Diode-Connected Transistors
in [Design]
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From: Jim Thompson on 21 Jul 2010 23:33 On Wed, 21 Jul 2010 19:59:24 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Thu, 22 Jul 2010 12:44:18 +1000, Grant <omg(a)grrr.id.au> wrote: > >>On Wed, 21 Jul 2010 19:10:04 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Wed, 21 Jul 2010 15:18:13 +1000, Grant <omg(a)grrr.id.au> wrote: >>> >>>>On Tue, 20 Jul 2010 18:02:54 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >>>> >>>>>Got an E-mail from Grant advising that I promised a comparison of the >>>>>various ways of doing Diode-Connected Transistors. So here it is... >>>>> >>>>>www.analog-innovations.com/SED/DiodeConnectedTransistors.pdf >>>>> >>>>> ...Jim Thompson >>>> >>>>John L. was going to check BJT diode performance back on the thread: >>>> >>>>>Subject: Re: Types of diode-connected BJTs >>>>>Date: Fri, 09 Jul 2010 08:04:56 -0700 >>>>>Message-ID: <1hee36hr0l0mm2dri4eml98is3mntfidld(a)4ax.com> >>>>>... >>>>>I'll try the BFT25 with the emitter open and shorted to the base, when >>>>>I get a chance. I'd be interested in comparing both leakage and >>>>>capacitance. As I mentioned, fA leakage testing is tedious. >>>> >>>>Hope you find time for this one John? >>>> >>>>I though Jim T. was going to do something with these too, now he has. >>> >>>He ran Spice. I measured actual transistors. >>> >>>> >>>>Might've been a different thread that I can't find? >>>> >>>> >>>>Time flies differently when one is having all their teeth ripped >>>>out :) >>>> >>>>Grant. >>> >>>OK, I gave it a try. My measurement resolution is 1 fA, but things get >>>dicey down there and things take minutes to settle. My best guess is >>>that, at 5 volts reverse, the c-b junction of a BFT25A leaks about 4 >>>fA at room temp. If I repeat with b-e shorted, it increases to about 7 >>>or maybe 8 fA, close to twice the leakage. >>> >>>I did parallel the transistor with a 1T resistor (which leaks 5 >>>whopping picoamperes at 5 volts) and got close to the expected >>>current, so my measurement rig isn't being too obviously silly. >>> >>>Even if I'm off some, the BFT25 c-b junction is the lowest leakage >>>diode I've ever encountered. >> >>Thanks, John. >> >>I was unsure whether you had connected the emitter back when you >>mentioned how low the c-b junction leakage was. Someone wrote that >>connecting the emitter (can't find the thread, though) changed >>things -- can't remember if it was good or bad, I thought it was >>good, possibly in a totally different context. >> >> >>So now we now about the very sensitive end of low leakage diodes :) >>Complete with sanity check! >> >> >>I settled for a 1N3595 recently as it leaks a lot less (1nA) than the >>'HC4053 analog mux chip (100nA, both datasheet 25'C numbers) it is >>protecting from signal over-voltage. >> >>For the curious, I have two signals, out of OP07s running from +12V >>and -5V, via 1k6 resistor, a 1N3595 diode to 4V stops the signal >>exceeding 5V as it goes into a 'hc4053 mux switch. There's a cap >>to analog ground as well. The 4V is a TL431 with scaling resistors >>(could use 3.9V zener) sinking several mA so it has low impedance. > >Good. Don't count on the HC40xx mux ESD diodes to clamp signals, even >if the source resistance is high. Once you get a little beyond the >rails, the internal series switch fets can be turned on, and all hell >breaks loose, many nanoamps of it. > >Central Semi makes some nice dual super-low-leakage surface-mount >diodes, 10s of fA. > >John > Or do the job properly :-) ...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 | Spice is like a sports car... Only as good as the person behind the wheel.
