Custom FIR Filter on AD4170-4

Hi GabWe ,

We will look into this and get back to you.

Thanks,
Francis

Hi GabWe ,

1. The different FIR modes in the AD4170‑4 define how the ADC internally interprets and reconstructs the coefficient set based on symmetry. In the symmetric modes, the device assumes the coefficients are mirrored about the center, so only half of the taps (plus the center tap for odd length) need to be loaded, with the remaining coefficients generated internally to reduce computation. In the antisymmetric modes, the same approach is used but the mirrored coefficients are applied with opposite sign, so only the independent half must be programmed while the ADC handles the sign inversion internally. In contrast, the asymmetric mode has no symmetry assumption, so all coefficients must be explicitly written.

Below is the summarized version of the coefficient settings.

• Odd Symmetric Coefficients
  Order Calculation: 2 * FIR_LENGTH - 2
  Symmetry: Coefficients are mirrored symmetrically (odd symmetry)
  Maximum Coefficients: 72
• Even Symmetric Coefficients
  Order Calculation: 2 * FIR_LENGTH - 1
  Symmetry: Coefficients are mirrored symmetrically (even symmetry)
  Maximum Coefficients: 72
• Odd Antisymmetric Coefficients
  Order Calculation: 2 * FIR_LENGTH - 2
  Symmetry: Coefficients are mirrored anti-symmetrically (odd anti-symmetry)
  Maximum Coefficients: 72
• Even Antisymmetric Coefficients
  Order Calculation: 2 * FIR_LENGTH - 1
  Symmetry: Coefficients are mirrored anti-symmetrically (even anti-symmetry)
  Maximum Coefficients: 72
• Asymmetric Coefficients
  Order Calculation: FIR_LENGTH - 1
  Symmetry: No symmetry constraints (fully asymmetric coefficients)
  Maximum Coefficients: 36 (approximately half of the other FIR types)

In the FIR_CONTROL register, you'll have to select which type you are going to use together with the number of coefficients to use and the length. This are discussed in page 59-60 under User-Programmable FIR Filter.

2. The FIR coefficients for the AD4170-4 are stored in a 24-bit two's complement format, allowing for both positive and negative values. It has one sign bit and 23 integer bits, but the range is from -2²³ to (2²³ - 1).

3. When designing or loading a custom FIR filter, the following constraints and considerations must be addressed:

• Only one channel can be enabled when using the FIR filter. Multi-channel configurations and the sequencer are not supported.
• The FIR_LENGTH is capped at 72 coefficients.
• Any write to the coefficient memory disables the default FIR filter. To restore the default filter, a device reset is necessary.

Regards,
Francis

Hi FMarasigan 

Thank you for your detailed answer. This really clarifies a lot.

I have one follow-up, regarding the scaling of the coefficients:

Referring to the datasheet for the AD7768-1 (I understand this is a different ADC, but there is some additional material for FIR design on this ADC to be found. I just want to make sure I'm not missing anything for the AD4170-4), on page 61 of the Rev. B version:

Filter coefficients are scaled such that the 56 coefficients must sum to 2^22. The total (112) coefficients, therefore, sum to 2^23.

Is there a similar formatting / scaling constraint to be followed also for the AD4170-4? 

Regards, 
Gabriel