Title: Hantek 2D72 - CH1 and CH2 not sampled simultaneously [Print this page] Author: gf1 Time: 2019-2-15 04:51 Title: Hantek 2D72 - CH1 and CH2 not sampled simultaneously Edited by gf1 at 2019-2-15 05:52
Dear Support,
when both channels are enabled, I notice a time offset/difference between CH1 and CH2, in the amount of about 50ns at >= 10us/div and about 5ns at faster time base settings. This is obviously 1/2 of the ADC clock period, which is used at the particular time base setting.
This is nasty, and it makes particulary X-Y mode quite unreliable (-> what I measure does not reflect the truth)
Example:
The same 1MHz sine wave signal is fed into both, CH1 and CH2, and displayed in X-Y mode. One would expect to see a diagonal line, however, an ellipse is displayed, although there is no phase shift between the signals fed into CH1 and CH2 - it is the same signal (-> same kind of PP-80 probes used for both channels, I also tried alternatively with two 50 Ohm terminated coax cables of the same length - no differece).
[attach]2608[/attach]
The same in the time domain (captured at 10us/div, then frozen and zoom-in):
[attach]2606[/attach]
Do you see any possibility to fix this problem in the FPGA and/or firmware?
Versions of my device:
SOFT 2018120601
PCB 0000000001
FPGA V02
Thanks,
gf1
Author: amy Time: 2019-3-2 14:11
Sir, I've tested and didn't find this problem.
How do you connect the signal to the oscilloscope? Please take a picture to me to study.
And check whether there is the phase difference between the two channels on the oscilloscope display. Author: gf1 Time: 2019-3-2 20:33 Edited by gf1 at 2019-3-5 03:58
Hi Amy,
the AWG is connected to both scope channels with BNC-cables of equal length (25cm). 50 Ohm terminated at the scope inputs.
[attach]2651[/attach]
It is easy to reproduce. Set AWG to say 1.95 MHz sine wave. Set scope timebase to 10us/div and enter X-Y mode. An ellipse appears instead of the expected diagonal line => about 50ns time offset between CH1 and CH2.
[ For time base <= 5us/div, the time offset is smaller - only about 5ns. But I just need to increase the signal frequency to say 10..20 MHz, and 5ns offset is still sufficient to turn the expected diagonal straight line into an ellipse. ]
[attach]2654[/attach]
[attach]2656[/attach]
I think that the root cause is related to the following observation:
I did measure the clock signals of the two channels of the AD9288, and obviously the clocks have always a phase difference of 180° (independent of the number of enabled channels). When the two ADCs are interleaved, for doubling the sampling rate in single-channel mode, this is indeed desired. But if both channels are enabled, then the ADCs must not be interlaved, but both channels must be sampled by the track &hold of the two ADCs at the same time.
Author: amy Time: 2019-3-6 16:39
Thanks for your feedback, we will modify.
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