Table of Contents
This prototype module connects to the DLITE-0100A1 (DL0100A1) board to allow you to wire transducers to the board using direct tinned bare wire connection to screw terminal blocks, or even cables terminated in RJ45 plugs (two are required to connect to the 8 channels). [This article stub is in progress – please inquire directly about any items not yet covered.]
Two (2) Board Variants: Basic and Basic+RJ-45 (-RJo)
There are 2 versions of this interface board: One has the RJ45 jack footprint (“-RJo”), and the other does not. Both boards have the screw terminal block option. There are a few options for implementing these interface boards.
Voltage Considerations
The RJ45 board has smaller adjacent trace clearance parameters on its PCB because of the standard RJ45 pin-pin spacing. The idea is to allow for concept testing at the bench. The suitability of any clearances – in fact all component selection for your integration, including wiring and component voltage (and current) rating – depends on your application and is your responsibility – just a friendly reminder.
The trace clearance and general voltage consideration is worth noting because a typical application could use signal pulses of a few hundred negative volts.
Typically, the trace clearance required for such voltages is very large. However, it may be application-specific, depending on your application and certification requirements.
In this particular case, we are talking about extremely small duty cycles, that is a negative high voltage pulse for a few hundred nanoseconds, repeating every, say 16 milliseconds, maximum (under current system continuous acquisition maximum capabilities).
Typical risks relate to voltage-induced creep type damage on a circuit board and arcing. Arcing is of less concern here because that requires a much higher voltage and smaller clearance. The creep is usually a consideration for applications with constant high voltage application to the conductors, even at a duty cycle of 50% for example.
The specifics really depend on your application and the required certification and/or safety standard. It is important to understand the risks and communicate with a consultant or agency specializing in your application.
Encapsulant can be used to decrease risks associated with smaller clearances.
The non-RJo (no RJ-45 jacks) option transducer connector interface board uses larger trace clearances because that is possible without the RJ-45 jack pin spacing. RJ-45s are used in lower voltage digital signalling (or lower voltage analog applications). Even the trace clearance on the non-RJo board may not be sufficient for constant or high duty cycle, high voltage application, depending on the voltages used in your application. Mask and encapsulant may also have an impact.
The same considerations apply for your choice of wiring and connectors, etc.
Coax vs. Non-Coax
Coaxial cable with a single center conductor, as has been typically used with the standard SMA connectors on the DL0100A1, is designed to:
- Carry a single-ended signal on the center conductor, and
- Provide shielding of that sensitive centrally-conducted signal by surrounding it with a cylinder of grounded conductor that is grounded somewhere.
The coax shield is grounded very well using the edge-launch SMA connectors as designed into the DL0100A1 board, providing shielding all the way up to the analog electronics because of the way the connector terminates onto the board and ties in directly to ground planes.
Non-coaxial cable, like for example, twisted pair, say Cat6 as came up in discussion, is fundamentally different. Twisted pair cable may have a shield surrounding the bundle of all 4 twisted pairs. Twisted pair cable is more generally and commonly used for digital transmission using a differential signaling scheme, not a single ended scheme. Though of course, depending on what criteria are most important, any cable can be used in any way, with various cautions and trade-offs. In the differential signaling scheme, the same “content” or “data sequence” is sent simultaneously on each of the two wires in a single pair, but in opposite polarity. When the differential signal is combined at the receiving circuitry to re-create the raw “content” or “data sequence”, the combining process can cancel out any common mode noise this is presented on or picked up by the cable.
So, when testing signals on the two different types of cable connection schemes, you may find signal quality differences. Or you may not, depending on factor like the analog input section of the hardware, the environment, the nature of the transmitted signals, cable length, etc.
This is a brief overview of general considerations. There are many more details to consider under some circumstances. Examples of relevant considerations: voltage rating of the cable and voltages used in the signals and cable capacitance, resistance, and inductance.
How To Assemble And Integrate This Board
Ground Exposure
The XD IFace boards use a sandwich approach to mate to PCBs in a coplanar and adjacent fashion. The outer sandwich layer, the “bread”, are two machined (or stamped) copper plates.
It is important to understand that these copper plates on the top and bottom surface of the assembled unit (XD IFace to DL0100A1) are connected to the ground signal on the main board. The copper around the mounting holes of the main board is not. To prevent inadvertent shorting of anything to the exposed ground of the copper plates, you will probably want to coat the copper plates with mask or some encapsulant designed for such applications.
Masking/Encapsulant
As per the above section, you will probably want to coat the copper plates with a mask or encapsulant suitable for protecting exposed signals on a circuit board.
Mounting Holes
Two mounting holes are included in the XD IFace board. The copper around the holes is not connected to any signal. The drill size is 150 mil and the outer copper is to a 250 mil diameter. They are spaced, in the Y-dimension, at the same distance as the mounting holes on the target host board, the DL0100A1.
Mechanical Integrity
Temperature-Induced Contraction/Expansion Cycles
Because there is a heavy (relatively) copper plate mated via solder joints to the boards, there is a fairly different average temperature coefficient between the PCBs and the copper plates. In large temperature cycles, this can create some stresses at the joints. This is normal for PCBs as well, but more pronounced here due to the solid copper plates of greater thickness. The presence of similar top and bottom plates may mitigate any out-of-plane flexion or asymmetrical expansion/contraction behavior.
Mechanical Loading
The copper plates and solder joints may provide some level of mechanical support. However, using the XD IFace board solder joints to the main board as a mechanical support for cabling without strain relief could pose problems.
It is probably a good idea to add additional mechanical support, using the mounting holes on the XD IFace board and perhaps additional channel along the sides of the PCB mating joint.
How To Wire This Board (Pinout)
Silkscreen Indicators
Silkscreen text and markers indicate the signal and ground for each channel pair.
Not TIA/EIA-568A/B
The implementation of the signals on the -RJo option board is not TIA/EIA-568A/B. Rather signals are paired signal and ground, adjacent, all the way through.
Shielding and Grounding
Extra Ground-Connected Terminal Block Positions
There are extra ground positions marked at the terminal block positions if you wish to tie the cable shield to the board assembly ground plane.
It may be better to tie the shield to your Earth ground net instead.
It is helpful to consider your grounding scheme. For example, if you were to connect the cable shield’s ground at both ends (the transducer assembly end and the electronics/board assembly end), you may create a ground loop that can degrade signal quality at the least. You may wish to diagram your grounding scheme, including cable shield(s), and ground appropriately.