[volt-nuts] Matched resistors

Randy Evans randyevans2688 at gmail.com
Mon Jul 21 15:01:55 EDT 2014


Andreas,

Of course, I would also expect the leakage currents to change over
time/temperature and I would expect them to be the dominant error source.

Randy


On Mon, Jul 21, 2014 at 9:38 AM, Randy Evans <randyevans2688 at gmail.com>
wrote:

> Andreas,
>
> Since you are familiar with the LTC1043, do you know what stability over
> time and temperature one could expect for the X2 circuit, assuming a high
> quality low leakage capacitor was used?   Since the circuit does not appear
> to be sensitive to the capacitor value, the primary change over time and
> temperature would be expected to be the switch resistance I would think.
>  If the circuit were fed into a high impedance buffer (LTC1151), then I
> would expect the switch resistance to have minimal impact.  What do you
> think?
>
> thanks,
>
> Randy
>
>
> On Sat, Jul 19, 2014 at 8:58 AM, Andreas Jahn <Andreas_-_Jahn at t-online.de>
> wrote:
>
>> 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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