[sdiy] THAT300

Tom Bugs admin at bugbrand.co.uk
Thu May 7 10:43:05 CEST 2015


Was going to suggest contacting THAT direct - they tend to be pretty 
good at replying to questions.


On 07/05/2015 01:26, Dave Manley wrote:
> A while back, THAT replied to an email I sent...you can always try that.
>
> -Dave
>
> On May 6, 2015 3:51:53 PM PDT, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
>> "Substrate biasing is not required for normal operation, though the
>> substrate should be ac-grounded to optimize speed and minimize
>> crosstalk."
>>
>> This sounds like the substrate is electrically insulated from the
>> workings (probably by an oxide layer.) So it doesn't matter what dc
>> voltage is on there.
>>
>> However they are aware that there is still significant capacitance
>> between the substrate and various nodes in the workings (probably
>> because the oxide insulating layer is very thin.) Therefore they advise
>> grounding it at least for ac signals to reduce high-frequency
>> crosstalk. Personally I'd just tie it directly to a good *clean*
>> analogue ground.
>>
>> -Richie,
>>
>> Sent from my Xperia SP on O2
>>
>> ---- rsdio at audiobanshee.com wrote ----
>>
>>> Did anyone answer this, Crystal?
>>>
>>> The substrate is typically connected to the source for 3-pin FETs, but
>> this is BJT technology so we should disregard FET conventions.
>>> The data sheet description holds a clue: "Substrate biasing is not
>> required for normal operation, though the substrate should be
>> ac-grounded to optimize speed and minimize crosstalk."
>>> My interpretation of that is you should connect a capacitor to the
>> substrate pins and ground the other end of the capacitor. No idea what
>> value it should be.
>>> I'm surprised that there aren't example circuits in the data sheet,
>> but perhaps there are application notes on the THAT Corp web site. The
>> closest I could find is a mic preamplifier based on the THAT120A array,
>> but it has a different pinout. They show pin 14 grounded, but its data
>> sheet only mentions "grounded" not "ac-grounded," so I think they
>> learned something before discontinuing the THAT120A.
>>> Have you made any progress since January? Keep us posted…
>>>
>>> Brian
>>>
>>>
>>> On Jan 2, 2015, at 3:44 PM, crystal <crystals at sonic.net> wrote:
>>>> Thanx Tim Ressel for the tip of using a THAT300 for a matched pair.
>> Haven't tested my circuit out for tracking accuracy quite just yet but
>> i bet it's a significant improvement. my question: what to do with the
>> substrate pin?   -V, or perhaps ground?
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