introductory remarks - indiana university...

STAR FGT Electrical Connections and Grounding Scheme

Gerard Visser

Indiana University

3/23/2010

Introductory Remarks

This document describes the FGT but many aspects will directly carry over to the IST. An IST document is in preparation.

This needs some improved presentation here, but for now please just refer to the following two figures which are taken from FGT collaboration meeting and safety review presentations:

With reference to the above figures: The FGT detector consists of 24 quadrants arranged in 6 disks. The grounds, i.e., reference points for the preamplifier and for the decoupling capacitors of the GEM bias voltage, are tied within a disk.[footnoteRef:1] The disk grounds are electrically isolated from each other and from the support structures including the rails and the WSC. [1: The induced voltage in this loop in the event of a magnet crash has been considered. It is sufficiently low, of order 10 mV for a 0.4 m radius loop at the worst-case ramp rate of 0.5 T / 30 s. Such a low voltage will not pose any threat to the electronics and will not generate sufficient current in the thin copper foil ground plane of the FGT to warrant concern about magnetic forces on the structure.]

The 6 disks are served by 24 FEE assemblies, each FEE assembly serving two half-quadrants. There are also 24 HV divider boards each serving one quadrant.

Each FEE assembly has a power/signal cable (type 1 in the figures above) going to the patch box mounted at the TPC wheel just outside the WSC. [Or mounted as part of the WSC structure, tbd pending discussions ~now.] Each HV divider assembly has a coax HV cable going to a patch (cable-cable, most likely) just outside the WSC.

In the cable type 1 there is a dedicated ground conductor which carries no currents and is inside the aluminum foil shield with the other conductors. Additionally there is the aluminum foil shield and drain wire which carry no currents and which optionally may be placed in parallel with the dedicated ground conductor. These ground conductors (1 per FEE and thus 4 per disk running together) serve as the ground reference for the disk. They tie to the TPC structure ground immediately outside the WSC.

The cable type 1 has independent conductors for +1.8 power feed and return, and -1.8 power feed and return, and sense lines for these. None of these are shared with the ground conductor.

A detailed listing of connections on each cable type is given later in this document.

From the patch point immediately outside WSC there are 24 power/sense cables (type 3 in the figure), 24 signal cables (type 4 in the figure), and 24 HV coaxial cables (type 4) running about 55 feet to rack 2C9. The reason for splitting the power/sense cables from the signal cables is simply due to materials/construction: The signal cables require low dielectric loss which practically requires foam polyethylene or foam teflon insulation. The power/sense cable is a custom construction with mixed wire gauges to minimize diameter and mass in the constrained routing space over the TPC electronics. A custom construction with polyethylene that would meet burn requirements would be expensive and carry unpleasant schedule risk. Foam teflon is simply very expensive technology. So, we opt for a commercial foam polyethylene cable carrying the required burn rating, and a custom PVC cable for the power/sense connections.

Grounding scheme

As described in the following three sections. A diagram will be prepared but it should be fairly clear already from this description.

Grounding scheme – at detector

Simply stated in three bullet points:

· All signals and power connections are ground referenced to the ground at the detector. That is to say, they have a low AC impedance to ground at the detector. The LVDS input signals and the current-mode APV chip differential output signals have their common-mode level set at the detector by a split resistor termination scheme.

· The HV cable ground is isolated from the detector ground with a modest resistance (1 kΩ) safety resistor.[footnoteRef:2] The HV center conductor is isolated with at least 1 MΩ, the primary purpose being to protect the detector from stored energy in the cable and power supply. [2: Which has 1 V drop at the maximum output current of the HV supply.]

· The detector ground, as mentioned above, is carried in the power/signal cable on an independent conductor out to the patch point.

Grounding scheme – patch point / ground reference

Here the ground conductor terminates to the TPC structure ground (west wheel) immediately outside the WSC. Presumably the WSC structure will be grounded similarly.

The power/sense/signal cable shields all tie to ground at the patch point.

The other conductors (power, sense, signal, HV) are all isolated from ground at the patch point. This includes the HV coax outer conductor – patch connectors (SHV) will be enclosed in insulated sleeving to maintain ground isolation.

Grounding scheme – at the rack

The bullet points here:

· All signal and power connections have a high common-mode impedance to ground at the rack.

