[time-nuts] Voltage standards
bruce.griffiths at xtra.co.nz
Sat Nov 29 20:27:16 EST 2008
Bruce Griffiths wrote:
> The LTZ1000 datasheet states that 100ppm change in this resistor
> produces a 1ppm shift in the LTZ1000 reference voltage.
> It is possible to use a 4 terminal resistor for this part to improve
> accuracy somewhat as the base current of th compensation resistor is
> around 1uA and its emitter current is 100uA.
> This isn't ideal but the emitter current is about 40x lower than the
> zener current.
On reflection this resistor operates in quasi 3 terminal mode with
about 200uA current flowing in one of the sense leads.
If this resistor is replaced by a modified Howland current source
(reference voltage derived from the temperature compensation transistor
Vbe) then the 120 ohm resistor can be employed in 4 terminal mode within
the Howland circuit. Depending on the degree of elaboration the
sensitivity to lead resistance of the 120 ohm resistor can be reduced by
a factor of 10-100 or more.
Other possibilities include sinking a 4mA current from the zener
transistor base node using a current source (with diode clamp to protect
the transistor base emitter junction against reverse breakdown) whose
value is determined by the 7V reference output of the LTZ1000. However
this requires a precision current mirror and a startup circuit.
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