[time-nuts] Hp10811 parts ref

[Charles Steinmetz]

One more LMx08 equivalent: the most obvious of all, the LT1008.  I 
didn't think about it earlier because I assumed it would have been 
discontinued long ago.

The LT1008 was designed specifically to upgrade LMx08 sockets, so it 
uses Pin 8 for overcompensation -- just like the LMx08.  It is one 
generation older than the LT1012/OP97, so its DC specs are not quite as 
good as theirs' (but still *much* better than the LMx08).

If I were choosing an opamp for this type of circuit today, with a new 
PC layout, I would use either the LT1012 or OP97.  On the other hand, if 
I were fixing an HP10811 that had a defective U3, I'd use an LT1008 to 
avoid the need to re-route any traces.

Best regards,

Charles


On 9/13/2019 6:04 PM, Charles Steinmetz wrote:
> You are very welcome, Gilles.  One point of clarification, and one
> additional note:
>
> I mentioned the LT1012 and OP97 together, then said I've settled on the
> OP97 without further comment.  My choice had nothing to do with
> differences between these two parts.  As far as I can tell from using
> them extensively, there are no differences -- for all I know, they could
> be made using identical masks.  I chose the OP97 for supply-chain
> reasons that won't be relevant to anyone else.
>
> Looking at the 10811A/B schematic, the bridge sampling resistors (R22,
> R23) are 1Mohm.  This is what accounts for choosing U3 with extremely
> low input bias current.  Note that the LT1012/OP97 are roughly 1000
> times better than the LMx08 in this respect (pA for the LT1012/OP97, nA
> for the LMx08) as well as input input offset voltage (mV for the LMx08,
> uV for the LT1012/OP97).
>
> One thing to note when replacing an LMx08 with an LT1012/OP97:  Notice
> C19 (0.01uF from Pin 8 to Ground).  This is an *overcompensation*
> capacitor, used to slow U3 down for lower noise and better transient
> response of the oven control loop.  The LT1012/OP97 use *Pin 5* for
> overcompensation instead of Pin 8, so C19 should be connected from Pin 5
> to Ground.
>
> Best regards,
>
> Charles
>
>
> On 9/13/2019 11:17 AM, Club-Internet Clemgill wrote:
>> Charles,
>> Thank you for this very useful information !
>> My plan is to « clone » as close as possible the initial 10811 design,
>> as it is well proven.
>> Except changing the crystal and possibly use modern components if they
>> bring any improvement, and adapting the « mass » to the crystal format
>> and manufacturing it through 3D printing.
>> Did anybody tried something similar ?
>> Best,
>> Gilles.
>>
>>
>>
>>> Le 11 sept. 2019 à 22:05, Charles Steinmetz <csteinmetz at yandex.com> a
>>> écrit :
>>>
>>> Didier wrote:
>>>
>>>> The LM208 is a single op-amp with low offset (<3mV) and low bias
>>>> current.
>>>
>>> Even 3mV offset is verging on horrible for a "precision" opamp these
>>> days -- it's hard to find any respectable opamp that doesn't do lots
>>> better.
>>>
>>> The secret sauce of the LMx08 series is the super-beta input pair,
>>> which matches FET opamps for input current but also holds that low
>>> (room-temp) input current over the full temperature range (the input
>>> current of FET opamps generally rises by at least 10x to more than
>>> 1000x at warmer to hot temperatures).
>>>
>>> There is no need for extraordinary measures -- there are a number of
>>> more modern opamps specifically designed to upgrade LMx08 sockets.
>>> The OP77 is one:
>>>
>>> <http://pdf.datasheetcatalog.com/datasheet/analogdevices/206654345OP77_c.pdf>
>>>
>>>
>>> The LT1012 and OP97 are two more.  I've used both extensively, and
>>> finally settled on the OP97 for this sort of application:
>>>
>>> <http://pdf.datasheetcatalog.com/datasheet/analogdevices/140679914OP97_d.pdf>
>>>
>>>
>>> Best regards,
>>>
>>> Charles
>>>
>>>
>>>
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