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DAC output level over frequency

Posted: Sat Jan 13, 2018 3:01 am
by ctrofin
When running the 'Oscilloscope & Generator' app, I noticed that the DAC'c output amplitude varies with frequency with a level exceeding twice the set amplitude somewhere in the middle of maximum bandwidth.

So, at 10KHz and a level set to 0.5V it outputs 0.5V/10KHz, changing just the frequency to 20MHz, will output a voltage level of 1.6V. Going to 40MHz will output ~0.5V/40MHz. This is on 'Low Voltage' jumper setting.

Generating a frequency sweep from 1 to 40MHz using the SCPI API, shows that there is a amplitude hump in the middle of the bandwidth, which saturates the ADC if DAC-to-ADC loopback is used.

This is a brand new board. Is there a calibration/setting I should do to correct this or should I suspect something being wrong with the board?

The output level/frequency was measured on a Tektronix scope.

Thank you,

Re: DAC output level over frequency

Posted: Wed May 23, 2018 1:12 am
by GeoffAU
The DAC has output filtering in it that assumes a 50 ohm connection. If you don't terminate it with 50 ohms you will end up with peaking as you've found.

Re: DAC output level over frequency

Posted: Thu May 24, 2018 12:32 pm
I use a 50 ohm connection. At 1kHz to 100KHz and level=0.1V, the output is OK. But I checked it at 7MHz, output level reduced to 80%(0.08V). Is it normal with Redpitya ? I don’t think the load resistor 50 ohm I use is affected by frequency around 7MHz.
Please let me know about it.

Thank you.