[volt-nuts] Matched resistors

Andreas Jahn Andreas_-_Jahn at t-online.de
Sat Jul 19 11:58:51 EDT 2014


Hello Randy,

some information you will get on eevblog. (its much easier to post 
(larger) pictures there).
Namely within the LTZ1000, LM399 and T.C. Measurements threads:

http://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/
http://www.eevblog.com/forum/projects/lm399-based-10-v-reference/
http://www.eevblog.com/forum/projects/t-c-measurements-on-precision-resistors/
http://www.eevblog.com/forum/projects/oshw-24bit-adc-measurement-system-for-voltage-references/
http://www.eevblog.com/forum/projects/building-a-7-decade-voltage-calibrator/
http://www.eevblog.com/forum/projects/ppmgeek!-5-5-digit-dvm-volt-ref-cal-%28for-arduino-or-any-uc-w-spi%29/msg296127/#msg296127

With best regards

Andreas

Am 19.07.2014 16:33, schrieb Randy Evans:
> Andreas,
>
> Thanks for the information.  Do you have the drift chart, etc. posted
> anywhere?  that would be very interesting reading.
>
> Thanks,
>
> Randy
>
>
> On Fri, Jul 18, 2014 at 10:11 PM, Andreas Jahn <Andreas_-_Jahn at t-online.de>
> wrote:
>
>> Hello Randy,
>>
>> I think the only difference is in oscillator section (and thus power
>> consumption)
>> and of cause the TSSOP-package.
>> The LTC1043 is easily available from stock e.g. from digikey.
>> The LTC6943 is more difficult to get.
>> Within the Keithley 2002 LTC1043 is used.
>> http://www.eevblog.com/forum/testgear/keithley-2002-8-5-
>> digit-dmm-review-and-teardown/
>> http://dev.xdevs.com/projects/kei2002/repository/entry/
>> photos/K1/small/K2002_1-2251.jpg
>>
>> I have added a drift chart with longterm drift data.
>> Note: the drift is for the whole measurement arrangement.
>> It consists of 3 7V references (2 LTZ1000A and 1 LM399)
>> measured via a LTC1043 divider with a 24 bit LTC2400 ADC with
>> temperature compensated voltage reference AD586LQ.
>> So most of the drift is related to the AD586LQ reference.
>> (X-axis is in days, Y-axis in ppm)
>>
>> I get around 2 ppm drift for the LTZ1000A  over 1 year
>> which I guess is mostly humidity related
>> from the ADC printed cirquit board + AD586 reference drift
>> and usually below 0.25 ppm standard deviation over 1000 hours.  (42 days).
>> All at unstabilized room temperature.
>> I guess with resistors you will need ovenized temperature stabilisation to
>> achieve this.
>>
>> with best regards
>>
>> Andreas
>>
>> Am 19.07.2014 05:57, schrieb Randy Evans:
>>
>>   Andreas,
>>> That is good information, I appreciate it.  I have contacted LT
>>> application
>>> support but they have yet to get back to me on my questions except they
>>> did
>>> recommend to use the LTC6943 instead of the LTC1043.  Later generation I
>>> guess.
>>>
>>> I think i am going to try both the LTC6943 and the LT5400 resistor array
>>> and characterize them.  The LT5400 matching ratio looks pretty good over
>>> temp (0.2ppm/C) but the absolute resistor change over temp is -10 to +25
>>> ppm/C, a little larger than I would like for the circuit I am using.
>>>
>>> Randy
>>>
>>>
>>> On Thu, Jul 17, 2014 at 10:37 PM, Andreas Jahn <
>>> Andreas_-_Jahn at t-online.de>
>>> wrote:
>>>
>>>   Hello Randy,
>>>> I am using the LTC1043 in 1/2 VIN or 1/3 VIN configuration.
>>>> A matching of the caps is not necessary.
>>>> In the 1/2 VIN or 1/3 VIN configuration a matching would give
>>>> the advantage that the settling time of the cirquit is reduced.
>>>> But in 2* VIN or inverting configuration a matching gives no advantage.
>>>> So perhaps it is better to put a 1/2 VIN divider into a feedback loop.
>>>>
>>>> The most important point: you will need a low leakage buffer amplifier at
>>>> the output.
>>>> The caps should be low leakage foil capacitors. (polypropylene would be
>>>> best).
>>>> The ESR is negligible against the switch resistance of around 1000 Ohms
>>>> And dielectric absorption would also affect only settling time.
>>>>
>>>> In 1/2 VIN configuration I am using cheap small mylar capacitors (WIMA
>>>> MKS02)
>>>> (isolation time constant is given only with  >1250 sec (3000 sec typ)).
>>>> Buffer amplifier is a LTC1050.
>>>> The circuit is very stable over temperature (10 - 40 deg C).
>>>> The absolute amplification error is usually some ppm lower than exact 2:1
>>>> value.
>>>> (depends somewhat on the pinning which is used so I am not shure wether
>>>> the pins are mixed up regarding the charge compensation)
>>>>
>>>> So I dont know wether the ±1 ppm is more a stability figure than a
>>>> absolute value.
>>>> Even polypropylene capacitors do not change the amplification error.
>>>>
>>>> With best regards
>>>>
>>>> Andreas
>>>>
>>>> Am 17.07.2014 17:26, schrieb Randy Evans:
>>>>
>>>>   Frank,
>>>>> The high cost is my concern, although high performance demands high
>>>>> price
>>>>> typically.  I am trying to double the voltage reference from either an
>>>>> LM399 or LTZ1000, hence the need for precision matched resistors for a
>>>>> x2
>>>>> non-inverting amplifier (using a LT1151 precision op amp).  An
>>>>> alternative
>>>>> I am investigating is using the LTC1043 in a voltage doubling circuit as
>>>>> shown in Linear Technology app note AN 42, page 6, Figure 16.  It states
>>>>> that Vout = 2xVin ± 5 ppm.  I am less concerned about the absolute
>>>>>
>>>>> accuracy than I am about the long term stability.  I assume that a high
>>>>> quality capacitor is required (low leakage, low ESR, low dielectric
>>>>> absorbtion, etc.) but the circuit does not appear to be dependent on the
>>>>> absolute value of the capacitors.  I'm not sure if the two 1uF caps
>>>>>   need
>>>>> to be matched.  If they do then that would be a show stopper.
>>>>>
>>>>> Does anyone have any experience using the LTC1043 in such a circuit?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Randy
>>>>>
>>>>>    _______________________________________________
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