Re: FDC/CDC ASIC meeting minutes

[Gerard Visser <gvisser@indiana.edu>]

Hi Yves,
	--->
	Thanks,
		Gerard

Yves Van Haarlem wrote:
> Hi Gerard,
> 
> Thanks for your feedback, here are my answers:
> 
> On Fri, 5 Sep 2008, Gerard Visser wrote:
> 
>> Hi guys,
...
>> It seems to me rather
>> subjective, and perhaps there is no reason to expect this procedure to
>> asymptote to a constant gain as you lower the bias voltage.
> Those spectra correspond pretty well with the fADC spectra taken
> with the Fe-55 source. The only thing is that I fit a gaussian by eye.
> So fitting a Gaussian the right way (which is as far as I know not
> possible on the scope) can change the gain values a bit but not the trend
> as a funtion of HV.

Er, well, I beg to differ. The goodness of fit using a Gaussian is 
plainly a function of HV, just compare for instance 8(a) and 8(b). I 
don't think you can rightly claim that this will not skew the trend as a 
function of HV, especially at the low end which you are worried about.

> 
> 
> 
>> I agree with your
>> conclusion that the plainly evident change of preamp pulse shape at higher
>> bias voltages affects the effective gain. Is it not possible that the shape
>> continues to change at lower voltages? (You could capture a set of waveforms,
>> normalize _each_ individually to same peak amplitude, and then average them,
>> and compare the results of this procedure at different bias voltages. This
>> could provide some quantitative indication of how much the shape is changing
>> from saturation of the preamp or whatever other effects.)
> Yes it could be (I was thinking that at low HV the signal to noise comes
> into play and change the signal shape). This behavior needs to be
> understood. Lets see if I understand your proposal: you want a set of
> waveforms (let's say 10) for each HV setting, normalize each waveform to
> one and than take the average of the 10 waveforms. Shape-compare those
> waveforms for different HV. Those traces need to be taken with the scope,
> saved and than analysed offline. 

Yes, this is exactly what I am suggesting. I don't know if it will 
really help illuminate the issue, but (possibly w/ more than 10 wfms 
each) it should at least see if the "shape change" carries on at lower 
HV or is only really happening at the top end where saturation is plain 
to see with the naked eye.

I will look into this. I will also try to
> measure the gain of signals (cosmics) wich are in the linear range of the
> preAMP (<500 mV amplitude) vs HV.
...

>> p.s. Are the histograms (I presume in a narrow time window) centered on the
>> peak time of the pulse in each of the 6 cases independently, or at fixed
>> time, not therefore picking up the peak amplitude? I presume/hope you have
>> done the former.
> I do not remember the time window that was set (it was not very narrow)
> but the scope displays the measurment every time it measures an amplitude
> and that looked correct.

Your answer confuses me... Are these histograms in all cases an accurate 
representation of the peak amplitude? I was assuming these are just 
histograms of the waveform crossing a box, and that the box was set to 
be at the observed typical time of the peak... Maybe I just have that 
wrong and the scope is here histogramming the measured peak amplitude 
where that measurement happens independently on each acquisition. Please 
clarify this.

> 
...