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

[r j]

I may by mistake have sent a mail to the group containing nothing but the
text "LM399 good". If so I am sorry - I was just making some notes (in the
wrong editor).

Roy


On Sat, Mar 29, 2014 at 6:37 PM, r j <rjinspace at gmail.com> wrote:

> 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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>
>