[volt-nuts] (no subject)

[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.