In a 4-circuit, 6-circuit, or other multi-circuit operational amplifier, what should I be designed with the free pins of unused circuits?

Operational Amplifier

If there are unused circuits in a multi-circuit operational amplifier, they must be designed properly so that they do not affect the other circuits in use.
Ideally, the unused circuits should be left in their normal operating state. The easiest way to do this is to constitute a voltage follower. In this case, the input terminal voltage must be apply at a level that allows normal operation for the Op Amp, usually at the midpoint of V⁺/V^-(V⁺/GND for a single supply Op Amp) (Fig1. Fig2.)

Note: Some op amps cannot be constituted as a voltage follower circuit. In this case, there is a description on the datasheet such as "Use with a gain of 6dB or more is recommended".

• 
  Fig1.a. single supply Op Amp
• 
  Fig1.b. On case of unavailable to constitute the voltage follower circuit.

• *As an example, the VICM of NJM12904 has an upper limit of about V+ -1.5V, so if +INPUT is set to the midpoint (V+/2) relative to V+ when using a low supply voltage (≤3V), the VICM will be exceeded. In such a case, apply the common mode voltage applied to +INPUT terminal even lower.

• 
  Fig2. dual supply Op Amp

• Note: See a section entitled, "a way to treat the unused Op amp" on using an Op amp. The comparator can not constitutes the voltage follower circuit.