AD8605 schematic with ICS-40300 questions?

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

Questions:

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?

  • 0
    •  Analog Employees 
    on Mar 14, 2018 1:10 AM

    Hi,

    We are currently into this thread. We will keep you posted.

    Regards,

    Jino

  • 0
    •  Analog Employees 
    on Mar 14, 2018 11:26 PM

    Hi Andy,

    ICS-40300 Product SPEC v1.0 design considerations:

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

       --->>> Sounds that you wanted to optimize the input range of the ADC you have. With that, you have to design around it. Assuming your ADC can accept inputs from 0 - 3.0 V and the input input coming from the microphone is 0.355 Vrms, you will be needing a pre-amp prior to the ADC inputs. But take note of the units. 0.355 Vrms is equivalent to 1 Vp-p. With that, you only need a pre-amp gain of x3 V/V not x8.45 V/V.

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

    --->>> This 1uF minimum coupling cap was recommended if your are going to design your HPF at 3.6 Hz in conjunction with the 47k resistor connected to the VREF. So with this information, you could actually vary the value of that capacitor depending on the target cutoff frequency of the HPF design you like. In your current schematic, the Fc is 19.89 Hz and not 10 Hz. You have to consider the parallel combination of 16k resistors to compute for the Fc. In general, you can play around the value of the capacitor depending on your desired HPF response.

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

    --->>> NO. The output of the microphone is 0.355 Vrms which is equivalent to 1.0 Vpp with a dc offset of 0.8 Vdc. Microphone output will look like this:

    Schematic Diagrams:

    (Power supply 3V; OP: AD8605)

     

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

       --->>> If you want full signal integrity, don't bias the amplifier at 0V, it will limit the Input and output ranges of the opamp in single-ended supply operation. Please see the specs table of AD8605 for more details. So stay with 1.5 V dc bias to have the amplifier optimized.

    2. Input preamp gain and 1-order RIAA filter

       --->>> Few corrections on the schematic. Please remove C3 and connect R3 to Vbias (1.5 V). R3 and R4 are too large, reduce these values to 1-tenth of each present value or smaller. Large resistors constitute to larger voltage noise. Gain of 10 is too much for the pre-amp. As mentioned above, you only need a gain of 3 to optimize the inputs of the ADC: use C4 = 5 nF,  R3 = 7.5k, and R4 = 15k instead.

    3. Sallen-Key low pass filter

       --->>> Yes!

    4. High pass filter and DC bias

       --->>> You don't need this stage. Please remove C7, R7 and R8. The 1.5 V ADC input bias was already provided in the pre-amp stage.

    5. Anti-Aliasing low pass filter

       --->>> I am not quite familiar with the ADC you are using, however it's a good practice to protect the amplifiers from the fly-back voltage when the ADC is switching. So it's okay to have the LPF (AAF) before the ADC. You can as well design it where R9 = 7.5k, you know for uniformity of resistors used in your circuit plus cost-savings.

     

    One more thing, just use AD8606 rather than AD8605 since it's already in dual package.

     

    Regards,

    Jino