The Designer's Guide Community Forum https://designers-guide.org/forum/YaBB.pl Design >> Mixed-Signal Design >> Harmoincs in DAC https://designers-guide.org/forum/YaBB.pl?num=1336057210 Message started by engrMunna on May 3rd, 2012, 8:00am

Title: Harmoincs in DAC Post by engrMunna on May 3rd, 2012, 8:00am Hi, I have a system with 21 digital outputs. The outputs are sort of thermometer coded. But in thermometer code 00001 is 1 and 00011 is 2 and I guess 00010 is invalid. But in my case both 00001 and 00010 are equal to 1 and  00011 and 10001 are equal to 2. That is the total number of 1's decides the output of DAC and not the order in which the one's are placed. So to convert this type of coding to analog, I used a DAC with 21 current sources which are matched. Now I need a differential output from the DAC and for this purpose I used a current conveyor at the DAC output which convets the current from DAC into a positive and negative current. I am doing all this in cmos integrated circuit. And the DAC is part of a Sigma Delta ADC. When I simulate the whole Sig-Del ADC with Ideal DAC (by ideal DAC i mean a DAC coded in VerilogA),I get very small harmoincs. but when I use the DAC that I designed, I get a small 2nd harmoinc and a large third harmoinc. Can you please suggest a better implementation for the DAC or point me to some literature about reducing DAC harmoincs?

Title: Re: Harmoincs in DAC Post by sheldon on May 5th, 2012, 7:03pm Greetings,   Have you tried looking tried looking at each of the block, individually? It is not fully clear whether your ideal DAC includes the current conveyor or not. How much distortion does the DAC have? How much does the distortion does the current conveyor have? It sounds like you have a differential to single-ended conversion in your design. If the you use an all differential signal path, what happens to the distortion?                                                                    Best Regards,                                                                       Sheldon

Title: Re: Harmoincs in DAC Post by engrMunna on May 6th, 2012, 6:36am Hi Sheldon, I have found out that the harmonics are coming from the current conveyor. Rather the current mirrors at the output of the current conveyor. In my band of interest they give an snr of 40dB. Since there is no other noise in the conveyor except from the harmoinc distortion so i guess i can say sndr = 40dB. now i am trying to linearize the mirrors

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