How do I design a cascade of op-amps in terms of high impedance?

Question

An operational amplifier has a high-impedance input. That means that its current consumption would be really small, so, we can say it can measure voltage without affecting the circuit in terms of consumption. I have got a design like this:

15 V - LDO - 5 V - VOLTAGE DIVIDER (for Vref) - OPAMP (BUFFER)

And then, this one:

OPAMP (COMPARATOR) (previous 5 V to power it, and Vref to compare) - OUTPUT OF THE SYSTEM

So, I have a input that I am going to compare to Vref and set a logic 1 when the threshold is reached. This design is not mine.

The comparator already has a high-impedance input (it is an op-amp), so why would I need another op-amp as a buffer before it? Notice that I have got:

Reference value --> Buffer --> Comparators (more than one)

For me, it would be the same if I delete the buffer. What do I have to do to check this? Check the input bias current of the comparator? Multiply the input bias current by teçhe number of comparators reading Vref and check if it is lower than my LDO output current? I am a bit lost in terms of high-impedance and the need for buffers.

The input bias current per comparator is 80 pA. I have got 14, so the result would be a total of 1.12 nA. The output current of the LDO is 100 mA. The divider is formed by 300Kohm and 200Kohm

Answer 1

If you omit the buffer, the input bias current of the comparators will flow through the voltage divider (R1, R2) and cause an additional voltage drop.

It is up to you to determine if this additional voltage error is tolerable or not. It can be mitigated by reducing the divider impedance, but that will draw more current from the LDO.

For most comparators, and e.g. 10 kΩ divider impedance, this is absolutely fine for everything other than the highest precision tasks.

On a sidenote, powering the buffer from 15 V and feeding its output without protection into a 5 V IC is dangerous. If the buffer outputs a high voltage (because of some transient or malfunction), it can fry the 5 V component.

Answer 2

For sure, you need to check if the combined input current of the comparators can change the voltage of the voltage divider. This is an obvious thing to do.

There could be another reason to use a buffer. Comparators can give a kick-back to the input. Whenever the output toggles, it can cause a glitch on the input.

If you are going to remove the buffer, check if the comparators toggling can affect the LDO or cause false triggerring of the other comparators due to such input glitches.