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#04
Combining AKM's current sensor with an A/D converter (1)

Episode 4. Introducing current sensing solutions by combining AKM's coreless current sensor and ADC.

Young A

Shunt resistors emit large amounts of heat and they increase mounting area when arranged in parallel.

Senior K

In that case, AKM's coreless current sensors are recommended. Primary conductor resistance is about 1 mΩ, thus reducing heat generation dramatically!

Young A

"Wow! Such a small package!"

Senior K

Common current sensors have magnetic cores, but this is, "coreless", because there is no, "core". It can be very small because it uses a surface mount type package with no core.

Expert M Manager

Besides size, there are two major advantages compared with a cored current sensor.

Young A

What are they?

Expert M Manager

First, it can be surface mounted. This will lead to lowering the cost of the substrate manufacturing process. Second is there's no magnetic hysteresis in the output error at zero current input, which is very important in motor current detection.

Young A

Magnetic hysteresis? Output error with zero current input, meaning the offset voltage of the current sensor? Do you have hysteresis? "

Expert M Manager

Magnetic hysteresis occurs in principle because the core is magnetic. Due to the magnetic hysteresis, the offset amount changes between when current flows in the positive direction and returned to zero current and when the current flows in the negative direction and then returned to the zero current.

Young A

I find that extremely difficult. It would be easy to correct if it has just offset, but with hysteresis …

Expert M Manager

Yes, this makes current detection near impossible. Therefore, a coreless current sensor with no magnetic hysteresis has a big advantage.

Young A

Wow, I'm about to use it. Which ADC for the output stage of the current sensor should be selected? "

Senior K

Since the current sensor has an internal amplifier, it is ok to choose a simple ADC rather than products with built-in preamplifiers.

Young A

What types of simple ADCs are available?

Expert M Manager

In general, ΔΣ and SAR types are popular, but recently new types are beginning to emerge. One example is the ZDS type and the AKM released ZDS-NS type. Since SAR and ZDS types are used in the same way, there are 3 types if it is roughly divided.

Figure 1. AKM ADC Selection Guide

Young A

With 3 kinds, how do I choose?

Senior K

For isolated ADC's with a shunt resistor, the ΔΣ type with a 1-bit output is mainstream. The ΔΣ type AK9223MK is the best match for this. Changing the digital circuits is unnecessary because the digital interface is the same.

Young A

That sounds simple!

Expert M Manager

However, in the ΔΣ case, a digital filter is needed in the subsequent stage, which increases filter delay. If higher speeds are required, the AK923X series, with SAR and ZDS type, would be better. In addition, if you want to add an oversampling function also, the ZDN-NS type in the AK925X series is suitable.

Young A

Well... I understand there are a lot of choices.

Senior K

Let's try to combine a current sensor with the most basic SAR type ADC first. The recommended circuit when combining a coreless current sensor, (CQ3303), and 12-bit SAR type, (AK9235NK), is below.

Figure 2. Connection Example of CQ3303 and AK9235

Young A

I see. This is also good as a reference of peripheral circuits.

Senior K

There are two types of power supply IC's, linear regulator and switching regulator types. A linear regulator should be used for current sensors and ADCs. AKM's AP1150ADS50 LDO we recommend. This is an LDO with constant output of 5V.

Young A

An operational amplifier is connected to the VREFP and the AINN pins.

Senior K

The current sensor CQ3303 has a single ended output, but the ADC AK9235NK has a differential input, so a reference voltage is necessary. So, I made 2.5V from the 5V power supply with a resistor divider on the board, and attached it to the AINN pin after an op-amp buffer. Ideally, the ADC will output just the middle code at zero current input.

Young A

Is a buffer necessary at the VREFP pin?

Senior K

Since these pins are configured as a switched capacitor circuit internally, current is drawn based on the operating clock frequency. I am using a voltage follower configuration to drive with low impedance. The AK2975H is recommended when choosing from AKM op-amp products.

Young A

Let's try it for a moment.

Senior K

A reference board with a current sensor and an ADC is also available. You can easily start evaluating.

Young A

It is appreciated!

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