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From: Phil Hobbs on 26 Mar 2010 12:31
On 3/26/2010 11:25 AM, Hammy wrote:
> On Fri, 26 Mar 2010 11:08:24 -0400, "Michael Robinson"
> <nospam(a)billburg.com> wrote:
>
>> On page 4 of this datasheet
>> http://www.nxp.com/documents/data_sheet/BCV62.pdf
>> the manufacuterer gives 0.7 to 1.3 for Ic1/Ic2.
>> Other manufacturers' datasheets use the same number.
>> How many sigmas away from the mean the 30% accuracy statistic is makes a
>> huge difference in the attractiveness of this current mirror as a part I
>> want to buy. But the datasheets don't give that kind of information.
>> Have you used these current mirrors and can give me an idea what kind of
>> accuracy could I expect from the actual physical parts?
>>
> Have you seen diodes matched pnpn's?
>
> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=DMMT3906W-FDICT-ND
>
> http://www.diodes.com/_files/news/DMMT3906W_DMMT3904W.pdf
>
> All diodes matched pairs
>
> http://www.diodes.com/products/catalog/list.php?parent-id=28

Those are isolated dice--meaning you have to keep the dissipation way,
way down to avoid nasty offsets and drifts. A 1 degree C temperature
difference--say 3 mW differential dissipation--will give you a 9% I_C
mismatch.

Monolithics are much better.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
From: Hammy on 26 Mar 2010 12:51
On Fri, 26 Mar 2010 12:31:16 -0400, Phil Hobbs
<pcdhSpamMeSenseless(a)electrooptical.net> wrote:

>On 3/26/2010 11:25 AM, Hammy wrote:
>> On Fri, 26 Mar 2010 11:08:24 -0400, "Michael Robinson"
>> <nospam(a)billburg.com> wrote:
>>
>>> On page 4 of this datasheet
>>> http://www.nxp.com/documents/data_sheet/BCV62.pdf
>>> the manufacuterer gives 0.7 to 1.3 for Ic1/Ic2.
>>> Other manufacturers' datasheets use the same number.
>>> How many sigmas away from the mean the 30% accuracy statistic is makes a
>>> huge difference in the attractiveness of this current mirror as a part I
>>> want to buy. But the datasheets don't give that kind of information.
>>> Have you used these current mirrors and can give me an idea what kind of
>>> accuracy could I expect from the actual physical parts?
>>>
>> Have you seen diodes matched pnpn's?
>>
>> http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=DMMT3906W-FDICT-ND
>>
>> http://www.diodes.com/_files/news/DMMT3906W_DMMT3904W.pdf
>>
>> All diodes matched pairs
>>
>> http://www.diodes.com/products/catalog/list.php?parent-id=28
>
>Those are isolated dice--meaning you have to keep the dissipation way,
>way down to avoid nasty offsets and drifts. A 1 degree C temperature
>difference--say 3 mW differential dissipation--will give you a 9% I_C
>mismatch.

I didnt know it was that bad.

>Monolithics are much better.
>
>Cheers
>
>Phil Hobbs
From: Joerg on 26 Mar 2010 13:02
Michael Robinson wrote:
> On page 4 of this datasheet
> http://www.nxp.com/documents/data_sheet/BCV62.pdf
> the manufacuterer gives 0.7 to 1.3 for Ic1/Ic2.
> Other manufacturers' datasheets use the same number.
> How many sigmas away from the mean the 30% accuracy statistic is makes a
> huge difference in the attractiveness of this current mirror as a part I
> want to buy. But the datasheets don't give that kind of information.
> Have you used these current mirrors and can give me an idea what kind of
> accuracy could I expect from the actual physical parts?
>

It's usually better but they do not guarantee it. Not sure if they'd mix
devices from different wafers but that can make for a huge difference.

If you need closer tolerance straight out of the gate it'll be a lot
more expensive:

http://www.analog.com/static/imported-files/Data_Sheets/SSM2220.pdf

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Michael Robinson on 26 Mar 2010 13:34

"Joerg" <invalid(a)invalid.invalid> wrote in message
news:8147ltFmnU1(a)mid.individual.net...
> Michael Robinson wrote:
>> On page 4 of this datasheet
>> http://www.nxp.com/documents/data_sheet/BCV62.pdf
>> the manufacuterer gives 0.7 to 1.3 for Ic1/Ic2.
>> Other manufacturers' datasheets use the same number.
>> How many sigmas away from the mean the 30% accuracy statistic is makes a
>> huge difference in the attractiveness of this current mirror as a part I
>> want to buy. But the datasheets don't give that kind of information.
>> Have you used these current mirrors and can give me an idea what kind of
>> accuracy could I expect from the actual physical parts?
>
> It's usually better but they do not guarantee it. Not sure if they'd mix
> devices from different wafers but that can make for a huge difference.
>
> If you need closer tolerance straight out of the gate it'll be a lot more
> expensive:
>
> http://www.analog.com/static/imported-files/Data_Sheets/SSM2220.pdf
>
> --
> Regards, Joerg

The lack of promised accuracy was annoying but I can deal with it.
Temperature is another matter. In all the threads I've seen here about
current mirrors and dual transistors, nobody was ever able to tell for sure
whether the parts were built on a single die. It would be nice if somebody
did some more testing of various parts, like that infra-red photo John
Larkin just posted. God forbid they should tell you anything useful on the
datasheet.


From: Phil Hobbs on 26 Mar 2010 13:57
On 3/26/2010 1:34 PM, Michael Robinson wrote:
> "Joerg"<invalid(a)invalid.invalid> wrote in message
> news:8147ltFmnU1(a)mid.individual.net...
>> Michael Robinson wrote:
>>> On page 4 of this datasheet
>>> http://www.nxp.com/documents/data_sheet/BCV62.pdf
>>> the manufacuterer gives 0.7 to 1.3 for Ic1/Ic2.
>>> Other manufacturers' datasheets use the same number.
>>> How many sigmas away from the mean the 30% accuracy statistic is makes a
>>> huge difference in the attractiveness of this current mirror as a part I
>>> want to buy. But the datasheets don't give that kind of information.
>>> Have you used these current mirrors and can give me an idea what kind of
>>> accuracy could I expect from the actual physical parts?
>>
>> It's usually better but they do not guarantee it. Not sure if they'd mix
>> devices from different wafers but that can make for a huge difference.
>>
>> If you need closer tolerance straight out of the gate it'll be a lot more
>> expensive:
>>
>> http://www.analog.com/static/imported-files/Data_Sheets/SSM2220.pdf
>>
>> --
>> Regards, Joerg
>
> The lack of promised accuracy was annoying but I can deal with it.
> Temperature is another matter. In all the threads I've seen here about
> current mirrors and dual transistors, nobody was ever able to tell for sure
> whether the parts were built on a single die. It would be nice if somebody
> did some more testing of various parts, like that infra-red photo John
> Larkin just posted. God forbid they should tell you anything useful on the
> datasheet.
>
>

It's not that hard to figure out. For instance, check out Fig 4 of the
BCV61 datasheet--it shows that for zero emitter resistors, a 1 mA
collector current gives you 30% error (about 7 mV Vos or 3.5 C) with
Vce=9V (9 mW) and a 5 mA does the same at about 3V. Either way, it's
ballpark 200-300 K/W, which is way too high for a monolithic--it's
comparable to the theta_jc of the package.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net
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