The ADC only converts positive input signals, from 0 Volts (Vssa, ADC output
0) up to Vref (ADC output = 4095) (this is shifting the ADC data to the 12 LSBs,
like a "normal" ADC, we do this in all our applications). So, if you have a
sinusoidal signal that goes both positive and negative, you will have to process
it so it is always positive. Most people bias the "zero" value up to Vref/2, a
good circuit is the one from Alex Fusti's posting on the discussion group "Re:
Recommendation for +/-10V analog input to DSP56F805/7" on September 9, 2002.
I haven't used the external DAC at all, so I can't help you there.
Senior Systems Analyst
PMC Prime Mover Controls Inc.
3600 Gilmore Way
Burnaby, B.C., Canada
Phone: 604 433-4644
FAX: 604 433-5570
From: Roger Flor [mailto:]
Sent: Thursday, September 26, 2002 4:30 AM
Subject: [motoroladsp] ADC converter on DSP56F805 Hello!
I am using a DSP56F805 EVM. I am trying to pass a signal through the on chip
ADC then the DSP then on to the on chip SPI and external ADC (Maxim).
I have already tackled the DSP to SPI to external ADC side (lots of thanks
to Mr Art Johnson). I used the MISO signal from the external Maxim DAC to
set a receiver register full interrupt. I also used the WAIT instruction to
wait for the interrupt after sending the data to the SPDTR register.
For the ADC to DSP, I followed AN1947D (DSP56800 ADC) and AN1933D
(Synchronization of on chip ADC ...). At first I was not getting any data
from the ADC. I used an End of Scan Interrupt to signal the end of a scan
(and to begin retrieval of data). I used the WAIT instruction again to wait
for the interrupt from the ADC. During WAIT state, can the ADC not pass data
to its ADRLST register?
When I've finally made the ADC work and I was able to pass a signal through
up to the external DAC, I could see that only the positive half of a
sinusoid is displayed. What could be the reason for this? I am using a
single ended input to the on chip ADC. What is the conversion scheme used by
the on chip ADC? (is it successive approximation?). I don't really have much
experience on ADC's and I feel that I have to know the on chip ADC very well
If I'm going to use it.
Also, If I keep the system running without an input to the ADC, there is a
DC output at the DAC of around 640 mV. Where is this from?
Thanks a lot Roger
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