From: Grant on 22 Jul 2010 01:14 On Wed, 21 Jul 2010 20:33:03 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: >On Wed, 21 Jul 2010 19:59:24 -0700, John Larkin ><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: > >>On Thu, 22 Jul 2010 12:44:18 +1000, Grant <omg(a)grrr.id.au> wrote: .... >>> >>>I settled for a 1N3595 recently as it leaks a lot less (1nA) than the >>>'HC4053 analog mux chip (100nA, both datasheet 25'C numbers) it is >>>protecting from signal over-voltage. >>> >>>For the curious, I have two signals, out of OP07s running from +12V >>>and -5V, via 1k6 resistor, a 1N3595 diode to 4V stops the signal >>>exceeding 5V as it goes into a 'hc4053 mux switch. There's a cap >>>to analog ground as well. The 4V is a TL431 with scaling resistors >>>(could use 3.9V zener) sinking several mA so it has low impedance. >> >>Good. Don't count on the HC40xx mux ESD diodes to clamp signals, even >>if the source resistance is high. Once you get a little beyond the >>rails, the internal series switch fets can be turned on, and all hell >>breaks loose, many nanoamps of it. All sorts of chaos including power getting mixed with signals, I've read the warnings in the various datasheets :) No way do I want to temp that chip into making errors when I'm chasing 15 or 16 bit resolution. >> >>Central Semi makes some nice dual super-low-leakage surface-mount >>diodes, 10s of fA. Don't think I need to go that far, yet. Perhaps if I had to go to better class analog mux chips and +/-15V rails. >> >>John >> > >Or do the job properly :-) Me? I don't want to try clamping the output via feedback network around the OP07 as there's a set of four 0.1% resistors setting the gain. I figure smallish resistor plus diode clamp was less evil than passing signal through another opamp. Not too fussed about noise since the ADC on other side of analog mux is a dual slope thingy. Span is done via reference adjustment, null offset on each OP07. Only running about ten conversions per second. PIC chip will run the ADC front end chip, last piece of this little puzzle, I hope... So far: http://grrr.id.au/adc-hires/ Grant.
From: John Larkin on 22 Jul 2010 09:57 On Thu, 22 Jul 2010 15:14:13 +1000, Grant <omg(a)grrr.id.au> wrote: >On Wed, 21 Jul 2010 20:33:03 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote: > >>On Wed, 21 Jul 2010 19:59:24 -0700, John Larkin >><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >> >>>On Thu, 22 Jul 2010 12:44:18 +1000, Grant <omg(a)grrr.id.au> wrote: >... >>>> >>>>I settled for a 1N3595 recently as it leaks a lot less (1nA) than the >>>>'HC4053 analog mux chip (100nA, both datasheet 25'C numbers) it is >>>>protecting from signal over-voltage. >>>> >>>>For the curious, I have two signals, out of OP07s running from +12V >>>>and -5V, via 1k6 resistor, a 1N3595 diode to 4V stops the signal >>>>exceeding 5V as it goes into a 'hc4053 mux switch. There's a cap >>>>to analog ground as well. The 4V is a TL431 with scaling resistors >>>>(could use 3.9V zener) sinking several mA so it has low impedance. >>> >>>Good. Don't count on the HC40xx mux ESD diodes to clamp signals, even >>>if the source resistance is high. Once you get a little beyond the >>>rails, the internal series switch fets can be turned on, and all hell >>>breaks loose, many nanoamps of it. > >All sorts of chaos including power getting mixed with signals, I've >read the warnings in the various datasheets :) No way do I want to >temp that chip into making errors when I'm chasing 15 or 16 bit >resolution. >>> >>>Central Semi makes some nice dual super-low-leakage surface-mount >>>diodes, 10s of fA. > >Don't think I need to go that far, yet. Perhaps if I had to go to >better class analog mux chips and +/-15V rails. > >>> >>>John >>> >> >>Or do the job properly :-) > >Me? I don't want to try clamping the output via feedback network >around the OP07 as there's a set of four 0.1% resistors setting the >gain. I figure smallish resistor plus diode clamp was less evil >than passing signal through another opamp. Not too fussed about >noise since the ADC on other side of analog mux is a dual slope >thingy. Span is done via reference adjustment, null offset on >each OP07. > >Only running about ten conversions per second. PIC chip will run >the ADC front end chip, last piece of this little puzzle, I hope... > >So far: http://grrr.id.au/adc-hires/ > >Grant. I used to do my own dual-slopes. Capacitor dielectric absorption was usually the major error source. Nowadays, you just buy a 24-bit delta-sigma and talk horrible SPI commands to it. John
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