On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>whit3rd wrote: >> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote: >>> whit3rd wrote: >>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote: >>>> >>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB >>>>> pad, let alone a trace. >>>> >>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot, >>>> so you can get your 0.1 pf by soldering a short length onto the board, and then >>>> with flush nippers, cut it off. >>> I invite you to try getting the fringing capacitance of an actual piece >>> of RG-174, connected to a circuit, to be that low. Show your work. ;) >>> >>> (The capacitance per unit length only applies when the fringing >>> capacitance is negligible.) >> >> But, because this is a trimming technique, there's an end correction BEFORE the >> snip as well as after. At least, there is until the two ends of the cable coincide... >> >> I'm not concerned with high frequencies much, so I might apply a negative capacitor >> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative >> impedance converter, you know... >> >> <https://wiki.analog.com/university/courses/electronics/text/chapter-4> >> > >Yes and no. Given invariant fringing capacitance, changing the length >incrementally does give a nice predictable delta-C. Trombone line is >good that way down at low frequency, for instance. > >This is because, in a nice long piece of coax, the E field inside is >purely radial almost everywhere, and any departure from the pure TEM >mode at the ends dies off exponentially, roughly as exp(-2 pi L/r), >where L is the distance from the open end and r is the radius. (That's >a consequence of Laplace's equation, and is also why perforated metal >makes good electrostatic shielding.) > >However, John asked for a small absolute capacitance. A picofarad or so >of end effect doesn't fit that bill.And I want something that can be manufactured, not a hobby fiddle thing. Something like this: https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1 It needs extreme CMRR to work. If C1 is just a resistor parasitic, I can make C2 a bit larger and the CMRR error becomes positive feedback, which is OK in moderation. High voltage Schmitt trigger. I could just use a digital isolator to drive the GaN fet, but they are slow. The modulated ones may add jitter, too. Of course, detonators probably don't care about a bit of jitter.
smallest 0603 cap
Started by ●January 24, 2023
Reply by ●January 25, 20232023-01-25
Reply by ●January 25, 20232023-01-25
On Wed, 25 Jan 2023 05:45:41 GMT, Jan Panteltje <alien@comet.invalid> wrote:>On a sunny day (Tue, 24 Jan 2023 10:37:01 -0800) it happened John Larkin ><jlarkin@highlandSNIPMEtechnology.com> wrote in ><n390th9l7dkb25tai5g3aj4cesoe6n0u5u@4ax.com>: > >>I need a tiny cap. >> >>This one is interesting >> >>https://www.mouser.com/ProductDetail/Walsin/RF03N0R1B100CT?qs=sGAEpiMZZMvsSlwiRhF8qtsBU8Zhqm2Ra%2Fa5698GTEQZP2uSIBTS%2FQ%3D%3D > >LOL looks like a type by them >value .1 pF >tolerane .1 pFYes, that was the interesting part.
Reply by ●January 25, 20232023-01-25
On a sunny day (Wed, 25 Jan 2023 10:49:52 -0500) it happened Joe Gwinn <joegwinn@comcast.net> wrote in <jpj2tht5cdbd666u4pu937ftqki68s3ah2@4ax.com>:>On Wed, 25 Jan 2023 05:48:33 GMT, Jan Panteltje <alien@comet.invalid> >wrote: > >>On a sunny day (Tue, 24 Jan 2023 11:17:06 -0800) it happened John Larkin >><jlarkin@highlandSNIPMEtechnology.com> wrote in >><o5b0thlfg6oiep66l7om8tcnns6hldcbfq@4ax.com>: >> >>>On Tue, 24 Jan 2023 13:41:52 -0500, Phil Hobbs >>><pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>>John Larkin wrote: >>>>> I need a tiny cap. >>>>> >>>>> This one is interesting >>>>> >>>>> >>>>> >>>>> https://www.mouser.com/ProductDetail/Walsin/RF03N0R1B100CT?qs=sGAEpiMZZMvsSlwiRhF8qtsBU8Zhqm2Ra%2Fa5698GTEQZP2uSIBTS%2FQ%3D%3D >>>>> >>>>> Below 0.1 pF, maybe I can use a 10 meg resistor, which is around 0.04 >>>>> pF. Maybe a couple in series? Maybe a network? >>>>> >>>>> >>>>> >>>> >>>>I have some 0.1 pF caps in stock too, primarily for adjusting the >>>>transfer functions of high-Z TIAs. >>>> >>>>You could maybe make a poor man's 3-terminal cap using two of those in >>>>series--the capacitance to ground from the midpoint would reduce the >>>>end-to-end capacitance. >>>> >>>>Cheers >>>> >>>>Phil Hobbs >>> >>>One end of my cap will be ground already. >>> >>>I could just use PCB capacitance, but that's hard to tune. I'd just >>>have to get it right first try. >> >>Bended wire, twisted wire... >>done it. > >This used to be called a gimmick capacitor. > >.<https://en.wikipedia.org/wiki/Gimmick_capacitor>http://panteltje.com/pub/twisted_wire_oscillator_IMG_6629.JPG http://panteltje.com/pub/2.4GHz_twisted_oscillator_IMG_3629.GIF http://panteltje.com/pub/GPS_jammer_board_twisted_wire_1.57GHz_oscillator_IMG_3622.GIF ...
