A2050D Problems

Description: The Quad Laser Head (A2050) is a LWDAQ device that drives four laser diodes. Designed by Brandeis University for general use with BCAMs, the Weizmann Institute adapted the A2050 schematic to create their own version, known as the A2050D, for use with the ATLAS end-cap trigger chambers. The A2050D features various components including laser diodes, an alignment corridor aperture, a 9-pin D-type socket for laser connections, and the electronics mounted on a metal base. Issues arose during testing at CERN, where the A2050D exhibited inconsistent functionality. Upon inspection, several design problems were identified, including improper grounding methods that could introduce ground loops, vulnerabilities of the laser diodes to static electricity, and inadequate decoupling in the power supply, which could lead to unreliable operation.

The Quad Laser Head (A2050D) serves as a sophisticated device designed to operate four laser diodes within data acquisition systems. Originally developed for compatibility with BCAMs, the A2050D has been tailored for integration with the ATLAS end-cap trigger chambers, showcasing its versatility in high-energy physics applications. The device is characterized by its modular design, which includes components such as a 9-pin D-type socket for laser connections, a laser mounting board, and a series of inductors and capacitors that are integral to its operation.

During initial testing at CERN, it was noted that the A2050D exhibited intermittent functionality, prompting a thorough evaluation of its design. The inspection revealed several critical issues, particularly concerning the grounding configuration. The A2050D connects the RJ-45 cable shield to the local 0-V power through an inductor (L1), which deviates from the established LWDAQ specifications. This connection could potentially create a ground loop, introducing significant noise into the data acquisition system and resulting in operational instability.

Additionally, the separation of the laser diodes from their driver circuits via a connector poses a risk of static discharge, which is detrimental to the sensitive laser components. The design also includes a low-pass filter (L3) that could inadvertently increase the voltage supplied to the laser diodes during power-up, risking overheating and damage. The introduction of an additional resistor in series with the power supply complicates the current delivery to the lasers, particularly over longer cable lengths, which may lead to insufficient power for reliable operation.

The logic power supply of the A2050D is another area of concern, as it employs a simplistic decoupling method that lacks the necessary robustness for stable operation. The absence of a ground plane or power grid exacerbates this issue, leading to voltage drops during rapid switching events in the logic circuits. This instability can result in erratic behavior of the device, making it unreliable in critical applications.

To mitigate these issues, it is recommended to remove the inductors L1 and L2 to eliminate the ground loops, secure the laser connections to prevent static damage, and enhance the decoupling of the logic power supply by adding additional capacitors. These modifications would restore the A2050D's functionality and ensure its compatibility with the LWDAQ system, allowing for reliable operation in high-energy physics experiments.The Quad Laser Head ( A2050 ) is a LWDAQ device that drives four laser diodes. The A2050 was designed by Brandeis University for general use with BCAMs. The Weizmann Institute took the A2050 schematic and created their own version of the A2050 for use with the ATLAS end-cap trigger chambers. We call their version the A2050D, or Quad Laser Head Ve rison D. Figure: The Quad Laser Head (A2050D) from the Weizmann Institute. Marked are (1) one of four laser diodes, (2) aperture for alignment corridor in end-cap muon system, (3) 9-pin D-type socket for laser connections, (4) the Quad Laser Head electronics on a metal base with the lid removed, (5) the LWDAQ device socket, (6) 9-pin D-type plug for laser connections, (7) the laser mounting board, and (8) J2 shield grounding wire. We learned of the existence of the A2050D when we received this e-mail from our employee Ben Kaplan at CERN.

Meir Shoa of the Weizmann Institute sent a set of A2050Ds (more than two) to Ben Kaplan in CERN`s Building 180, asking that Ben test the boards. We knew the Weizmann Institute was planning to produce a version of the A2050. We designed the A2050B for them in 2002, and sent them the schematic and a parts kit (see here ). Our understanding was that the Weizmann Institute would lay out a printed circuit board for the A2050D using our schematic, and so make an LWDAQ board compatible with our ATLAS MDT end-cap alignment system.

