Sometimes when we design electronic devices we are left with extra components, or parts thereof. This most often occurs if there are multiple components in a single package and we don’t need to use them all.
Let’s start with logic gates. Obviously (I hope) unused extra inputs of used logic should be tied to logic high or low depending on the function (Figure 1). For example, if we need want to OR together three signals using a 4-input OR gate, the extra input should be tied low. Similarly, if we are ANDing three signals using a four-input AND gate the extra input has to be tied high for the logic to work. This article is really focussed on completely unused elements, for example if we use three gates out of a four-gate package, how do we deal with the fourth unused one?
Unused inputs of logic gates, such as the fourth input of the gates shown here, should be connected high or low to maintain the proper function of the gate.
You should leave the outputs of unused logic elements open, but you generally should tie unused inputs somewhere (Figure 2). The exception is inputs with built-in pullups or pulldowns which can be left open. Almost all modern logic is CMOS, which means they have high-impedance inputs, which if left unconnected, can float to an indeterminate voltage or pick up noise. This can cause the logic element to change state which consumes power and can cause unwanted glitches. Worse, these open inputs are more susceptible to electrostatic discharge (ESD) which can damage the component or reduce its working life.
Unused gates should have their outputs left open and their inputs tied high or low as shown. Using a pull-up or pull down can be a good idea in prototypes in case the gate needs to be used.
Should inputs be tied high or low and should they be tied directly to the power rails or via pullup/pulldown resistors? The answer is that it depends. For CMOS input gates it usually does not matter if the input is pulled high or low, although there are exceptions where one option may have slightly lower power consumption than the other. Always consult the data sheet if you are unsure. I generally tie unused inputs low if I can.
I think the decision on whether to use pullups/pulldowns or to tie unused inputs directly to the rails is one of cost vs flexibility. If you are building a prototype and think you may want to make use of the “spare” gates, using a resistor makes this much easier especially if you use a separate pullup/pulldown for each input. If your design is fixed and you want the lowest cost or smallest footprint then you can connect unused logic inputs directly to the rails.
Unused op amps are a different story. We can’t leave their inputs open since its possible for stray signals to drive them outside their common-mode range which can cause latch-up (although, this is less of a risk with modern op amps). We don’t want to tie them together since the output will saturate at one or other of the supply rails due to the input offset voltage. This can lead to increased power consumption.
This leaves us with the solution of configuring the op amp as a unity gain buffer and connecting the non-inverting input to a fixed voltage somewhere between the power rails (Figure 3). If your design uses split positive and negative rails, connecting this input to the mid-point (zero volts) is perfect. If not, you can try to find a suitable point in your circuit to connect it to. Failing this you might have to use a dedicated resistive divider.
Unused op amps should be connected as unity gain buffers with the non-inverting input connected to a voltage level inside the op amp’s common mode voltage range.
If you want to maximise the flexibility of the circuit for a prototype you might want to close the loop with a resistor and use a resistor to connect the non-inverting input to ground. As above, this will make it easier for you to use the spare op amp if that becomes necessary.
Unused analog switches or multiplexor inputs are another interesting case. The control input is usually a logic input like any other and so can be connected to one of the rails (optionally via a resistor). The analog terminals are both inputs and should therefore be treated as inputs and connected to a fixed voltage within their common-mode range. Again, zero volts is ideal in circuits with split supplies.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
