Electronic – Why do some voltage reference ICs (LM385B) have 8 pin packages where 6 pins are NC

Per Stevenh's recommendation, I am placing my comment as an answer and extending it.

Could be desired compatibility with a competing chip.. I used be in Semi and we have done this all the time. The idea was to offer a pin drop out and under cut them to get the biz. Wirebond is a sturdy piece of technology and last I checked people are moving to copper from gold. Most of the chips are tested at 50g drop tests, I don't remember us failing them at all.(BTW, these tests are never ever mentioned or released unless you are a serious customer due to hassle of answering questions)

In terms of signaling crossing wires is generally a no no, however don't assume there is a single pad for that pin. Depending on the marketing input, I remember we made several bond out options where we use different pads for the same functionality.

Generally-speaking, larger capacitor packages increase the current loop through the part, so the inductance (ESL) is greater. Similarly, the extra material means the resistance (ESR) is higher. When you put the ESL and capacitance together for decoupling applications, you get an LC tank circuit with a resonant frequency that decreases with increasing inductance and capacitance. The ESR in this circuit represents the minimum impedance at resonance.

When decoupling, you normally want to get below a certain impedance over the operating frequency range of the device in question. To achieve this you need multiple LC circuits covering different parts of that frequency spectrum. This is why you need a range of different capacitor sizes.

In order to achieve your desired ESR, you may also need several capacitors in parallel rather than one, as the ESR goes in parallel as well and will therefore be lower.

As a last note, also bear in mind that the escape pattern you use (position and number of vias & traces) from decoupling caps can dramatically affect the decoupling performance as well, because they add to the inductance. When you get below 0201 caps you can find that the overall inductance actually increases with smaller cap size because of this.

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