AC-coupling capacitor values

Thread Starter

Tim Fernandez-Hart

Joined Nov 30, 2018
12
Dear All,

I'm building a DSP circuit using the ADAU1701 from Analogue Devices. It has two analogue inputs, of which the datasheet states:
"The 47 μF capacitors are used to ac-couple the signals so that the inputs are biased at 1.5 V".

I am wondering what would be the effect of using a capacitor of a smaller value e.g. 10uF or 22uF? Am I right in thinking that these caps are to block any DC current? If so then surely a much smaller value cap would suffice since DC signals don't have a frequency? The other thing to bear in mind is this circuit will be used for audio signals only, so any frequencies below 20Hz could be safely blocked.

Any help in my understanding of this concept would be much appreciated.

I attach the relevant excerpt from the datasheet.
 

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danadak

Joined Mar 10, 2018
3,572
The caps also implement a V divider applied to input signal.

a 1 uF cap at 20 Hz = 8 K ohms.

Sol it all depends on how much signal loss at low end you can live with.


Regards, Dana.
 

JWHassler

Joined Sep 25, 2013
273
47uF is specified because it's a standard value whose X(c) will be under 200 ohms at 20 hertz, which is less than 10% of the 2K ohm input impedance.
As noted above, if you can handle more low-end loss, lower the value.
It might be instructive to do a simulation of the circuit.
 

crutschow

Joined Mar 14, 2008
23,296
The capacitor does block DC and carries the AC, but it doesn't carry all AC equally.
The impedance of a capacitor to an AC signal is inversely proportional to frequency, so the signal will start to drop off at some low frequency, where the capacitor impedance approaches the resistance it is driving (voltage divider effect).
The frequency of interest for this is typically called the corner frequency (-3dB rolloff point), where the capacitance reactance equals the resistance.
Mathematically this frequency is F = 1 / (2*Pi*R*C).
So from that you can determine the minimum capacitor size to get the low frequency corner frequency you want (typically 20Hz for full range audio).
 

mvas

Joined Jun 19, 2017
538
For more information on a Passive High Pass Filter ...
https://www.electronics-tutorials.ws/filter/filter_3.html

Scroll down to the Bode Plot - Frequency Response of a 1st Order High Pass Filter
Changing the value of the capacitor will change the Cut-Off Frequency and the Phase Shift in the graphs.
Not only does it block DC, it also attenuates low frequencies.
The "Pass Band" must be appropriate for your application.
 
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