[time-nuts] V refs

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Nov 30 04:26:28 UTC 2008


Richard Moore wrote:
> RE comments from Bruce --
>   
> Bruce, thanks for the clarification, and also for your earlier  
> suggestion on the chopper amp. I think I've got a couple of LT1150 or  
> 1151s in a drawer. If I remember correctly, noise should improve  
> roughly 3dB for every doubling of devices, which means that 8 399s  
> should yield a 9dB improvement, which is nothing to sneeze at, and is  
> still pretty cost-effective. Seems like using a chopper on each one  
> and then paralleling those outputs into a chopper gain stage would  
> make effective LP filtering relatively easy and would further average  
> out noise and drift contributions from the various discrete parts,  
> which means that using 1% MF resistors, for example, might be  
> practical for sub-ppm results.
>
> I'm beginning to like this, and I can see replacing the dual zeners  
> in one of my 7-1/2 digit Datron 1082s  might be a lot of fun (the one  
> with the bad VF display that's going to get 7-seg LEDs instead,  
> 'cause I HATE LCD displays that aren't backlit -- no wonder HP went  
> to VF in the 3458 -- they sure must have got tired of the complaints  
> about the 3457). That 1082 shifts its reading of my Fluke 732A's 10V  
> output about 8-10 ppm/year or a bit more, when I can get everything  
> at the same temp for repeating the measuremement...
>
> Nevertheless, if I can get LTZ1000s for $60 a pop, that's a much  
> simpler way to go mechanically and electrically. Are you sure that  
> the price you stated is for singles? I wouldn't be averse to having a  
> couple of them around.
>
> I appreciate very much the depth of both your theoretical and  
> practical experience and the insight you bring to everything you  
> comment on here.
>
> Dick Moore
>
>   

Dick

The prices are taken from the Linear Technology website but do not
include shipping.
They ship via UPS (at least thats what they used when I ordered an
LTC1407A demo board).
They can be ordered on line and charged to a credit card.
However you may need to register with them (a quick and fairly trivial
process - at least when I did it).
Note you will need to use a few high stability (wire wound or Vishay
bulk film or equivalent performance) resistors with the LTZ1000 to get
the best out of it.
One thing to watch is thermoelectric emfs at resistor junctions (solder
joints and lead to resistance element). careful thermal matching of the
thermal capacitance and resistance of connections to resistors is
required. Resistors should have equal lead lengths and the resistor body
should be parallel to the PCB.

Usually the zener flicker noise far exceeds the noise of any associated
amplifier so there's probably not too much point in using chopper
stabilised opamps just to bias each LM399.
However a chopper stabilised opamp buffering the resistively averaged
output is worthwhile.

The LTC1151 is probably more straight forward to use.
The LTC1150 has a noise peak that the LTC1151 lacks, it also has a
smoother gain rolloff.

Does the Datron use a series stack of zeners or does it use the once
popular ring of 2 reference circuit?

Bruce




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