Reply by ●January 25, 20232023-01-25
On a sunny day (Wed, 25 Jan 2023 08:02:22 -0800) it happened John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote in <tlj2th5frqrbrvhf8mjrgup4df34lgq2j2@4ax.com>:>Something like this: > >https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=12 twisted wire caps?
Reply by ●January 25, 20232023-01-25
John Larkin wrote:> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> whit3rd wrote: >>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote: >>>> whit3rd wrote: >>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote: >>>>> >>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB >>>>>> pad, let alone a trace. >>>>> >>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot, >>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then >>>>> with flush nippers, cut it off. >>>> I invite you to try getting the fringing capacitance of an actual piece >>>> of RG-174, connected to a circuit, to be that low. Show your work. ;) >>>> >>>> (The capacitance per unit length only applies when the fringing >>>> capacitance is negligible.) >>> >>> But, because this is a trimming technique, there's an end correction BEFORE the >>> snip as well as after. At least, there is until the two ends of the cable coincide... >>> >>> I'm not concerned with high frequencies much, so I might apply a negative capacitor >>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative >>> impedance converter, you know... >>> >>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4> >>> >> >> Yes and no. Given invariant fringing capacitance, changing the length >> incrementally does give a nice predictable delta-C. Trombone line is >> good that way down at low frequency, for instance. >> >> This is because, in a nice long piece of coax, the E field inside is >> purely radial almost everywhere, and any departure from the pure TEM >> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r), >> where L is the distance from the open end and r is the radius. (That's >> a consequence of Laplace's equation, and is also why perforated metal >> makes good electrostatic shielding.) >> >> However, John asked for a small absolute capacitance. A picofarad or so >> of end effect doesn't fit that bill. > > And I want something that can be manufactured, not a hobby fiddle > thing. > > Something like this: > > https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1 > > It needs extreme CMRR to work. If C1 is just a resistor parasitic, I > can make C2 a bit larger and the CMRR error becomes positive feedback, > which is OK in moderation. High voltage Schmitt trigger. > > I could just use a digital isolator to drive the GaN fet, but they are > slow. The modulated ones may add jitter, too. > > Of course, detonators probably don't care about a bit of jitter.Well, not repeatable jitter, anyway. ;) Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●January 25, 20232023-01-25
On Wednesday, January 25, 2023 at 5:00:04 AM UTC-8, Phil Hobbs wrote:> whit3rd wrote: > > On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote: > >> whit3rd wrote: > >>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote: > >>> > >>>> It's already not that easy to get less than 0.1 pF to ground from a PCB > >>>> pad, let alone a trace. > >>> > >>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot, > >>> so you can get your 0.1 pf by soldering a short length onto the board, and then > >>> with flush nippers, cut it off. > >> I invite you to try getting the fringing capacitance of an actual piece > >> of RG-174, connected to a circuit, to be that low. Show your work. ;) > >> > >> (The capacitance per unit length only applies when the fringing > >> capacitance is negligible.) > > > > But, because this is a trimming technique, there's an end correction BEFORE the > > snip as well as after. At least, there is until the two ends of the cable coincide...> However, John asked for a small absolute capacitance. A picofarad or so > of end effect doesn't fit that bill. > > Plus when the coax gets too short, the approximation you're relying on > starts getting inaccurate because (a) there are varying axial fields > throughout the length of the coax, and (b) the fields at the two ends > interact.Not so; this (or any) trim isn't intended to hit only one absolute value, it's for test-and-adjust finagling. If you want to hit near-zero, any adjustable capacitor plus a negative offset circuit (negative capacitance) seems to me to be a suitable solution. Having a suitable length to snip on, does eliminate the concern with an invariant element, like the end correction.
