[time-nuts] Questions about TAC frontend, and some measurements

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Dec 24 19:15:24 UTC 2012

Fabio Eboli wrote:
> Il 2012-12-23 11:36 Bruce Griffiths ha scritto:
>> The simulation indicates that the TAC capacitor charging current is
>> far from constant whilst charging.
>> This is due to the use of saturated switches rather than current
>> steering switches.
>> The capacitor charging current is poorly controlled.
>> So in this respect your circuit fails miserably.
> So if I'm understanding correctly, the non linearity is due
> to the uncontrolled current flowing from the collectors to
> the bases of saturated transistors, that changes the magnitude
> of charging current in uncontrolled way.
> I was trying to use a sungle supply, but now I'm starting to
> see the limits of my implementations.
> Fabio.
R6 = 250 ohm
R14 = 125 ohm
R10 = 250 ohm
R5 = 250 ohm
R2 = 2k ohm

The current source tempco is around 2000ppm/C
This can be reduced to about 100ppm/C (limited by hfe tempco of the 
transistors) by replacing the LEDS with emitter followers the base of 
which is driven by an opamp.


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