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

[Attila Kinali]

On Sun, 23 Dec 2012 22:45:40 +0100
Fabio Eboli <FabioEb at quipo.it> wrote:

> Il 2012-12-23 07:42 Bruce Griffiths ha scritto:
> 
> > The classic TAC using current mode switching is similar to the
> > attached circuit schematic.
> 
> http://pastebin.com/EkgqmgfE

I have a couple of small questions about this circuit.

Why are Q2 and Q13 driven by Vth? As there are inverted versions
of the Discharge and Ramp signals available, wouldn't it make sense
to use those to drive Q2 and Q13? I guess it would enhance switching
speed.


What is the reason behind the emitter followers Q1 and Q9?
Respecitvely, why shouldnt R3/R4, R7/R8 be connected directly to V+/V-?


Is there a special reson why the current source around Q9 is set to 20mA
and the one around Q4 to 10mA? Is it because Q14/Q15 are driven by a 20mA
current source while Q18/Q19 by a 10mA source?


Am i correct, that the only current source whos value really matters is
the one around Q11? If so, wouldn't it be beneficial to use a stable
reference voltage (probably coupled with the ADC reference) to be used
in an opamp based current source against GND or V- and a current mirror
(cascode or wilson) to drive Q18/Q19 (while leaving the other LED based
current sources as they are, including Q16)?


			Attila Kinali

-- 
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