[volt-nuts] (no subject) -> LM399 Reference

[r j]

LM399 good


On Sat, Mar 29, 2014 at 5:18 PM, Andreas Jahn <Andreas_-_Jahn at t-online.de>wrote:

> Hello Jan,
>
> The LM399 is being far from bad as stated in the datasheet.
> The 10ppm/khr (8-20ppm) is a figure which is very conservative.
> (perhaps directly after soldering process).
>
> Most of my LM399 are within 1-2 ppm/khr after a run in phase of 1-2 khrs.
> And the typically drift of instruments with LM399 as reference
> have about 1-2ppm/year drift after some ageing.
>
> A large part of power supply sensitivity on output voltage comes
> from heater temperature setpoint dependancy on heater voltage/current.
> The effect is larger with lower supply voltages. (especially in the 9-10V
> range).
> You should always use a well stabilized (some applications specify 0.1%)
> power supply >= 15V with low tempco for the heater.
>
> With best regards
>
> Andreas
>
>
>
>
> Am 29.03.2014 10:47, schrieb Jan Fredriksson:
>
>> I was looking at a few different alternatives for a transportable,
>> non-permanently powered on, DIY, voltage reference and I picked the LM399
>> as one of the candidates.
>>
>> The LM399 is a buried zener reference with built-in temperature regulation
>> and an integrated thermal insulation cover. It is very simple to
>> implerment, only one resistor needed.
>>
>> The downsides:
>> - it has a nominal drift in the order of 10ppm/sqrt1000h (as good as any
>> except the LTZ1000)
>> - Its a bit sensitive to input volgage, due to a 1ohm input impedance.
>> - Its a bit noisy, about 10uVptp.
>> - Output voltage tolerance is poor and at around 6.9V
>>
>> I set one of these up and, after a day of stabilizing, found the noise
>> about as stated in the datasheet. However, noise in the datasheet is
>> specified for 10Hz and up. The real problem to was at lower
>> frequency. Overnight, the 10s averaged values where slowly drifting around
>> about 10uV ptp.
>>
>> Now I made twelve LM399 parallelled on a simple breadboard, running at
>> 1.8mA each, plus heating, a total of around 200mA.
>>
>> Like the first, I use simple 2K7 1% metal film resistors for current
>> limiting at 12V supply. The resistors do not need to be very high-spec
>> as errors are attenuated by a factor 1:2700 (1R/2K7). There is another 2K7
>> resistor per zener for output averaging.
>>
>> This board, after a few hours stabilizing, measures 0.2uVptp, 10s
>> averaged,
>> over 6 hours.
>>
>> My 7.5digit NI PXI-4071 DMM has 0.1uV resolution on the 10V range. So I
>> need a better method... But anyhow, id say it looks promising. It seems
>> like the low frequency noise is cancelling well and does not come from the
>> power supply.
>>
>> I will now leave this board for a half-year burn-in.
>>
>> Back to the original idea; I am assuming that much of the long term
>> drift is due to the high, fixed to 90C, internal heating temperature in
>> the
>> LM399. To find out; after the half-year burn-in, I will turn off half of
>> the board (six zeners) and turn them on for something like a few hours /
>> times per week only, while leaving the other half on all the time. Over
>> another half year, I should be able to see if there is any significant
>> long
>> term difference between the two sides.
>>
>> If whole idea turns out sour, it's still be interesting to me as a
>> volt-nut
>> wannabe. The construction is simple and the price of a dozen LM399 is less
>> than one LTZ1000 with required precision resistors...
>>
>> At some point I will probably make a proper board with built-in power /
>> voltage regulator, output buffer etc.
>>
>> In the mean time, I will build a few other boards with other ICs,
>> including
>> the LTZ1000.
>>
>> I will be posting progress now and then.
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>
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