Tell me more about ZDS-NS A/D Converters!

Episode 9. Introducing the AK925X series, new "ZDS-NS Type ADCs" developed by AKM.

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

Please tell me more about ZDS-NS type ADCs!

Expert M Manager

ZDS-NS is an abbreviation for, "Zero-latency Delta Sigma with Noise Suppression". In order to know the ZDS - NS type in detail, first you must understand the ΔΣ type ADC.

Young A

Delta-sigma is difficult, correct? I have am impression that they are somehow high performance.

Expert M Manager

Oversampling and noise shaping are the main things about delta-sigma type.

Young A

I can capture the image of oversampling. It seems that noise will improve if you sample the data many times and get an average. But what is noise shaping?

Expert M Manager

Oversampling and noise shaping are often confused, but they are totally different things. Normally, the quantization noise is uniformly distributed over the entire frequency band, but in the ΔΣ ADC, the negative feedback and the integrator are used to change the noise distribution so that the quantization noise is decreased in the low frequency band and increased in the high frequency band. This method is called noise shaping. As a result, the quantization noise can be reduced more from the digital filter. (Figure 1)

Figure 1. Disadvantages of ΔΣ A/D Converters

Young A

Well ... It is a combination technique of oversampling and noise shaping, is it that the ΔΣ ADC is improving noise characteristics and accuracy?

Expert M Manager

That's it. And noise shaping is the cause of poor response of a ΔΣ ADC. The noise characteristics improve since the quantization noise is modulated to a high frequency band which can be suppressed by a steep digital filter. Conversely, if it is a gentle digital filter, noise cannot be eliminated. There is a strong trade-off between responsiveness and noise characteristics.

Young A

So, if you use a ΔΣ ADC, the delay of the digital filter is naturally large?

Expert M Manager

Yes, also the ΔΣ ADC achieves resolution improvements by using previously converted data. This means that characteristics will change due influence from previous data.

Young A

Well, then when you use an analog multiplexer, you have to be careful.

Expert M Manager

Yes, using a ΔΣ ADC after the multiplexer is difficult unless it is a low-speed application. As summarized in Figure 1, it is common that the ΔΣ type cannot be used for applications requiring immediate responsiveness. Unfortunately, there are not many applications that can utilize the oversampling characteristic which is an advantage with conventional ΔΣ ADCs.

Young A

That's a shame.

Expert M Manager

Turning back to the principle here, there is an advantage that the oversampling can make a LPF at the input stage easier and the input capacity becomes smaller. On the other hand, noise shaping only has the purpose of improving the noise characteristics of the ADC itself. From the user's point of view, there is an advantage in oversampling but there is no need for noise shaping.

Young A

I see...

Expert M Manager

Therefore, AKM came to an idea of a ΔΣ ADC which incorporates the advantage of oversampling and improves responsiveness without relying on noise shaping. This is the basic concept of ZDS-NS type ADCs.

Young A

Wow! It sounds like a secret story of the development.

Expert M Manager

By optimizing the calculation method on the premise that does not rely on noise shaping and making the digital filter delay very small, the responsiveness has been improved to the same level as SAR ADCs. Adopting a configuration that resets internal data on every conversion completion is an important point of a ZDS-NS type ADC. Because of this, the ZDS-NS ADC does not use previous data. In other words, it can be used like a SAR ADC even for multiplexer applications. (Figure 2,3)

Figure 2. Internal Configurations of Δ∑ and ZDS-NS Types
Figure 3. Conversion Timing of ZDS-NS ADC

Young A

That's great!!

Expert M Manager

The ZDS-NS type ADC is a new ADC which has both the oversampling characteristics featured in ΔΣ types and immediate response equivalent to that of the SAR type. This product is very Asahi Kasei that advocates "Creating for Tomorrow".

Young A

Come to think of it, it also contains a variable digital filter, doesn't it?

Senior K

The variable digital filter in the AK925X series can be switched between 4 levels. Check the filter characteristics on the, "technical information", on AKM's site.