly) slow to turn on. They also have lots of
capacitance.
If you're doing something slow, and working at very low impedance
levels, then a 1N4007 could be the right part in this circuit.
Personally if it was as slow as that I'd probably be wanting to use a
Polyfuse and a Transzorb.
A diode has a nonzero conductance at zero bias--for an ideal diode, the
zero-bias conductance is Is*kT/e, as you can easily verify by
differentiating the diode equation. Bootstrapping will reduce the
current, but won't reduce the noise. Pick something sensible--you've
had several suggestions--and build something. Then tell us how it worked.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net