[Halld-tracking-hw] CDC measurements

[Naomi Jarvis]

Hi,

The amplitude plots end at 2200ch because the preamp output does not go 
much above (approx) 0.5V and the fADCs were set up with a range of 1.1V 
and a small negative offset.

Just to make sure of this, I put a square wave into them (example 
attached) and calculated channel 0 is -220mV and 4096 is 870mV for ADC 
channel 1.
The shaper output was set to put the baseline at around -130mV, channel 
300 for channel 15 (our most convenient straw for Fe55 access).
There are example scope traces on the wiki under Preamp4 'scope 
traces.   We have not been using the interposer board for any of these 
measurements.

To match the final fADCs range, we reset our fADCs today so that the 
gain is halved, and remeasured with the square wave again.  We now have 
channel 0 at approx 50mV and channel 4096 at approx 600mV (example 
attached - the voltages were measured on the 'scope). 
The shaper output is now set to put the baseline at 80mV.

We are collecting cosmic data with this new range over the weekend.

Best regards,

Naomi.



Fernando J. Barbosa wrote:
> Hi Naomi,
>
> I am still puzzled by the fact that your amplitude plots show a limit 
> at ~2200 channels. Could this be because of a combination of the 
> shaper offset and the dynamic range of the ADC you have?
>
> If I understand your setup, the ADC has a negative polarity range 
> (-1V?) and the output from the shaper has positive-going pulses into 
> the ADC. You set the shaper offset negative so that the baseline is at 
> the highest scale of the ADC and a large pulse would underflow. You 
> then perform the subtraction off line. Is this correct?
>
> Thanks and best regards,
> Fernando

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