Regarding ADC in DSP 2S180 board

[Altera_Forum]

hi all, 

i have been working on altera dsp board (2s180), where i am sampling simple sine wave ( using sine wave generator) using on board adc (ad9433). i am using a pll setting my sampling frequency to 25khz, giving the output of the pll to the clock pin of adc   for now i am just checking it on using signal tap and pio ( nios)  i found out that  [list] adc works fine for sampling frequency greater than 25khz and the input frequency greater than 5khz. [/list] [list]  when i set the sampling frequency to 25khz and set the input frequency to 1khz, all i get is 0, -1 and occasionally -2,  [/list] [list]  i checked the data sheet of the adc used on my board (ad9433) it doesnot say anything about any cutoff( lower cut off) for the sampling frequency set or the input frequency given [/list]   so was wondering if i am missing anything? or is there something i misinterpreted  any help would be appreciated.   regards dspboard

[Altera_Forum]

Hello, 

 1. AD9433 has a specified minimum encode rate of 10 MSPS 2. DSP Development Board has AC-coupled clock to ADC 3. DSP Development Board analog input is transformer coupled with about 0.4 MHz cutoff frequency (Mini-circuits ADT2-1T and ADT1-1WT).  so there are several reasons why acquiring of low frequency signals may not work. It would be no disadvantage to use minimum specified sampling frequency and resample the digital input, but transformer coupling can't be disabled by software. Generally SNR would be improved using a higher sampling rate and rate conversion, e. g. by a CIC filter.  Regards, Frank

[Altera_Forum]

--- Quote Start ---  Hello,  1. AD9433 has a specified minimum encode rate of 10 MSPS 2. DSP Development Board has AC-coupled clock to ADC 3. DSP Development Board analog input is transformer coupled with about 0.4 MHz cutoff frequency (Mini-circuits ADT2-1T and ADT1-1WT).  so there are several reasons why acquiring of low frequency signals may not work. It would be no disadvantage to use minimum specified sampling frequency and resample the digital input, but transformer coupling can't be disabled by software. Generally SNR would be improved using a higher sampling rate and rate conversion, e. g. by a CIC filter.  Regards, Frank  --- Quote End ---    Can you explain a little bit more about the AC coupling and transformer coupled ? or is there a place i could get more insight on the ADC working   Sorry I guess these are basic, i would appreciate if you could tell me so that i could document it :confused:   Thank you so much Frank You rule :):)

[Altera_Forum]

The ADC circuit is shown at page 39 and 40 of Stratix II DSP Board schematics. Clock AC coupling is not an issue, cause cutoff frequency is much lower than specified minimum sampling frequency, but analog in AC coupling is. 

 Transformer coupling is an effective way to produce a differential signal with good common mode rejection, if DC and low frequency components aren't of interest. To transmit DC signals, a differential amplifier would be used instead.

[Altera_Forum]

--- Quote Start ---  The ADC circuit is shown at page 39 and 40 of Stratix II DSP Board schematics. Clock AC coupling is not an issue, cause cutoff frequency is much lower than specified minimum sampling frequency, but analog in AC coupling is.  Transformer coupling is an effective way to produce a differential signal with good common mode rejection, if DC and low frequency components aren't of interest. To transmit DC signals, a differential amplifier would be used instead.  --- Quote End ---    Ok let me look it up, I will come back and bug you after i read it  thank thank you so much  Happy easter :) Regards DSPboard