diodes are such rotten diodes
in [Design]
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From: Joerg on 15 Jun 2010 17:45 John Larkin wrote: > On Wed, 16 Jun 2010 07:15:17 +1000, Grant <omg(a)grrr.id.au> wrote: > >> On Tue, 15 Jun 2010 11:25:17 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>> On Wed, 09 Jun 2010 14:58:42 -0700, John Larkin >>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> >>>> 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 >>> >>> I got some samples of the Central CMPD6001S dual SOT-23 diodes. >>> Through a local rep, since their registration/password thing is so >>> absurd. >>> >>> I measured about 50 fA leakage at -5 volts. I say "about" because this >>> is sorta hard to measure... 5 mV across a 100G resistor. That's a lot >>> better than most diodes at around 5 na. Pricing is 0.099 by the reel, >>> as compared to 0.085 for the BAV199, which I haven't measured yet. >> And just how does one go about measuring 5mV across 100G? Seems like >> an open circuit that might not collect a static charge, to me ;) >>> Looks like 1.8 pF at zero bias, pretty nice part. >> Grant. > > > Here's my pA parts tester: > > ftp://jjlarkin.lmi.net/99A260A1.JPG > > ftp://jjlarkin.lmi.net/99A260A3.JPG > Oh, people would kill for those ceramic solder posts. > ftp://jjlarkin.lmi.net/99S260A.JPG > > I plugged a 100G resistor into the Z2 slot, and put the diode in as > Z1. > > With a 100M or 1G resistor as Z2, nanoamps or tens of pA are easy to > measure. When you get to fA, everything has to be very well shielded, > and you have to wait a long time for the inherent time constants to > settle down. Without a solid shield, just moving around in the > vicinity will induce huge offsets. > > Those damned RatShack banana terminals are conductive as hell, so I > had to machine the hole and add the lexan *after* it was all built. > Grrrrr. > So how do you make the San Francisco fog disappear? Tape the windows shut and ask all your employees not to breathe out for a few minutes? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Larkin on 15 Jun 2010 18:12 On Tue, 15 Jun 2010 14:45:34 -0700, Joerg <invalid(a)invalid.invalid> wrote: >John Larkin wrote: >> On Wed, 16 Jun 2010 07:15:17 +1000, Grant <omg(a)grrr.id.au> wrote: >> >>> On Tue, 15 Jun 2010 11:25:17 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>>> On Wed, 09 Jun 2010 14:58:42 -0700, John Larkin >>>> <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> >>>>> >>>>> 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 >>>> >>>> I got some samples of the Central CMPD6001S dual SOT-23 diodes. >>>> Through a local rep, since their registration/password thing is so >>>> absurd. >>>> >>>> I measured about 50 fA leakage at -5 volts. I say "about" because this >>>> is sorta hard to measure... 5 mV across a 100G resistor. That's a lot >>>> better than most diodes at around 5 na. Pricing is 0.099 by the reel, >>>> as compared to 0.085 for the BAV199, which I haven't measured yet. >>> And just how does one go about measuring 5mV across 100G? Seems like >>> an open circuit that might not collect a static charge, to me ;) >>>> Looks like 1.8 pF at zero bias, pretty nice part. >>> Grant. >> >> >> Here's my pA parts tester: >> >> ftp://jjlarkin.lmi.net/99A260A1.JPG >> >> ftp://jjlarkin.lmi.net/99A260A3.JPG >> > >Oh, people would kill for those ceramic solder posts. Old Tek scope things. I got a bunch at the Foothill Flea Market. But the critical node is not on a strip... all air. > > >> ftp://jjlarkin.lmi.net/99S260A.JPG >> >> I plugged a 100G resistor into the Z2 slot, and put the diode in as >> Z1. >> >> With a 100M or 1G resistor as Z2, nanoamps or tens of pA are easy to >> measure. When you get to fA, everything has to be very well shielded, >> and you have to wait a long time for the inherent time constants to >> settle down. Without a solid shield, just moving around in the >> vicinity will induce huge offsets. >> >> Those damned RatShack banana terminals are conductive as hell, so I >> had to machine the hole and add the lexan *after* it was all built. >> Grrrrr. >> > >So how do you make the San Francisco fog disappear? Tape the windows >shut and ask all your employees not to breathe out for a few minutes? I'm not seeing a lot of leakage across the lexan, or even on the dual pomona banana plugs where I mount the test resistors. My measurement uncertainty seems to be about 10 or 20 fA. My 1T resistor is an 0805, soldered across a pomona plug! 1 Tohm and 1 pF is a 1 second time constant, so measurements can get slow. Some transistor c-b junctions, as diodes, are logarithmic from 10s of fA to 10s of mA, and leak ballpark 20 fA reverse-biased. For serious current measurement, you could let an unknown current charge a polystyrene cap for some time (minutes, hours, days) and then measure the voltage on the cap. 1 fA would charge a 1 nF cap at 1 uv/sec, tons of signal. John
From: Jamie on 15 Jun 2010 19:44
John Larkin wrote: > On Wed, 16 Jun 2010 07:15:17 +1000, Grant <omg(a)grrr.id.au> wrote: > > >>On Tue, 15 Jun 2010 11:25:17 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >> >>>On Wed, 09 Jun 2010 14:58:42 -0700, John Larkin >>><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >>> >>> >>>> >>>>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 >>> >>> >>>I got some samples of the Central CMPD6001S dual SOT-23 diodes. >>>Through a local rep, since their registration/password thing is so >>>absurd. >>> >>>I measured about 50 fA leakage at -5 volts. I say "about" because this >>>is sorta hard to measure... 5 mV across a 100G resistor. That's a lot >>>better than most diodes at around 5 na. Pricing is 0.099 by the reel, >>>as compared to 0.085 for the BAV199, which I haven't measured yet. >> >>And just how does one go about measuring 5mV across 100G? Seems like >>an open circuit that might not collect a static charge, to me ;) >> >>>Looks like 1.8 pF at zero bias, pretty nice part. >> >>Grant. > > > > Here's my pA parts tester: > > ftp://jjlarkin.lmi.net/99A260A1.JPG > > ftp://jjlarkin.lmi.net/99A260A3.JPG > > ftp://jjlarkin.lmi.net/99S260A.JPG > > I plugged a 100G resistor into the Z2 slot, and put the diode in as > Z1. > > With a 100M or 1G resistor as Z2, nanoamps or tens of pA are easy to > measure. When you get to fA, everything has to be very well shielded, > and you have to wait a long time for the inherent time constants to > settle down. Without a solid shield, just moving around in the > vicinity will induce huge offsets. > > Those damned RatShack banana terminals are conductive as hell, so I > had to machine the hole and add the lexan *after* it was all built. > Grrrrr. > > John > Oh, you mean those things they sell with high grade plastics loaded with fillers of what ever was hanging around at the time or extrusion ? :) |