IMP23ABSU interface with microcontroller

[vselvaraj]

Hello, 

I am working on integrating IMP23ABSU into my hardware. Hence, I reviewed the evaluation schematic (steval-stwinkt1b) to understand its setup. I have a few questions regarding the schematic, in addition to what has been answered here, it would be great if you could answer them.

1. There are three MREF in the schematic for steval-stwinkt1b, part of which is attached to this message as an image below. I understand what the ones circled in green correspond to, but what does the MREF circled in red correspond to?

2. In the response here, it was mentioned that the amplification factor of 1 + R6/R7 = 2.34 is to take advantage of the microcontroller's 3.3V. But isn't the Opamp's VCC+ and microcontroller's VREF+ tied to 2.7V? Meaning the analog voltage range will be from 0 to 2.7V and the output M1P can only range from 0 to 2.7V, am I correct in interpreting the setup?

Thank you.

 

[Peter BENSCH · Accepted solution]

Welcome @vselvaraj, to the community!

1. as @Federica Bossi has already described, the signal MREF marked red by you is the output of the opamp U8, which provides the voltage VDD/2 buffered at OUT2. If you redraw the schematics around M1 and U8 again using pen and paper, you will recognise its function.
2. The second, non-inverting opamp, which amplifies the audio signal M1P_FILT, also refers to this reference. Unfortunately, Federica has misread something here, because the designation and value partially overlap: R7 has the value 4.7k (not 74.7k), which results in an amplification factor of 22.28. Yes, your objection with the ADC range 0...2.7V, into which the microphone signal is shifted, is correct.

Hope that answers your questions?

Regards
/Peter

[Moe_Rice]

Hello,

I'm new to this field, so please excuse any mistakes I might make. I'd like to make sure I understand correctly. Is my schematic accurate? If so, why is the 10k resistor not taken into account in the calculation? I was expecting a formula like 1 + (100 / (4.7 + 10)).

Thank you in advance for your help.

Best regards,

[Peter BENSCH]

@Moe_Rice 

As mentioned in my previous answer: If you redraw the schematics around M1 and U8 again using pen and paper and using common symbols for microphone and the opamps, you will recognise its function.

But I will try to explain this in more detail:

• MREF is the output OUT2 of the second opamp of U8 (pins C3, B3, A3), which is connected as a voltage follower and amplifies the voltage from the voltage divider R8/R9, which is the halved 2.7V. A voltage of 1.35V is therefore driven by the opamp at MREF.
  
  
• The first opamp of U8 (pins C1, B1, A1) initially forms a non-inverting amplifier with R6 and R7. R28 is used here to shift the microphone signal by MREF in order to bring it to the reference level (so-called centre voltage).
  
  
• Instead of being connected to GND, the base point of R7 is also connected to the reference voltage MREF, so that MREF is set exactly at the output of the amplifier opamp when there is no microphone signal. Each variation of the microphone signal is amplified by the factor 1+R6/R7, the output signal then follows symmetrically by the value MREF. MREF therefore represents a DC offset for the amplified signal, which is important for subsequent processing with the ADC of an MCU because an ADC can only convert positive signals.

Clear now?

Regards
/Peter

[Moe_Rice]

Thank you, sir, for your explanation, which is now very clear. Have a great day!