Is there a way for measuring or know, input impedance of MICIN1 pin of Allwinner H3?

[Ratin]

hi all!

my problem is simple!

Is there a way for measuring or know, input impedance of Microphone input pin (MICIN1) ?

in case C.H.I.P , but because it's prossesor is Allwinner A13(=R8), it must be same.

thanks in advance  

[Ford Prefect]

Hi

According to the schematic there is a pair of  2 K resistors supplying DC to the mic

so impedance is about 4K differential or 2K if you ground one side.

[Ratin]

Hi

According to the schematic there is a pair of  2 K resistors supplying DC to the mic

so impedance is about 4K differential or 2K if you ground one side.

Hi Ford ! thanks for your answer.

Do you said that because of Matching Impedance Theory?

But there is a problem! we need a Decoupling Cap in input of MICIN1 pin of Allwinner; That i saw in here it is 100nF. [see attached image]

With that 100nf and even 4Kohm, we make a HightPass Filter with F-3dB = 400 Hz!?

So, Sounds like somthing go wrong! because the Amount of That HPF must be designed to acheive as low as possible frequency (Like 1 Hz or even lower).

[Ford Prefect]

I believe that the real impedance of the chip input is very high.

Thus the .1 µF cap doesn't cause low frequency loss and seen from the microphone connexions

it is the bias resistors that determine the impedance.

Those electret mikes are of low impedance because of the incorporated amplifiers.

Here is the schematic I have:

[Ratin]

Special Thanks Ford!.

for direct use of An Electret Mic , I think your are right. But how about indirect input(Pre Amplified Microphone Signal to MICIN1 pin)? 

I mean like whats I did, when we connect Output of an PreAmplifier to MICIN1,Which Determine F-3db freq?

As you know, output impedance of  Amplifiers are very low, so I think  input resistanse of MICIN1 pin should be considered . 

Maybe that Input impedance combined with decoupling Cap (in this case 100nF and in my schematic C6=1uF)  make HPF ? 

So whats your Idea? What is Maximum capacitor(C6) value that i can use for this circuit?