Re: FADC progress

[Frederick Gray <fegray@berkeley.edu>]

Dear Rene,

> This means that the FADC will accept real differential 
> signals (with 12 bits conversion) or mono-polar signals with eventual 
> overshoot  (= single ended) but with only 11 bit conversion or more in 
> case of overshoot.

I would just like to be sure that I understand the statement above: I think that you're 
pointing out that a monopolar signal will only use half the range of the ADC, since
half of the codes are reserved for positive input voltages and half for negative ones.  
If there is an overshoot of the opposite polarity, then you occasionally use some of the 
codes from that side.  Is that correct, or are you saying something else?

Unfortunately, I think we don't have a single device in this experiment that produces a 
true differential analog output.  While I agree that this design simplifies production, it 
really doesn't match either the TPC or the DEMON detectors -- both of which have frontend
electronics that produce positive single-ended signals with small negative overshoots.  
As you know, fast 12-bit ADCs are really quite expensive compared with, say, 8-bit 
versions.  We should try to use all the bits we pay for if possible.  I like the 
single-ended input stage on the current prototype, which allows a pedestal offset voltage
to be added to the signal.

> To adapt the range of the ADC, we need to know if the highest signals 
> of 0.8 volts coming from the TPC are  saturated ones and if 0.8 volts 
> will correspond to the maximum converted value. Or do we need more gain 
> to look at low signals and accept to saturate the ADCs? 

I will reconfirm with Malte tomorrow that 0.8V is really the maximum pulse height
possible from the TPC preamps -- that is the information that I received from Claude.
Let's assume for discussion that we should set the ADC scale to go from -0.15V to +0.85V. 
Then one ADC count corresponds to 0.24 mV, which is comparable to or less than the noise 
floor.  There is therefore no reason to deliberately saturate the ADCs.

Thanks,

-- Fred

-- Fred Gray / Visiting Postdoctoral Researcher                 --
-- Department of Physics / University of California, Berkeley   --
-- fegray@berkeley.edu / phone 510-642-2438 / fax 510-642-9811  --