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Hallo DSP'lers, I'm not very happy with the ADC of the DSP56803. It converts 12Bit, but I get only 7-8 Bits stable. The manual says: Signal-to-Noise min. 55 dB. This means for me, that at least 9 Bits should be stable. I put a filter with a 56H coil, a 4.7F and a 100nF capacitor to Vref and I filter the analog inputs directy at the chip, too. Is this noise a feature of the DSP or has somebody better results? My settings are: DSP56F803_ADCA_CR1 = 0x0001; // Disable interrupts, simultan samples // (single-ended, non-differential) DSP56F803_ADCA_CR2 = 0x0004; // ADC-frequency fADC = 4MHz // IPBus_Clk = ZCLK / 2 = 40Mhz // -> fADC = IPBus_Clk / 2 / (DIV+1) = 4MHz |
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ADC-Noise of DSP56803
Started by ●January 7, 2004
Reply by ●January 7, 20042004-01-07
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Hello Thomas, Here is a part of my message of January 16, 2003 (we use DSP56F807): The AD converter is extremely noisy (although we followed all the Motorola recommendations about board design) and alinear and has a lot of temperature drift (we use it from -10 to 70 degrees Celcius and have about 30 AD steps drift). To overcome these problems, we did the following: 1) Oversampling to eliminiate the noise (so take a lot of samples and calculate the average) In this way we also raised the resolution (not the accuracy). We take 512 AD samples, calculate the sum and divide the sum by 64. In this way we get 15 bit AD values. (So if you need 12 bit AD values, take 32 or 64 samples and calculate the average) 2) We do a 4 point calibration for each channel to overcome offsets and alinearity 3) We measure always at a sepearate channel (with analog switches) a 250 mV and a 2850 mV reference value (also at calibration time). Temperature drift can be corrected by shifting and multiplying the scale dependant on these reference values kind regards, Wim de Haan Exendis B.V. W.J. de Haan P.O.box 56, 6710 BB Ede Keesomstraat 4, 6716 AB Ede The Netherlands. Tel: +31-(0)318 - 676305 mailto: URL: http://www.exendis.com <http://www.exendis.com/ -----Original Message----- From: thomaskracke [mailto:] Sent: woensdag 7 januari 2004 1:53 To: Subject: [motoroladsp] ADC-Noise of DSP56803 Hallo DSP'lers, I'm not very happy with the ADC of the DSP56803. It converts 12Bit, but I get only 7-8 Bits stable. The manual says: Signal-to-Noise min. 55 dB. This means for me, that at least 9 Bits should be stable. I put a filter with a 56H coil, a 4.7F and a 100nF capacitor to Vref and I filter the analog inputs directy at the chip, too. Is this noise a feature of the DSP or has somebody better results? My settings are: DSP56F803_ADCA_CR1 = 0x0001; // Disable interrupts, simultan samples // (single-ended, non-differential) DSP56F803_ADCA_CR2 = 0x0004; // ADC-frequency fADC = 4MHz // IPBus_Clk = ZCLK / 2 = 40Mhz // -> fADC = IPBus_Clk / 2 / (DIV+1) = 4MHz _____________________________________ Note: If you do a simple "reply" with your email client, only the author of this message will receive your answer. You need to do a "reply all" if you want your answer to be distributed to the entire group. _____________________________________ About this discussion group: To Join: To Post: To Leave: Archives: http://www.yahoogroups.com/group/motoroladsp <http://www.yahoogroups.com/group/motoroladsp> More Groups: http://www.dsprelated.com/groups.php3 <http://www.dsprelated.com/groups.php3 _____ > . |