Reply by ●January 25, 20232023-01-25
On Wed, 25 Jan 2023 13:45:07 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> whit3rd wrote: >>>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote: >>>>> whit3rd wrote: >>>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote: >>>>>> >>>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB >>>>>>> pad, let alone a trace. >>>>>> >>>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot, >>>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then >>>>>> with flush nippers, cut it off. >>>>> I invite you to try getting the fringing capacitance of an actual piece >>>>> of RG-174, connected to a circuit, to be that low. Show your work. ;) >>>>> >>>>> (The capacitance per unit length only applies when the fringing >>>>> capacitance is negligible.) >>>> >>>> But, because this is a trimming technique, there's an end correction BEFORE the >>>> snip as well as after. At least, there is until the two ends of the cable coincide... >>>> >>>> I'm not concerned with high frequencies much, so I might apply a negative capacitor >>>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative >>>> impedance converter, you know... >>>> >>>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4> >>>> >>> >>> Yes and no. Given invariant fringing capacitance, changing the length >>> incrementally does give a nice predictable delta-C. Trombone line is >>> good that way down at low frequency, for instance. >>> >>> This is because, in a nice long piece of coax, the E field inside is >>> purely radial almost everywhere, and any departure from the pure TEM >>> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r), >>> where L is the distance from the open end and r is the radius. (That's >>> a consequence of Laplace's equation, and is also why perforated metal >>> makes good electrostatic shielding.) >>> >>> However, John asked for a small absolute capacitance. A picofarad or so >>> of end effect doesn't fit that bill. >> >> And I want something that can be manufactured, not a hobby fiddle >> thing. >> >> Something like this: >> >> https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1 >> >> It needs extreme CMRR to work. If C1 is just a resistor parasitic, I >> can make C2 a bit larger and the CMRR error becomes positive feedback, >> which is OK in moderation. High voltage Schmitt trigger. >> >> I could just use a digital isolator to drive the GaN fet, but they are >> slow. The modulated ones may add jitter, too. >> >> Of course, detonators probably don't care about a bit of jitter. > >Well, not repeatable jitter, anyway. ;)Right. Measuring jitter requires a statistically valid number of pulses. We used to ship CAMAC systems that went down-hole near a nuke. They shipped a bunch of time stamps up a long cable. Good replacement market. Just drilling the hole cost a million dollars.
Reply by ●January 25, 20232023-01-25
John Larkin wrote:> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> whit3rd wrote: >>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote: >>>> whit3rd wrote: >>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote: >>>>> >>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB >>>>>> pad, let alone a trace. >>>>> >>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot, >>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then >>>>> with flush nippers, cut it off. >>>> I invite you to try getting the fringing capacitance of an actual piece >>>> of RG-174, connected to a circuit, to be that low. Show your work. ;) >>>> >>>> (The capacitance per unit length only applies when the fringing >>>> capacitance is negligible.) >>> >>> But, because this is a trimming technique, there's an end correction BEFORE the >>> snip as well as after. At least, there is until the two ends of the cable coincide... >>> >>> I'm not concerned with high frequencies much, so I might apply a negative capacitor >>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative >>> impedance converter, you know... >>> >>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4> >>> >> >> Yes and no. Given invariant fringing capacitance, changing the length >> incrementally does give a nice predictable delta-C. Trombone line is >> good that way down at low frequency, for instance. >> >> This is because, in a nice long piece of coax, the E field inside is >> purely radial almost everywhere, and any departure from the pure TEM >> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r), >> where L is the distance from the open end and r is the radius. (That's >> a consequence of Laplace's equation, and is also why perforated metal >> makes good electrostatic shielding.) >> >> However, John asked for a small absolute capacitance. A picofarad or so >> of end effect doesn't fit that bill. > > And I want something that can be manufactured, not a hobby fiddle > thing. > > Something like this: > > https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1 > > It needs extreme CMRR to work. If C1 is just a resistor parasitic, I > can make C2 a bit larger and the CMRR error becomes positive feedback, > which is OK in moderation. High voltage Schmitt trigger.Okay, for something like that, just dorking it to one side a bit makes sense, assuming that the worst-case RC time constant is small enough that it doesn't distort the next transition. I'm actually working on a proof-of-concept that needs two separate TIAs to have very closely similar phase and amplitude response out to a couple of hundred kilohertz, which requires excellent phase matching. We're only making 10 or so boards, so I'm planning to put a couple of DNP caps in parallel with the main one, to get the tolerance down from ~5% to ~0.5%. (Yes, this will require a bit of measurement and selection on each board, but there aren't very many, and we have all the stuff in stock.) Maybe it's possible to use a dpot with a shunt cap on the wiper to balance the two sides. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●January 25, 20232023-01-25
On Wed, 25 Jan 2023 21:26:38 -0500, Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:>John Larkin wrote: >> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs >> <pcdhSpamMeSenseless@electrooptical.net> wrote: >> >>> whit3rd wrote: >>>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote: >>>>> whit3rd wrote: >>>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote: >>>>>> >>>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB >>>>>>> pad, let alone a trace. >>>>>> >>>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot, >>>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then >>>>>> with flush nippers, cut it off. >>>>> I invite you to try getting the fringing capacitance of an actual piece >>>>> of RG-174, connected to a circuit, to be that low. Show your work. ;) >>>>> >>>>> (The capacitance per unit length only applies when the fringing >>>>> capacitance is negligible.) >>>> >>>> But, because this is a trimming technique, there's an end correction BEFORE the >>>> snip as well as after. At least, there is until the two ends of the cable coincide... >>>> >>>> I'm not concerned with high frequencies much, so I might apply a negative capacitor >>>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative >>>> impedance converter, you know... >>>> >>>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4> >>>> >>> >>> Yes and no. Given invariant fringing capacitance, changing the length >>> incrementally does give a nice predictable delta-C. Trombone line is >>> good that way down at low frequency, for instance. >>> >>> This is because, in a nice long piece of coax, the E field inside is >>> purely radial almost everywhere, and any departure from the pure TEM >>> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r), >>> where L is the distance from the open end and r is the radius. (That's >>> a consequence of Laplace's equation, and is also why perforated metal >>> makes good electrostatic shielding.) >>> >>> However, John asked for a small absolute capacitance. A picofarad or so >>> of end effect doesn't fit that bill. >> >> And I want something that can be manufactured, not a hobby fiddle >> thing. >> >> Something like this: >> >> https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1 >> >> It needs extreme CMRR to work. If C1 is just a resistor parasitic, I >> can make C2 a bit larger and the CMRR error becomes positive feedback, >> which is OK in moderation. High voltage Schmitt trigger. > >Okay, for something like that, just dorking it to one side a bit makes >sense, assuming that the worst-case RC time constant is small enough >that it doesn't distort the next transition. > >I'm actually working on a proof-of-concept that needs two separate TIAs >to have very closely similar phase and amplitude response out to a >couple of hundred kilohertz, which requires excellent phase matching. >We're only making 10 or so boards, so I'm planning to put a couple of >DNP caps in parallel with the main one, to get the tolerance down from >~5% to ~0.5%. (Yes, this will require a bit of measurement and >selection on each board, but there aren't very many, and we have all the >stuff in stock.) > >Maybe it's possible to use a dpot with a shunt cap on the wiper to >balance the two sides. > >Cheers > >Phil HobbsHow about a trimmer cap? Beats soldering. If it's TIA feedback, a trimpot can tweak the effective C value. DPOTs are usually slow with the slowness code-dependent. Real trimpots are great and easy to program.
