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