We answered Ben Kaplan`s questions about using the A2050D with the BCAM Instrument, and he was able to flash all four lasers on his test bench. Later, however, he found that the A2050Ds would work one day, and fail the next. He gave some to Jim Bensinger to bring back to Brandeis University in Boston. We received two A2050Ds from Jim on 14th June 2006. Until we received these two assemblies, we had never seen an A2050D, nor even a picture of one. We were unaware of the changes made to the circuit by Weizmann Institute, and unaware that the lasers had been separated from their driver circuits by a cable and connector.

We inspected the A2050Ds and found several problems with the design. Three problems are a result of changes made by the Weizmann Institute to the A2050B circuit, and a fourth is the result of using a two-layer printed circuit board instead of a four-layer board with a ground plane and power grid. We describe each problem below. You will find the first page of the A2050 schematic here. You will see that the RJ-45 connector shield is connected to the local circuit`s 0-V power line by a 10-nF capacitor.

The LWDAQ (Long-Wire Data Acquisition) specification lays down the following rules about cable shields. Rule: A device, multiplexer, or repeater enclosure may connect to the local circuit`s zero-volt potential through a resistance ‰¥ 1 k © and a capacitor ‰¤ 1 F.

The A2050D connects the J1 shield to the local 0-V power with an inductor marked L1 on the printed circuit board. This inductor is not on our schematic. We measured its impedance to be less than 1 ©. Figure: Close-Up of A2050D Circuit. Marked are (1) L1 connecting J1 shield to 0-V, (2) L3 in the power supply line for J2, (3) L2 connecting shield of J2 to ground, (4) additional power dissipation resistor.

The A2050D connects its RJ-45 cable shield to its 0-V power. The shield is likely, although not certain, to make contact with the A2050D chassis. In the assembly photographed above, the two were in contact. Because the chassis is connected to the local 0-V power, the A2050D introduces a ground loop into our LWDAQ system. The loop begins at the LWDAQ driver, passes along the 0-V power wire in the cable to the device, through L1, through the body of the connector shield, and so to the chassis, then to the chassis mounting structure, and so into the metal frame that supports the trigger chambers, through the frame to another A2050D, into its chassis, through its connector shield, through its own L1, onto its 0-V supply, and back all the way to the driver again along its 0-V power wire.

This loop could have a cross-sectional area of tens of square meters, and end up carrying a hundred milliamps of 50-Hz alternating current, leading to several volts of 50-Hz mains hum throughout the data acquisition system. Not only does L1 destroy the low-frequency star-grounding architecture of the LWDAQ, it also combines with C1 to compromise the high-frequency local-grounding architecture.

We don`t know the value of L1, but at some frequency it will combine with C1 to make a resonant tank circuit with high impedance, allowing high-frequency noise to induce voltage in the shield. Although less serious than the introduction of ground loops, the resonance of L1 and C1 will make the A2050D more sensitive to noise of a particular frequency.

If this sensitivity causes the circuit to malfunction, we will have a hard time isolating the noise as the cause of our problems. In addition to L1, there is also L2, which connects the J2 shield to the local 0-V supply. This shield is connected to a local chassis, we assume, by means of the J2 grounding wire (see photo ).

These parts guarantee that the A2050D will create a ground loop, because they connect the shield of both connectors to the local 0-V supply and to a local structure ground. Laser diodes are vulnerable to static electricity. The A2050D separates the laser diodes from the circuit board, and inserts a connector between the circuit board and the lasers.

The lasers are unprotected against static electricity, and therefore vulnerable. On a dry day, it will be easy to destroy a laser by touching one of its bare leads, or one of the pins on the 9-pin D-type plug. Laser diodes are vulnerable to over-heating when driven with more than their recommended operating currents.

In addition to the 30-cm cable and 10-cm PCB tracks that lie between the lasers and their driver circuits, there is also a new component, marked L3, in series with the +6-V connection to the laser mounting board. So far as we can tell, L3 is a low-pass filter made out of two inductors and a capacitor to ground. When the lasers turn on, this inductor will tend to push up the power supply on the laser mounting board beyond +6 V on turn-on, and so destabilize the driver control loop, driving the laser current above its designed value.