Reply by ●January 26, 20232023-01-26
John Larkin wrote:> On Wed, 25 Jan 2023 21:26:38 -0500, Phil Hobbs > <pcdhSpamMeSenseless@electrooptical.net> wrote: > >> John Larkin wrote: >>> On Wed, 25 Jan 2023 07:59:52 -0500, Phil Hobbs >>> <pcdhSpamMeSenseless@electrooptical.net> wrote: >>> >>>> whit3rd wrote: >>>>> On Tuesday, January 24, 2023 at 5:36:47 PM UTC-8, Phil Hobbs wrote: >>>>>> whit3rd wrote: >>>>>>> On Tuesday, January 24, 2023 at 12:47:09 PM UTC-8, Phil Hobbs wrote: >>>>>>> >>>>>>>> It's already not that easy to get less than 0.1 pF to ground from a PCB >>>>>>>> pad, let alone a trace. >>>>>>> >>>>>>> Yeah, but the usual trim techniques still work; RG174 is 30 pF/foot, >>>>>>> so you can get your 0.1 pf by soldering a short length onto the board, and then >>>>>>> with flush nippers, cut it off. >>>>>> I invite you to try getting the fringing capacitance of an actual piece >>>>>> of RG-174, connected to a circuit, to be that low. Show your work. ;) >>>>>> >>>>>> (The capacitance per unit length only applies when the fringing >>>>>> capacitance is negligible.) >>>>> >>>>> But, because this is a trimming technique, there's an end correction BEFORE the >>>>> snip as well as after. At least, there is until the two ends of the cable coincide... >>>>> >>>>> I'm not concerned with high frequencies much, so I might apply a negative capacitor >>>>> first, swamp it with the long-RG174 trimmer, then trim down to get near zero. Negative >>>>> impedance converter, you know... >>>>> >>>>> <https://wiki.analog.com/university/courses/electronics/text/chapter-4> >>>>> >>>> >>>> Yes and no. Given invariant fringing capacitance, changing the length >>>> incrementally does give a nice predictable delta-C. Trombone line is >>>> good that way down at low frequency, for instance. >>>> >>>> This is because, in a nice long piece of coax, the E field inside is >>>> purely radial almost everywhere, and any departure from the pure TEM >>>> mode at the ends dies off exponentially, roughly as exp(-2 pi L/r), >>>> where L is the distance from the open end and r is the radius. (That's >>>> a consequence of Laplace's equation, and is also why perforated metal >>>> makes good electrostatic shielding.) >>>> >>>> However, John asked for a small absolute capacitance. A picofarad or so >>>> of end effect doesn't fit that bill. >>> >>> And I want something that can be manufactured, not a hobby fiddle >>> thing. >>> >>> Something like this: >>> >>> https://www.dropbox.com/s/z4vevjyuflm732l/T578_LVDS.jpg?raw=1 >>> >>> It needs extreme CMRR to work. If C1 is just a resistor parasitic, I >>> can make C2 a bit larger and the CMRR error becomes positive feedback, >>> which is OK in moderation. High voltage Schmitt trigger. >> >> Okay, for something like that, just dorking it to one side a bit makes >> sense, assuming that the worst-case RC time constant is small enough >> that it doesn't distort the next transition. >> >> I'm actually working on a proof-of-concept that needs two separate TIAs >> to have very closely similar phase and amplitude response out to a >> couple of hundred kilohertz, which requires excellent phase matching. >> We're only making 10 or so boards, so I'm planning to put a couple of >> DNP caps in parallel with the main one, to get the tolerance down from >> ~5% to ~0.5%. (Yes, this will require a bit of measurement and >> selection on each board, but there aren't very many, and we have all the >> stuff in stock.) >> >> Maybe it's possible to use a dpot with a shunt cap on the wiper to >> balance the two sides. >> >> Cheers >> >> Phil Hobbs > > How about a trimmer cap? Beats soldering.If it can be adjusted, it can be mis-adjusted, leading to hard-to-diagnose misbehavior and reasonable-looking wrong answers. (My least favorite kind.)> > If it's TIA feedback, a trimpot can tweak the effective C value. > > DPOTs are usually slow with the slowness code-dependent. Real trimpots > are great and easy to program.Sure, if you can still get them. Those nice Murata PVA2 things are long gone. A dpot is a variable RC network, so by putting the cap at the wiper, you change the capacitive loading of the ends. There's some built in to the dpot as well, as you say, but it varies with code too. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
