diodes are such rotten diodes
in [Design]
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From: John Larkin on 9 Jun 2010 17:58 I have a signal that can potentially swing +-15 volts maybe, and I'm going into a 4-pole Sallen-Key filter, then an ADC with swing range 0 to 4.096. So I added a dual diode, BAV99, at the input of the first opamp, connecting to clamp rails of 0 and 4.1. The filter input resistors add up to about 10K. This works, but it's not safe over temperature. Turns out a BAV99 leaks around 5 nA at room temp alone. The collector-base junction of a cheap transistor, like a BCX70, leaks about 150 fA at room temp, -5 volts, kinda hard to measure. Transistors are so much better diodes than diodes. Do they still make diodes by dicing up featureless wafers, exposing the damaged edges? Barbaric. Or are they just big junctions? Maybe I'll test some high-voltage dual diodes; they might leak less. I could use the BCX70 or BFT25 junctions (we created a PADS schematic symbol for a transistor used as a diode) but it will take two parts. Central makes a "low-leakage" SOT-23 dual diode, samples coming. John
From: Fred Bartoli on 9 Jun 2010 18:02 John Larkin a �crit : > > I have a signal that can potentially swing +-15 volts maybe, and I'm > going into a 4-pole Sallen-Key filter, then an ADC with swing range 0 > to 4.096. So I added a dual diode, BAV99, at the input of the first > opamp, connecting to clamp rails of 0 and 4.1. The filter input > resistors add up to about 10K. > > This works, but it's not safe over temperature. Turns out a BAV99 > leaks around 5 nA at room temp alone. > > The collector-base junction of a cheap transistor, like a BCX70, leaks > about 150 fA at room temp, -5 volts, kinda hard to measure. > > Transistors are so much better diodes than diodes. Do they still make > diodes by dicing up featureless wafers, exposing the damaged edges? > Barbaric. Or are they just big junctions? > > Maybe I'll test some high-voltage dual diodes; they might leak less. I > could use the BCX70 or BFT25 junctions (we created a PADS schematic > symbol for a transistor used as a diode) but it will take two parts. > > Central makes a "low-leakage" SOT-23 dual diode, samples coming. > > John > BAS116, BAV199, BAW156, BAW170 -- Thanks, Fred.
From: Joerg on 9 Jun 2010 21:14 Fred Bartoli wrote: > John Larkin a �crit : >> >> I have a signal that can potentially swing +-15 volts maybe, and I'm >> going into a 4-pole Sallen-Key filter, then an ADC with swing range 0 >> to 4.096. So I added a dual diode, BAV99, at the input of the first >> opamp, connecting to clamp rails of 0 and 4.1. The filter input >> resistors add up to about 10K. >> >> This works, but it's not safe over temperature. Turns out a BAV99 >> leaks around 5 nA at room temp alone. >> >> The collector-base junction of a cheap transistor, like a BCX70, leaks >> about 150 fA at room temp, -5 volts, kinda hard to measure. >> >> Transistors are so much better diodes than diodes. Do they still make >> diodes by dicing up featureless wafers, exposing the damaged edges? >> Barbaric. Or are they just big junctions? >> >> Maybe I'll test some high-voltage dual diodes; they might leak less. I >> could use the BCX70 or BFT25 junctions (we created a PADS schematic >> symbol for a transistor used as a diode) but it will take two parts. >> >> Central makes a "low-leakage" SOT-23 dual diode, samples coming. >> >> John >> > > BAS116, BAV199, BAW156, BAW170 ^^^^^^ Bingo! Drop-in replacement, and cheap. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 9 Jun 2010 21:38 On Wed, 09 Jun 2010 18:14:00 -0700, Joerg <invalid(a)invalid.invalid> wrote: >Fred Bartoli wrote: >> John Larkin a �crit : >>> >>> I have a signal that can potentially swing +-15 volts maybe, and I'm >>> going into a 4-pole Sallen-Key filter, then an ADC with swing range 0 >>> to 4.096. So I added a dual diode, BAV99, at the input of the first >>> opamp, connecting to clamp rails of 0 and 4.1. The filter input >>> resistors add up to about 10K. >>> >>> This works, but it's not safe over temperature. Turns out a BAV99 >>> leaks around 5 nA at room temp alone. >>> >>> The collector-base junction of a cheap transistor, like a BCX70, leaks >>> about 150 fA at room temp, -5 volts, kinda hard to measure. >>> >>> Transistors are so much better diodes than diodes. Do they still make >>> diodes by dicing up featureless wafers, exposing the damaged edges? >>> Barbaric. Or are they just big junctions? >>> >>> Maybe I'll test some high-voltage dual diodes; they might leak less. I >>> could use the BCX70 or BFT25 junctions (we created a PADS schematic >>> symbol for a transistor used as a diode) but it will take two parts. >>> >>> Central makes a "low-leakage" SOT-23 dual diode, samples coming. >>> >>> John >>> >> >> BAS116, BAV199, BAW156, BAW170 > ^^^^^^ > >Bingo! Drop-in replacement, and cheap. Wow, that's nice. 10 pA typ at 60C! Thanks, I'll get some. I tried a couple of high-voltage dual diodes, but they all leaked ballpark 5 nA at room temp. I got to thinking, who makes a diode the same way that people make transistors, a real planar/lithographed/passivated process? The only answer I came up with was "varicap." Sure enough, a Skyworks SMV1234 leaks about 1.8 pA. John
From: Joerg on 9 Jun 2010 21:53
John Larkin wrote: > On Wed, 09 Jun 2010 18:14:00 -0700, Joerg <invalid(a)invalid.invalid> > wrote: > >> Fred Bartoli wrote: >>> John Larkin a �crit : >>>> I have a signal that can potentially swing +-15 volts maybe, and I'm >>>> going into a 4-pole Sallen-Key filter, then an ADC with swing range 0 >>>> to 4.096. So I added a dual diode, BAV99, at the input of the first >>>> opamp, connecting to clamp rails of 0 and 4.1. The filter input >>>> resistors add up to about 10K. >>>> >>>> This works, but it's not safe over temperature. Turns out a BAV99 >>>> leaks around 5 nA at room temp alone. >>>> >>>> The collector-base junction of a cheap transistor, like a BCX70, leaks >>>> about 150 fA at room temp, -5 volts, kinda hard to measure. >>>> >>>> Transistors are so much better diodes than diodes. Do they still make >>>> diodes by dicing up featureless wafers, exposing the damaged edges? >>>> Barbaric. Or are they just big junctions? >>>> >>>> Maybe I'll test some high-voltage dual diodes; they might leak less. I >>>> could use the BCX70 or BFT25 junctions (we created a PADS schematic >>>> symbol for a transistor used as a diode) but it will take two parts. >>>> >>>> Central makes a "low-leakage" SOT-23 dual diode, samples coming. >>>> >>>> John >>>> >>> BAS116, BAV199, BAW156, BAW170 >> ^^^^^^ >> >> Bingo! Drop-in replacement, and cheap. > > Wow, that's nice. 10 pA typ at 60C! > > Thanks, I'll get some. > > I tried a couple of high-voltage dual diodes, but they all leaked > ballpark 5 nA at room temp. I got to thinking, who makes a diode the > same way that people make transistors, a real > planar/lithographed/passivated process? The only answer I came up with > was "varicap." Sure enough, a Skyworks SMV1234 leaks about 1.8 pA. > But those will be really close to the cliff with your +/-15V requirement and then one sunny day ... *phut* Also, they set you back 30 cents. For that kind of money you can get two Girl Scout cookies of the "thin mints" kind (my favorite) _and_ a BAV199 :-) -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |