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.
