False asymmetries/Ground Loops

David Bowman4/27/12

Why do we need a filter?

• The signal consists of a large number of pulses with widths of ~ 2 micro second

• We sample at ~ 20 micro sec intervals (50 KHz rate)

• It is necessary to form a time average of the signal to avoid loss of information because if the pulse width is less than the sampling interval, some pulsed don’t contribute

Time response of different filters

Time

The desirable feature of the Bessel filter is that the timeresponse does not have oscillations.

Signal flow

• Detector, preamp• TRIUMF adjustable gain module• Sum and difference and filter Amplifier

– Ring sums and detector differences • VME 3

– ADC of Ring Sums, spin state and monitors• VME2

– ADC of detector differences• Different components are located in different crates and racks.

– Communication by ground loops– Communication within Sum, Difference, and Filter box via ground

loops and stray capicatance

Ground loops

If the loop is defined by conductors, E appears across the largest resistance

Reconfigure analog signal processing chain

• Eliminate TRIUMF gain box, Sum and difference box, Ring sums

• Build new Bessel filter box with (gain of 3 -> gain of 6) connected to detector signals by twinex (shielded twisted pair) and connected to ADC’s in VME2 by twinex.

Least bit ADC noise –staircase problem

G is bin width, s is RMS noise on analog signal.If s is ~ .5 G, average ADC signal ~ input signalIt is possible to achieve 6 V detector signal, ands ~ .5 .G