gvokalek@augean.ua.OZ.AU (George Vokalek) (11/07/90)
I have a query about the Motorola DSP56ADC16 Sigma Delta converter which someone may be able to answer. My datasheet has an errata sheet which was printed after the main data sheet, showing a circuit apparently connected between FSO and Vin(-). The circuit is a series 10uF and 680R. This represents quite a low impedance connecting a digital frame sync signal to the analogue signal input of the converter. The errata sheet states: 'For optimum small signal performance from your DSP56ADC16, the following circuit addition is recommended. This external circuitry may be omitted on the subsequent mask revisions.' The errata sheet applies to mask C41C only. Now I am using mask revision 2C35R. Does the RC circuit connect Vin(-) and FSO, or Vin(+) and FSO via the 0.0047uF cap? The omission of connection 'dots' in the wiring makes this unclear, though I am reasonably sure that the former is correct. What is the purpose of this circuit? (some sort of dither generator?) After wiring up my system, I find that I am getting noise in the system with an RMS value of about 20lsb, even when the inputs are shorted together. This strikes me as being rather poor. I have followed all of the guidelines (few) in the datasheet. Does anyone have any recommendations for minimising noise in the DSP56ADC? ..G..