i set MCLK speed with 16Mhz and 5V power supply.
when 9837 ouputs 1Khz sine,its VPP is 0.6v and the sine has a 0.3v DC offset.
But,when i wanna get 1Mhz sine,its VPP is 7mv and it has a 0.3v DC offset. Is it right?
I'd like to know how you terminated your output?
The measurement was taken by an digital agilent oscilloscope with 200Mhz bandwidth and 2*10^9 sampling points per second.
The power supply for AD9837 is +5V .(the maximum output current of the power supply is 0.04A)
The input impedance of my oscilloscope probe is 10M ohm.
Because AD9837 has an 200 ohm load resistor,i did nothing with its signal output point on my PCB but just hooked up it with a probe to get the waveform.
Have you tried intermediate output frequencies, like 10kHz or 100kHz or maybe a sweep?
This would give you an idea if it acts as if there was a low pass filter or if there are abrupt changes which would maybe indicate errors in updating the control registers...
thank you for you reminder！ I did try to write different intermediate output frequencies to control register.
The followings were what I got：
output frequencies VPP DC offset
1kHz 611mv 0.34v
8kHz 450mv 0.34v
100kHz 43mv 0.34v
1mHz 7mv 0.34v
first,through my analysis for the waveform and the measurements ,there was no evidences about the cancellation of high frequency signal. The 1mHz sine waveform was clean and stable but its vpp is too small.
second,the frenquency tested by oscilloscope was consistent with the value I loaded into the selected frequence register. And I observed the AD9837 real-time timing diagrams from my oscilloscope .So I am sure that my write operations to control or update registers is right.
This seems very odd. I can't think of any reason why the output of the part should attenuate with frequency like that. I would first make sure that your measurements system is doing its job correctly.
Do you have a sine wave generator available that you could probe with the same probe and scope settings as a sanity check? Maybe you have bandwidth limiting turned on in the scope somewhere...
Alternatively you could try a second scope and or probe.
Thank you for your reply.Maybe you analysis is right. I‘ll try it after my winter vocation and then share my results.
And how did you measure this?
What instrument did you use with what settings and how was it hooked up to your circuit?
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