CONFIDENTIAL Rev. 3.6

March 26, 2021

ES9018K2M Datasheet

ESS TECHNOLOGY, INC. 109 Bonaventura Drive, San Jose, CA 95134, USA. Tel (408) 643-

8800 • www.esstech.com

8

Programmable FIR filter

A two stage interpolating FIR design is used. The interpolating FIR filter is generated using MATLAB, and can then be

downloaded using a custom C code.

Example Source Code for Loading a Filter

// only accept 128 or 16 coefficients

// Note:

The coefficients must be quantized to 24 bits for this method!

// Note:

Stage 1 consists of 128 values (0-127 being the coefficients)

// Note:

Stage 2 consists of 16 values (0-13 being the coefficients, 14-15 are zeros)

// Note:

Stage 2 is symmetric about coefficient 13.

See the example filters for more information.

byte reg26 = (byte)(coeffs.Count == 128 ? 0 : 128);

for (int i = 0; i < coeffs.Count; i++)

{

// stage 1 contains 128 coefficients, while stage 2 contains 16 coefficients

registers.WriteRegister(26, (byte)(reg26 + i));

// write the coefficient data

registers.WriteRegister(27, (byte)(coeffs[i] & 0xff));

registers.WriteRegister(28, (byte)((coeffs[i] >> 8) & 0xff));

registers.WriteRegister(29, (byte)((coeffs[i] >> 16) & 0xff));

registers.WriteRegister(30, 0x02);

// set the write enable bit

}

// disable the write enable bit when’we're done

registers.WriteRegister(30, (byte)(setEvenBit ? 0x04 : 0x00));

OSF Bypass

The oversampling FIR filter can be bypassed, sourcing data directly into the IIR filter. ESS recommends using 8 x FSR as

the input. For example, an external signal at 44.1kHz can be oversampled externally to 8 x 44.1kHz = 352.8kHz and then

applied to the serial decoder in either I2S or LJ format. The maximum sample rate that can be applied is 1.536MHz (8 x

192kHz).

THD Compensation

THD Compensation removes the non-linearity of the DAC resistors and to a lesser degree the non-linearity of passive

components in the output stage. Taking the I-V characteristic curve of a real resistor you will notice that it as a slight

downward curvature. As more current flows, more power dissipates the resistor heats and the resistance rises.

Non-linearity of the DAC output resistors can lead to output distortion in two ways:

•

Amplitude modulation of the output current from the DAC

•

Gain modulation of the output stage as the output impedance of the DAC swings with the audio signal

The ES9018K2M includes models for its output resistors and can compensate for their characteristic curve by finely adjusting

the DAC codes for large and small signal amplitudes.

THD Compensation is effective if the base THD+N measurement with no compensation is less than approximately 70dBr. If

your system performs worse than this, check for other errors with the circuit before applying the THD Compensation.

Registers #13, and #22 to #25 are used for THD Compensation.