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Transmission and reception in same DSP

Started by sdqayyum December 10, 2005
Hello..

i want to send data from D/A converter and receive at the A/D
converter of same DSP kit .... i m using tms320c31 DSK... i am trying
some instructions but .. i didnt get exact value .. e.g. if i transmit
2000 i receive 2400 ..results tend to be even worst in most of the
cases wat could be the problem? do i need to take care of timings as
well.. plz guide me .. i need urgent help .. reply as soon as
possible ..

Saad Qayyum



Hello Saad

To properly analyze something like this you need to break down the problem into pieces that are a bit more manageable.  Basically your question is a bit open ended.

 The TLC32040 output can have up to 65 mv and the input 75 mv of error for a +/-3V single ended swing.  A total maximum error of 140 mV out of  6V is therefor 2.3% of the full swing.  If the _numeric_ range is +/-32K (this is not known in your message), you would have a numeric error of up to 1529 counts out of 65536.  An error of 400 would therefor seem to be easily within spec (and would also be true for a 2^14 scale).  But then again, 'typical' is not the same as 'maximum' so maybe something else is wrong.

A digital and analog loop back delay will also occur with this device.  In particular, note how the DAC output filter cannot be defeated.  Though implemented as an analog switched cap filter this is very much like an FIR filter.  Looking at the stop band slope and attenuation it would seem that this is a fairly high order filter.  So how much delay would be expected?  Also notice how the input ADC also has a bypassble 300-3Khz voice band filter.  Has that filter been disabled?

The next question might be if you are using the codec in or out of its specified and tested range.  Many codecs are much better than the data sheet indicates and the TLC32040 is no exception.  Typically at room temperature these devices can be over clocked rather successfully, but just like an over clocked PC, you cant really depend on what is in the data sheet since you would in fact not be operating within the tested range.

Or, maybe the TLC32040 got whacked in which case you will be looking for a replacement (which I dont have).  If you are lucky, and it is only the output that has been whacked, look at the ADC and you will see that the output is differential.   Basically you can try using the -V output from the DAC, or also noting that there is also an alternate output, use those +/-V outputs.  Obviously you will need to change your code!

Best regards
Keith Larson sdqayyum wrote:
Hello..

i want to send data from D/A converter and receive at the A/D
converter of same DSP kit .... i m using tms320c31 DSK... i am trying
some instructions but .. i didnt get exact value .. e.g. if i transmit
2000 i receive 2400 ..results tend to be even worst in most of the
cases  wat could be the problem? do i need to take care of timings as
well.. plz guide me .. i need urgent help .. reply as soon as
possible ..

Saad Qayyum