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#13
Which A/D converter is suitable for a photodiode output?

Episode 13. Introducing how to use the AK924X series with an optical encoder as an example.

Young A

Photodiodes are often used in optical encoders, but I have never designed a subsequence stage circuit. I have designed slits and photodiodes.

Senior K

The subsequence-stage circuit of the photodiode is normally using either (1) a transimpedance amplifier (TIA) or (2) an external resistor for I/V conversion.
(Figure 1)

Figure 1. Comparison of I/V Conversion Configuration

Young A

Is I/V conversion current-voltage conversion?

Senior K

That's right. This is necessary when the sensor uses current output. A representative example of a current output sensor is a photodiode.

Senior K

Although it is easy to understand how to buffer with high impedance after I/V conversion with the resistor in (2), it is disadvantageous that the reverse voltage of the photodiode fluctuates and the frequency response cannot be made high.

Young A

For current output sensors, how do you calculate low pass filters?

Senior K

It can be calculated from the resistance and capacitance connected to the output terminal. This capacitance is often dominated by the junction capacitance of the photodiode. For example, if the junction capacitance is 30 pF and the I/V conversion resistance is 130 kΩ, the cutoff frequency is 40 kHz. (Figure 2)

Figure 2. Equivalent Circuit of a Current Output Sensor

Young A

40 kHz is low. Assuming that the number of slits of the optical encoder is 500 and the output frequency is 50 kHz when the motor rotation speed is 6000 rpm, the output waveform will decay at high rotation speed. Capacitance and resistance should be smaller.

Senior K

The capacitance is almost determined by the photodiode, so it is difficult to make it smaller. But, decreasing the resistance decreases the output voltage, right? When the signal is small the output voltage is small. As a result, the noise characteristics deteriorate. When you want to improve responsiveness, it is recommended to use TIA in (1).

Young A

Why can I increase the resistance with TIA?

Senior K

In the TIA configuration, the amplifier keeps the photodiode output at a constant voltage. Determine the transfer function of the circuit yourself for details.

Young A

Well. Certainly, TIA is also used in our products.

Senior K

This is a point to care for TIA configuration, it could oscillate depending on the circuit design since TIA is sensitive to the input load capacitance.

Young A

Oscillation!! An unpleasant word...

Senior K

It is necessary to properly design the phase compensation of the operational amplifier. You also have to read the datasheet of the operational amplifier to determine whether it can be used or not.

Young A

Well. I do not feel confident.

Senior K

Besides, the TIA input line is quite delicate. If it is incorrectly routed on the PCB board, electromagnetic noise is easily picked up and the TIA characteristics degrade easily.

Young A

Seems for advanced users. Maybe still impossible for me.

Senior K

Well, there is a good news for you! TIA is integrated in the AK924X series which is AKM's ADC with pre-amplifier!

Young A

But, the AK924X supports instrumentation amplifier input, doesn't it?

Senior K

Instrumentation amplifier input mode is used when the AK924X is placed after a voltage output type sensor like Hall element or SMR element, but actually you can select TIA input mode for the output stage of a current output type sensor. (Figure 3)

Figure 3. AK924X Series Block Diagram

Young A

I can choose both the instrumentation amplifier and the TIA ..., how beautiful!

Senior K

Both micro-voltage and micro-current are compatible. The best ADC with a built-in preamplifier for the output stage of various sensors is the AK924X series!

Young A

Very catchy advertising.

Senior K

Because it is housed in a 4 mm x 3 mm QFN package, it can be placed very close to the photodiode. Besides, it supports both anode and cathode common connection with the AK9242/43.

Young A

However, since it is built-in, it is difficult to understand TIA's specification. I am concerned with possible oscillation...

Senior K

It is optimized in design so that the input load capacitance does not oscillate up to 100 pF.

Young A

Wow. So what I have to care about is only input load capacity? That’s easy. I can avoid designing a TIA.

Senior K

By the way, LED driver and auto power control circuits are also integrated in the AK9240NK.

Young A

It is stuffed.

Senior K

Moreover, it is an advantage that the AK9242/43 runs at 4.2V and the AK9240 runs at low power supply down to 2.7V.

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