· Signal lines are received by DC coupled active line receiver tolerant of ±3 V or more of ground noise. Termination is differential only.

· LVDS signals (CLK & TRG) are transformer coupled.

· I2C signals are optically coupled

· Power is from low noise push-pull DC/DC converter with remote sense circuits powered locally from the output. The complete set of 4 lines (power and return) taken together has high common-mode impedance to ground.

· The HV cable is ground referenced here at the power supply. This is ok because we isolated it at the other side. Supplies with floating output are available but the cost is excessive and other specifications are compromised a bit.

Inner signal cable (cable type 1)

The inner signal cable is a custom copper-clad aluminum construction per the following:

(SPECIFICATION: Calmont Wire & Cable #3007-3397-02-1036-Conductor Multi-Gage Shielded FEP/Silicone Special Purpose Cable. 8ea low voltage wire 22(1) Copper-clad Aluminum, FEP to nominal wall of .005". Color: Natural. 28ea signal wire 28(1) Copper-clad Aluminum, FEP to nominal wall of .007". Color: Natural. Twist: signal wires into 14 matched impedance pairs (115-ohms ± 15%). Overall: 1st layer, 8 ea. 22(1) CCA singles + 2 ea. unshielded twisted pairs of 28(1) CCA cabled with fillers using LHL. 2nd layer, 12 ea. unshielded twisted pairs of 28(1) CCA cabled using RHL. Shield: Aluminum-Mylar foil with 22(1) Copper-clad Aluminum drain wire (in contact with foil). Jacket: Silicone, Yellow, Nominal wall of 0.020" and nominal OD of 0.299". Note: Refer to drawing 3007-3397-02-10.)

The conductor usage is per the following table:

wire 1

22 CCA

ground

wire 2

22 CCA

+1.8 V force

wire 3

22 CCA

-1.8 V force (note 2 parallel conductors)

wire 4

22 CCA


wire 5

22 CCA

return (i.e. 0 V) force

wire 6

22 CCA

+1.8 sense

wire 7

22 CCA

return (gnd) sense

wire 8

22 CCA

-1.8 sense

pair 1

28 CCA

SCK & VSS (I2C)

pair 2

28 CCA

SDA & VSS (I2C)

pair 3

28 CCA

TRG (LVDS)

pair 4

28 CCA

CLK (LVDS)

pair 5

28 CCA

APV 0 output (diff. analog ±4 mA)

pair 6

28 CCA


pair 7

28 CCA


pair 8

28 CCA


pair 9

28 CCA


pair 10

28 CCA


pair 11

28 CCA


pair 12

28 CCA


pair 13

28 CCA


pair 14

28 CCA


drain wire

22 CCA

ground (at patch point)

HV cable (cable type 2)

This is same cable type is used from the detector (HVB) to the patch point, and from the patch point to the rack.

The cable is equivalent to Draka P/N HTC-50-1-1,0.5Lz/1.5,CEH50, datasheet appended here. This is a 3.2 mm OD high voltage coax rated for 5 kV DC. The actual manufacturer and P/N vary; Doug Hasell of MIT is responsible to get actual datasheets as required for safety approval.

Conductor usage as follows:

inner

HV bias (0 to −5 kV)

outer

ground

External power/sense cable (cable type 3)

This runs from the patch point to the rack, one cable per FEE group (24 total). Custom construction per the following specification:

The conductor usage is per the following table:

wire 1

#18

+1.8 V force

wire 2

#18


wire 3

#18


wire 4

#18

return (i.e. 0 V) force

pair 1

#24

sense (+1.8 V & gnd sense)

pair 2

#24

sense (-1.8 V & gnd sense)

pair 3

#24

SCK & VSS (I2C)

pair 4

#24

SDA & VSS (I2C)

drain wire

#20


External signal cable (cable type 4)

This is Belden # 1424A. Datasheet appended here. It is a 12 pair foam polyethylene cable with OD 0.418 inch.

Conductor usage is per the following table:

wire 1

#24

no connection

pair 1

#24

TRG (LVDS)

pair 2

#24

CLK (LVDS)

pair 3

#24


pair 4

#24


pair 5

#24


pair 6

#24


pair 7

#24


pair 8

#24


pair 9

#24


pair 10

#24


pair 11

#24


pair 12

#24


drain wire

#24


Appendix (standard item datasheets)

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