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AFE5401-Q1: Questions about Decimate-by-4 mode.

Kazuya Nakai59
Guru 21595 points
Part Number: AFE5401-Q1

Hello guys,

One of my customers is considering using AFE5401 for their next products.
They have the following questions about AFE5401. Could you please give me your reply?

Q1.
8.4.5.2 Decimate-by-4 Mode section on page 38 of AFE5401-Q1 datasheet (Ver. SBAS619A) says that
FIR parameters are needed to set like the follows. 

(1) Set 1: C1 = 5, C2 = 2, C3 = –13, C4 = –2, C5 = 38, and C6 = 66. Set 2: C1 = –5, C2 = –2, C3 = 7, C4 = 19, C5 = 30, and C6 = 34       

The datasheet explains that C1 means Digital Filter Gain = ( DIG_GAIN(=C1?) +32) / 32.
But we can't find any explanation or equation about FIR parameters, C2 ~C6.

Do you have any document or information about C2~C6?

Q2.
This is a question about Decimate-by-2 mode and Decimate-by-4 mode.
In these mode, I think the output data is calculated in FIR stage every two data in "decimate-by-2 mode" and every four data in "decimate-by-4 mode".

Is my understanding correct?

Your reply would be much appreciated.

Best regards,
Kazuya. 

  • 0
    TI__Expert 4890 points

    Hi Kazuya, 

    Q1. C1 to C6 are same as DIG_GAIN_C1_FIR, C2_FIR,..., C6_FIR as shown in the register map from 08h to 0Ah address locations. 

    Q2. Let me elaborate. The digitized data (at the rate of sampling frequency, Fs) is sent to the 11-tap symmetric FIR with the coefficients of C1, C2, C3, C4, C5, C6, C5, C4, C3, C2, C1. The output of the FIR is also at the rate of Fs.

    In Decimate by 2 mode, one in two samples at the FIR-output are dropped. So the output rate becomes Fs/2. 

    In Decimate by 4 mode, three in four samples at the FIR-output are dropped. So the output rate becomes Fs/4. 

    If this response has answered your question, kindly consider closing the thread from your side. 

    Thanks,

    Karthik

  • 0 in reply to Karthik Nagabhushana
    Guru 21595 points

    Hi Karthik,

    Thank you very much for your reply.

    Could I ask you an additional question?
    Based on your information, I think AFE5401-Q1 FIR filter equation is the follow.

    y[n]=C1*X[n-0]+C2*X[n-1]+C3*X[n-2]+C4*X[n-3]+C5*X[n-4]+C6*X[n-5]+C5*X[n-6]+C4*X[n-7]+C3*X[n-8]+C2*X[n-9]+C1*X[n-10]

    Is my understanding correct?

    Thank you again and best regards,
    Kazuya.

       

  • 0 in reply to Kazuya Nakai59
    TI__Expert 4890 points

    Hi Kazuya, 

    Yes, your understanding on y[n] is correct. 

    Thanks,

    Karthik

  • 0 in reply to Karthik Nagabhushana
    Guru 21595 points

    Hi Karthik,

    Thank you very much for your reply.
    It is very helpful for the customer and me.

    Thank you again and best regards,
    Kazuya.