What is the sampling operation of the A/D converter? (2)

Episode 15. Continuing from the episode 14, introducing details of sampling operation of SAR type and ZDS ADC.

This is a story that young A, who works in a fictional motor company deepens the knowledge of ADCs with a senior colleague K, and his boss, M, manager.

Young A

Well ... regarding kickback, I cannot keep up with it at all.

Senior K

Kickback means that the amount of charge from the previous input voltage stored in the input capacitance of the ADC that flows back to the outside of the ADC at the start of sampling.

Young A

Ok ...

Expert M Manager

To qualitatively understand the kickback and the acquisition time described earlier, you should imagine water.

Young A

Water?

Senior K

The relationship between the input capacitance, the input signal voltage and the charge amount is easy to understood if we consider the same image as water in a cup. There is a cup called input capacitance at the input part of the ADC, and think that you want to fill water to a desired level in the cup according to the analog input signal voltage.

Young A

Certainly in the class of electric circuits, we expressed the voltage as the water level and the current as the water flow.

Senior K

The difficulty driving the analog input of the ADC is that it is necessary to accurately fill water in this cup to the desired water level in a short time. It will be an error component by the amount of deviation, so either if it is not enough or too much, it will not work.

Young A

Is the short time related to the length of the acquisition time?

Senior K

Yes. So, if the cup is large, the amount of water to accumulate to the desired water level will increase. That means large input capacity.

Young A

The amount of water supplied by the input stage amplifier? It seems hard to do that in a short time.

Senior K

Operation image of the preamplifier is opening and closing the faucet so that the amount of water you want to fill and the amount of water currently filled in ADC are compared to make the difference disappear.

Young A

It seems that considerable speed is required for opening and closing it.

Senior K

There is an item called, "Gain Bandwidth", in the datasheet of the amplifier. The faster its characteristics, the more image you can make the tap opening and closing faster and stronger.

Young A

That means that the longer the acquisition time, the less load to the input stage amplifier. The smaller the input capacity, the less load to the input stage amplifier, I suppose.

Senior K

So, for example, when kickback is considered, the amount of water that was filled last time will flow back at the time sampling starts. So, filling water until the desired amount that you want to fill next will cause an error in the amount of water flowing backwards. Therefore, you will need a solid water faucet.

Young A

I understand.

Senior K

The analog input driving part of the ADC is hard to understand with logic only. You also need to use your hands and struggle with the circuits.

Young A

It sounds hard.

Senior K

So, returning to the ZDS and SAR types, the SAR type often has "large cups", "large reverse flow", and, "short filling time".

Young A

"Time filling water is short", is the acquisition time, right? I was taught last time why it can be longer with a ZDS type, (... well I did not understand it).

Expert M Manager

Let me explain why input capacitors can be smaller with a ZDS type than SAR type. To realize a high-resolution SAR ADC, it is necessary to reduce manufacturing variations of built-in capacitive elements. Many products increase the input capacitance in order to reduce variations. On the other hand, since the basic configuration of the ZDS ADC is delta-sigma, high resolution can be achieved from oversampling the signal inside. The resolution improvement of the ADC and production variation of the input capacitance are not directly connected. So, it is possible to achieve high accuracy while reducing the input capacitance.

Young A

Okay, I understand.

Young A

I feel I could grasp the outline of AKM 's ADC! Thank you very much!

Senior K

Finally, I summarized them in a table. All products have built-in 2 channel independent ADC.

Expert M Manager

Next, you should try to build products using an ADC by yourself. You will find many difficulties.

Young A

I need further growth. I will improve my knowledge at the seminar!

Senior K

When you are grown-up, you can present, "Foundation for Understanding of ADC - Intermediate".

Young A

Wow, at the end, what a burden you casually proposed ...