In the long run, this will damage lasers that are already running at close to their 5-mW maximum power. The A2050B uses two 47- © resistors in series with its 6-V regulator to dissipate some of the power lost when we step down from +15 V to +6 V.

These are R6 and R7 in the schematic. The A2050D introduces a third 47- © resistor (see item 4 in photo ). These resistors lie in series with the resistance of the +15-V and 0-V wires in the cable connecting the A2050D to its LWDAQ driver. If this cable is 100 m long, the resistance of each wires is roughly 10 ©. When we add all these resistances together, we arrive at a total of roughly 160 © in series with the +15-V supply on its way to the +6-V regulator.

The regulator itself uses a darlington transistor, and drops at least 1 V. The maximum voltage available for the resistors and the wires is 15 ’ 6 ’ 1 = 8 V. The maximum current the A2050D can supply to the lasers at +6 V is 50 mA. A single laser and its driver can consume up to 70 mA, which is the maximum current the A2050B, with only two 47- © resistors, can supply to a laser at the end of a 100-m cable. Total cable length from driver to device in ATLAS has grown to almost 140 m. If the turn-on threshold current of a laser and its driver is 60 mA, it is possible that the A2050D will be able to turn it on at the end of a short cable, but not at the end of a long cable.

The A2050 3. 3-V logic power supply consists of a simple radiation-resistant regulator and a single 22-nF decoupling capacitor. Look for U7 and C3 in the schematic. The logic circuits have propagation delays of around 2 ns, and their outputs switch in less than 1 ns.

We have learned from experience we must distribute 0-V to the logic chips with a solid copper plain, and 3. 3-V with a lattice of thick tracks, or else our single decoupling capacitor is inadequate to keep the logic power supply stable.

The inductance of a 40-mm 20-mil track is of order 10 nH. With 1 V across this 10 nH, the current will increase at 100 mA/ns. The logic chips are switching up to 30-pF loads in 1 ns, so they draw 100 mA for 1 ns when they switch. With a 40-mm 20-mil power supply track, the power supply voltage at a chip will drop by roughly 1 V during the 1-ns transition.

The chip`s input thresholds will also drop in proportion to the power supply drop. We observed such dips in the power supplies on our earliest LWDAQ devices. The result was a circuit that worked sometimes, but not at other times. The A2050D is a two-layer printed circuit board with no ground plane, and simple, isolated-track distribution of 3. 3-V and 0-V power. Although the circuit may work some of the time, spikes on the power supply are likely to stop it from working at other times.

The A2050D breaks the LWDAQ cable shield grounding rules, and almost guarantees the introduction of large ground loops into the LWDAQ. Such loops are likely to stop the data acquisition system from working, and may damage it also, through excessive mains-frequency current and voltage in the 0-V lines.

We recommend that no A2050D be connected to the ATLAS LWDAQ until both L1 and L2 have been removed. The A2050D isolates its lasers from their drivers, making them vulnerable to static electricity. We recommend that the laser cables be screwed into the main A2050D circuit boards, and the screws be locked with glue, so that they cannot be removed easily. Locking the connector in place will protect the lasers on the laser mounting board. The A2050D logic power supply is inadequately decoupled because it uses single tracks instead of a power plane or power grid.

In our experience, inadequate decoupling of exactly this form makes the circuit unreliable. We recommend that 10-nF radial ceramic capacitors be soldered across the power supply pins of some or all of the logic chips, and that Weizmann Institute examine the power supplies during operation to make sure that they are stable. It is possible, although unlikely, that A2050D lasers will work when the A2050D is on the end of a short cable, but not when it is on the end of a long cable.

This is because of an extra voltage-drop resistor added to the circuit by the Weizmann Institute. We recommend that this resistor be removed, and replaced with a 0- © jumper, to restore the full current capacity of the laser drivers. Lorne Levinson of the Weizemann Institute responded to our comments with a Word document. We are content with his proposed corrections to the A2050D.