Operational Amplifier

• Gain-Bandwidth Product (GBP): Indicates the frequency at which the gain of the op-amp is 1. Higher GBP values allow for higher frequency operation with adequate gain.
• Input Offset Voltage: The differential input voltage required to make the output zero. Lower values are preferable for accurate signal processing.
• Input Bias Current: The average of the currents entering the inverting and non-inverting inputs. Lower input bias currents are important for high-impedance applications.
• Input Impedance: The resistance seen by the input signal. Higher input impedance is desirable to minimize the loading effect on the preceding stage.
• Output Impedance: The resistance of the op-amp’s output. Lower output impedance ensures better drive capability and signal integrity.
• Slew Rate: The maximum rate of change of the output voltage. Higher slew rates are needed for applications involving rapid signal changes.
• Common-Mode Rejection Ratio (CMRR): Measures the op-amp’s ability to reject common-mode signals (signals present on both inputs). Higher CMRR indicates better performance in noisy environments.
• Power Supply Rejection Ratio (PSRR): Indicates how well the op-amp can reject variations in its supply voltage. Higher PSRR means less influence of supply noise on the output.
• Total Harmonic Distortion (THD): The level of unwanted harmonics introduced by the op-amp. Lower THD is important for high-fidelity signal applications.
• Output Voltage Swing: The range of output voltages the op-amp can provide. Ensure it meets the requirements of your application, especially if operating close to the power supply rails.
• Noise: The amount of electrical noise generated by the op-amp. Lower noise is critical for precision and low-signal applications.
• Package Type: The physical package (e.g., DIP, SOIC, QFN) affects how the op-amp can be mounted and integrated into a circuit.
• Power Supply Voltage Range: The range of supply voltages the op-amp can operate within. Ensure it is compatible with your power supply.
• Offset Voltage Drift: The change in input offset voltage with temperature. Lower drift is important for maintaining accuracy across varying temperatures.

Picking an OpAmp IC with as many internal OpAmps as possible - just in case - seems to make sense from an economic perspective: after all, ICs with four or more OpAmps are often not much more expensive than those with just one or two.

There are obvious disadvantages when you are using only part of the OpAmps in a chip, though: each individual OpAmp inside the IC requires a quiescent current of 100 µA or so (whether you use it or not), takes up space, and adds pins. Then again, you may not have space constraints, and you may not care about a couple hundred µA wasted power. Are you good then? Unfortunately not.

The real power loss may be a hundred times higher than the quiescent current, and you risk that your signal processing is not working as expected, or the chip gets destroyed entirely. Here is why (excerpt from an engineering discussion at Analog Devices):

Power Waste Or Damage

If the terminals of unused OpAmps are left unconnected, stray electromagnetic fields can cause an input to go outside the supply rails. This may cause latch-up and destroy the chip. If latch-up does not happen, a dc field may cause saturation, and a power waste much higher than the quiescent current results. The amplifier may also pick up and amplify an ac field and, if overdriven, heavily modulates its own supply current, causing crosstalk to other amplifier(s) on the chip. In short, an unconnected OpAmp can do unexpected things based on electromagnetic fields.

That’s why users sometimes try and connect unused terminals to known potentials in an effort to prevent stray electromagnetic influences. Here are some commonly seen approaches that you definitely should NOT do:

• Connect one input to the positive supply and the other input to the negative supply: saturates the output, wastes power and may exceed the differential input voltage rating and damage the device. Even if damage does not occur, some input stages draw several tens of milliamps under these conditions, wasting even more power.
• Grounding both inputs, or shorting them together: causes the output stage to saturate, since the offset voltage of an op-amp is never exactly zero; shorting them together and not biasing them has the same latch-up risks as mentioned before.

Recommendations

If you have unused OpAmps, then use them: employ them in a defined and safe way that draws the least energy:

• Follower: connect the output to the inverting input, and connect the non-inverting input to a potential somewhere between the supply rails. Connecting to the positive or negative supply of a single supply system will cause saturation (power waste) if the offset voltage has the wrong polarity.
• Buffer: use it in a simple buffer amplifier configuration for some other part of your circuitry (that does not need one but might even perform slightly better if it had one, at least isn’t hurt by an additional buffer).

By far the easiest solution is to pick an IC with just the number of OpAmps you need in the first place. For simple DIY projects and devices that are not critical when failing, try and at least pick an IC that is close to the number of OpAmps you need, minimizing the mentioned side effects: rather not use a Quad Op-Amp when you really just need one.

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