DAC/ADC not at full range.

Question

I am using the THDB_ADA daughter card and the DE2-115. I am feeding an NCO output (2Vpp sine wave) from the DAC straight into the ADC through an SMA cable. However, the values registered by the ADC only indicate an input value of ~730mV peak to peak, when it should be at full range.  Any ideas on what's going on?

altera_forum · Answer

So You get a chopped wave or the wave form is correct, but the amplitude is wrong?

altera_forum · Answer

Is the output frequency above the transformer cut-off frequency? Did you measure 2 Vpp with 50 ohm load or open circuit? The AD input has 50 ohm impedance, thus it's halving the magnitude.

altera_forum · Answer

My frequency from the NCO is 3 MHz. I send the signal from the first DAC into the ADC then feed back out through the second DAC, as follows:

assign comb2 = {~sin_out[13],sin_out[12:0]};

assign comb = {ADC_DA[13:0]};

Where comb and comb2 are the DAC output pins. It's coming out as a sin wave at the correct frequency just attenuated. I'm measuring the output of the 2 DACS with the same oscilloscope, high input impedance. Is it right to say that the output impedance of my DAC is high, so is attenuated by half at the ADC input?(given that the SMA has Zo of 50 ohms).

altera_forum · Answer

I reviewed the schematic, DAC output impedance is actually 100 ohm. When connecting it to a 50 ohm terminated input, the signal level is reduced to one third, very near the value you observed. Please don't ask me, why the DAC output has 100 ohm impedance ...

altera_forum · Answer

Thank you for the replies. Mystery solved!

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DAC/ADC not at full range.

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