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AD8605 schematic with ICS-40300 questions?

Question asked by Andydong1583 on Mar 14, 2018
Latest reply on Mar 15, 2018 by JinoL

Dear All,

The project is for acquiring heart sound device with MCU ADC. The input frequency range are from 20Hz to 2KHz.

For lower frequency performance, a high SPL analog microphone is selected. We also saw some documents about microphone preamp circuit from InvenSense. And AD8605 is used as preamp. The schematic check the followings.

So there are some questions, please give me some comments. Many thanks.

MCU and Analog Power supply is 3.0 V.


ICS-40300 Product SPEC v1.0 design considerations:

1, Dynamic range: MUST add 9db gain (about x2.8);

Questions: The preamp gain must be greater than 2.8 times?

Considering the Max output Voltage (0.355V), the total gain to ADC full range is 3/0.355 = 8.45;

So the total gain range is 2.8~8.45. Right?


2, Minimum output AC-coupling capacitor:  should >= 1uF

Why 1uF?


3, Maxium Output Voltage: 0.355Vrms (130 dB SPL);

Questions: the output range : 0~0.355V ?


Schematic Diagrams:

(Power supply 3V; OP: AD8605)


1, Input AC-coupling filer, Add DC bias;

 HP filter: f1c=1/2*pi*R2*C2 = 10Hz

DC bias = 3/2 =1.5V

Questions: Can the bias be set at zero (0V)? The output will be chopped wave?


2, Input preamp gain and 1-order RIAA filter

f2c=1/2*pi*R4*C4 = 2.1kHz

Gain = 750k/75k = 10

Questions: Any suggestions?


3, Sallen-Key low pass filter

f3c=1/2*pi*R5*C5 = 2.1kHz (Sallen-Key)

Q = 0.5


The AD8605 is ok for lower power consumption and low noise?


4, High pass filter and DC bias

f4c=1/2*pi*R8*C7 = 10Hz

Questions: Does it need to add DC bias?


5, Anti-Aliasing low pass filter

f5c=1/2*pi*R9*C8 = 2.1kHz

Questions: Any suggestions?