[volt-nuts] Traveling Standards

[Bob Smither]

Andreas Jahn wrote:

<snip>

> Practically I can power a LM399 with 8 NiMH cells (9,6V 2000mAH)
> for about 2 Days before I need to recharge.

Too short to mail it out and return.  I think a line powered unit makes 
the most sense (especially in light of your following data).

> I tried better thermal insulation but this has no significant influence.

Haven't tried it yet - thanks for the input.

<snip>

> On my LM399 with a 6,5 Digit instrument (10uV resolution)
> you will not see any thermal hysteresis.
> (except when comparing 2 of them in a higher resolution range).

This is what I gather from the data sheet.

> To the diagram: Y-axis is the output of my 5V  24 Bit ADC in mV. X-axis 
> is time in minutes.

Thanks for the data.  Looks encouraging.

> So I would recommend a power-up time of at least 1 hour or so.
> The instrument which is used to measure the value will need this too.

Noted. Thanks.

Here are my back of the envelope preliminary design / calculations:

Power transformer / rectifier / capacitor => ~35 Volts DC.
LM7815A => 15 Volts (regulated).

LM199A: Vout =~ 7 V.
Set Id =~ 1 mA with an Rs = 8.06 K 1% resistor from 15 V to diode.
Power on-chip heater from 15 V.
Rout =~ 1 ohm => delta Is of 7 uA causes 1ppm shift.

Require Zin of measuring VM > 10M => 0.7 uA => 0.1 ppm error.

LM7815 TC =~ 1 mV/C => 0.125 uA/C => 0.018 ppm/C.
LM7815 line regulation =~ 13 mV/V.
   For 35V +/- 10% => 46 mV => 5.7 uA => 0.813 ppm.

Rs TC =~ 100 ppm/C => dIs/dT =~ 100 ppm/C => 0.1 uA/C => 0.014 ppm/C.
Rs stability =~ .5% / 1000 hrs => 5.0 uA => 0.714 ppm.

Comments - feedback?

The two largest errors (other than 20 ppm stability of the lm199ah) are 
both under 1 ppm.  They could be reduce further but is it worth it given 
the stability of the lm199a?

Best regards,

Bob Smither

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smither.vcf
Type: text/x-vcard
Size: 140 bytes
Desc: not available
URL: <http://www.febo.com/pipermail/volt-nuts/attachments/20110826/adda7e34/attachment.vcf>