Yes, you need to properly terminate your op-amps stages. Improper connection or termination of unused op amp stages can lead to power and noise issues. An op amp is susceptible to noise, extra power consumption and dissipation, and electrical overstress if left improperly terminated. Malfunction of an unused amplifier channel does not only negatively impact system performance but also the performance of channels in use.
Factors to be considered in op amp termination:
1. Inputs specifications
Input specifications must be met to maintain predictable output behavior,
- Input common-mode range
Avoid unexpected current biases on diodes and transistors which can short circuit the supply rails by using two equivalent resistors in a voltage divider or any low impedance node (reference voltage within the common mode and output range) connected to the non-inverting input. This will ensure that the input falls within the common-mode range.
- Differential input voltage rating
Do not exceed the input voltage rating as it is stressful to the inputs and this overstress can damage op amps. In a configuration where the output is railed low, the op amp will consume more supply current.
- Input offset voltage
Make sure that the input offset voltage does not rail the output low to prevent the op amp from consuming more supply current.
The output stage should not be driven near supply rails otherwise it can saturate or rail out which makes the op amp consume more current and dissipate more power.
Gain should be limited to something relatively small so that noise is less likely to be amplified and the output does not saturate. Negative feedback is highly recommended to achieve amplifier stability.
Op amps have product datasheets that specify input and output ranges and serve as a guide to proper use and implementation. The input of an op amp will have a specified common mode range, and the output stage of an op amp will have a specified voltage range. These ranges must be met to ensure predictable op amp functionality.
Figure 1 shows the dual-channel 1A and 1B op amps powered by a single supply rail. Op amp 1A is in a non-inverting configuration while op amp 1B is unused in the circuit as a voltage follower where the input is set by a voltage-divider from the supply rail. Using two equivalent resistors (R3 = R4) sets the input voltage to mid-supply, ensuring it falls in common mode range. To provide negative feedback, the output is connected to the inverting input. This allows the output to follow the input and operate in its linear region.
Figure 2 shows op amps 2A and 2B powered by split supply rails. Op amp 2A is implemented into a circuit in a non-inverting configuration while op amp 2B is unused in the circuit. Connecting the non-inverting input directly to ground sets the voltage to mid-supply and connecting the output to the inverting input provides negative feedback. This is a cost-effective implementation as it is simple and does not require additional resistors.
Instead of using extra resistors with a single supply op amp, the non-inverting input can be connected to a reference voltage within the specified common mode and output ranges. With split supplies, a follower with grounded input is the simplest implementation.
- Mullins, E., Williams, I., Lo Voi, G. and Puzio, R. (2017). How to Properly Configure Unused Operational Amplifiers. TI TechNotes. Texas Instruments Inc.
- Trump, B. (2012). The Unused Op Amp—what to do?. E2E Support Forums. Texas Instruments Inc.
- Avoiding Noise and Power problems with Unused Op Amp. (2003). Maxim Integrated Products, Inc.