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 <[email protected]> wrote: > LM399 good > > > On Sat, Mar 29, 2014 at 5:18 PM, Andreas Jahn > <[email protected]>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. >>> _______________________________________________ >>> volt-nuts mailing list -- [email protected] >>> To unsubscribe, go to https://www.febo.com/cgi-bin/ >>> mailman/listinfo/volt-nuts >>> and follow the instructions there. >>> >> >> _______________________________________________ >> volt-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/volt-nuts >> and follow the instructions there. >> > > _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
