MSP58C20

The MSP58C20 is the analog portion of an audio-band sigma-delta analog-to-digital and digital-to-analog converter and is a companion part to the MSP58C80. The MSP58C20 is designed to operate only with the MSP58C80, which contains the digital portion of the audio-band converter. The circuit consists of three main blocks: the analog-to-digital converter (ADC), the digital-to-analog converter (DAC), and internal reference and bias voltages.

The analog-to-digital conversion chain of MSP58C20 consists of a continuous-time antialiasing stage, an analog oversampled modulator, and the modulator bias voltage. The antialiasing stage is a second-order low-pass filter with a cutoff frequency of typically 190 kHz. The modulator is a sigma-delta feedback loop, which oversamples the signal at 1.024 MHz and provides second-order noise shaping. MSP58C20 performs the conversion of the differential analog input signal to a pulse-density-modulated single-bit digital output (ADOUT). When a maximum positive differential input voltage (i.e., a maximum positive voltage difference of AIP AIM) is applied at the AIP and AIM inputs, the resulting code at the ADOUT output is all ones.

The digital-to-analog conversion chain of MSP58C20 consists of a fast DAC, an analog low-pass filter, and the filter's bias voltage. The two input bits (DIGS and DIGL), sampled at 0.512 MHz from a digital modulator on the MSP58C80, are the inputs of the DAC conversion chain. Based on the values for DIGS (the sign bit) and DIGL (the level bit), the following table shows the DAC voltage steps that are produced.

When DIGS = L, the AOM analog output of MSP58C20 has a more positive voltage than AOP. When DIGL = H, the absolute value of the voltage difference between AOP and AOM is greater than when DIGL = L. A band-gap voltage source is used to produce the DAC and ADC reference voltages. These two references of MSP58C20 are different to avoid crosstalk between the two converters.