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Circuit designer's guide to I/O signal grounding

19 Aug 2014  | Peter Wilson

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Power rail feed: The rule also applies to the power rail feed as well as to its return, and in fact to any connection where current is being shared between several circuits. Say the high-power load on PCB3 was also being fed from the +5 V supply VA+, then the preferred method of connection is two separate feeds (figure 8).

The reasons are the same as for the 0 Vreturn: with a single feed wire, a common voltage drop appears in series with the supply voltage, injected this time in the supply rail rather than the 0 V rail. Fault symptoms are similar.

Of course, the example above is somewhat artificial in that you would normally use a rather more suitable size of wire for the current expected. High currents flowing through long wires demand a low-resistance and hence a thick conductor is required. If you are expecting a significant voltage drop then you will take the trouble to calculate it for a given wire diameter, length and current. See Table 1.3 on page 24 for a guide to the current-carrying abilities of common wires. The point of the previous examples is that voltage drops have a habit of cropping up when you are not expecting them.

Conductor impedance: Note that the previous examples, and those on the next few pages, tacitly assume for simplicity that the wire impedance is resistive only. In fact, real wire has inductance as well as resistance and this comes into effect as soon as the wire is carrying AC, increasing in significance as the frequency is raised.

A one-meter length of 16/0.2 equipment wire has a resistance of 38mΩ and a self-inductance of 1.5µH. At 4 A DC the voltage drop across it will be 152mV. An AC current with a rate of change of 4 A/µs will generate 6 V across it. Note the difference! The later discussion of wire types includes a closer look at inductance.

Figure 8: Separate power supply rail feeds.

Input signal ground
Figure 2 shows the input signal connections being taken directly to PCB1 and not grounded outside of the PCB. To expand on this, the preferred scheme for two-wire single-ended input connections is to take the ground return directly to the reference point of the input amplifier, as shown in figure 9(a).

The reference point on a single-ended input is not always easy to find: look for the point from which the input voltage must be developed in order for the amplifier gain to act on it alone. In this way, no extra signals are introduced in series with the wanted signal by means of a common impedance. In each of the examples in Figure 9 of bad input wiring, getting progressively worse from (b) to (d), the impedance X–X acts as a source of unwanted input signal due to the other currents flowing in it as well as the input current.

Connection to 0 V elsewhere on the PCB: Insufficient control over pc layout is the most usual cause of arrangement (b), especially if auto-routing layout software is used. Most CAD layout software assumes that the 0 V rail is a single node and feels itself free to make connections to it at any point along the track. To overcome this, either specify the input return point as a separate node and connect it later, or edit the final layout as required. Manual layout is capable of exactly the same mistake, although in this case it is due to lack of communication between designer and layout draughtsman.

Connection to 0 V within the unit: Arrangement (c) is quite often encountered if one pole of the input connector naturally makes contact with the metal case, such as happens with the standard BNC coaxial connector, or if for reasons of connector economy a common ground conductor is shared between multiple input, output or control signals that are distributed among different boards. With sensitive input signals, the latter is false economy; and if you have to use a BNC-type connector, you can get versions with insulating washers, or mount it on an insulating sub-panel in a hole in the metal enclosure.

Incidentally, taking a coax lead internally from an uninsulated BNC socket to the PCB, with the coax outer connected both to the BNC shell and the PCB 0V, will introduce a ground loop (see Section 1.1.4) unless it is the only path for ground currents to take. But at radio frequencies, this effect is countered by the ability of coax cable to concentrate the signal and return currents within the cable, so that the ground loop is only a problem at low frequencies.

External ground connection: Despite being the most horrific input grounding scheme imaginable, arrangement (d) is unfortunately not rare. Now, not only are noise signals internal to the unit coupled into the signal path, but also all manner of external ground noise is included. Local earth differences of up to 50 Vat mains frequency can exist at particularly bad locations such as power stations, and differences of several volts are more common.

The only conceivable reason to use this layout is if the input signal is already firmly tied to a remote ground outside the unit, and if this is the case it is far better to use a differential amplifier as in figure 9(e), which is often the only workable solution for low-level signals and is in any case only a logical development of the correct approach for single-ended signals (a). If for some reason you are unable to take a ground return connection from the input signal, you will be stuck with ground-injected noise.

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