The advantages of oversampling

Episode 8. Continued from the episode 7. Introducing the advantages of oversampling ADCs.

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

What is the benefit of being able increase the analog low-pass filter (LPF) cut off frequency with an oversampling ADC?

Senior K

It is possible to decide the signal band from the digital filter side. Analog components have large manufacturing variations and temperature characteristics, correct?

Young A

Certainly, it is not surprising that an LPF made from resistive and capacitive elements can vary by several percent due to individual manufacturing variations and by several tens of percent from temperature characteristic variations. If the part is high accuracy, it will be expensive.

Senior K

The variation adds dispersion to the cutoff frequency of the LPF. From another viewpoint, it varies the phase delay. (Figure 1)

Figure 1. Comparison of Variations in Cutoff Frequency

Young A

By phase delay do you mean the delay caused from the filter?

Senior K

Generally it is OK. Strictly speaking, it is a value obtained by dividing the delay time for each frequency component by the frequency.

Young A

When the cutoff frequency varies, the phase delay amount also varies. What did you dislike?

Senior K

For example, with an encoder, the phase difference of 90 degrees between phase A and B is ideal. It will be a problem if there is a delay difference. So, it is also required to sample 2 channels simultaneously for ADCs.

Young A

It is because we obtain the angle θ by dividing each phase. Certainly, I do not want a different cutoff frequency in phase A and B!

Senior K

The absolute value of the phase delay can be greatly suppressed when the cutoff frequency is increased, so the tolerance to individual variation will be higher.

Young A

I see. ...So, no phase delay occurs in the digital filter?

Senior K

Actually it happens. But the variation is suppressed. Do you know why?

Young A

Well, the point is how does the cutoff frequency of the digital filter fluctuates, correct? Since it depends on the sampling rate of the ADC in the case of the digital filter, does it mean that it is sensitive to the deviation of the clock frequency input to the ADC?

Senior K

Yes, in general, the clock frequency is generated from a crystal oscillator, so its frequency variation is negligibly small even with including temperature characteristics. Furthermore, if the ADC integrates 2-channel, there is no possibility that the phase A and phase B are shifted.

Young A

I see. So in order to reduce the influence of device variation, you should use a digital filter.

Senior K

Also, there are cases where you want to change the cutoff frequency for each model. For example, in the case of an encoder, you want to make fine adjustments to the cutoff frequency since the upper limit of the rotation speed differs for each model.

Young A

Yes, there are. Even with slight changes, it takes time and labor to change parts on the board and verify.

Senior K

In such a case, you can use a digital filter. The analog LPF is fixed regardless of the model, and the digital filter is used to change the number of calculations, such as the number of averaging times, depending on the model. Is it easy to design a circuit that changes the number of calculations according to the logic signal?

Young A

That is easy! If you just change the input logic signal, you do not need to redesign the board and it is easy to verify. Since it can be the same board, it also leads to a large productivity improvement.

Senior K

But there aren't the only good things. The biggest disadvantage of the oversampling ADC is the output is delayed greatly due to a digital filter at the output stage. (Figure 2)

Figure 2. Data Output Delay with Each ADC

Young A

Hmm?

Senior K

The higher the order of the filter, the more noticeable the effect. Since it is necessary to increase the OSR when trying to make a steep filter, an oversampling ADC is inherently unsuitable for speeding up. Indeed, most delta-sigma type ADCs cannot help if a delay within 1 microsecond is required.

Young A

Well, then you cannot use it for fast encoder applications.

Expert M Manager

For this scenario, we have ZDS-NS type ADC by AKM!

Young A

Wow! M manager!

Expert M Manager

It has the same digital interface and immediate responsiveness as the SAR type, so its usability is the same as that of the SAR type. In addition, since it has a characteristic of delta-sigma type oversampling ADC, the Nyquist frequency can be high and the LPF cutoff frequency can be set high. Of course, the built-in digital filter will suppress high frequency noise. (Figure 3)

Figure 3. Cutoff Frequency Difference

Young A

In other words, you get advantages of both the ΔΣ ADC and the SAR ADC? This is amazing!

Expert M Manager

Moreover, it even has the function of variable high frequency noise suppression. It's a simple variable digital filter.

Young A

I certainly like to try this! Please tell me more about ZDS-NS!

Expert M Manager

Ok, we'll see in the next episode!