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Correct the offset of the FMC151


Hello,
I am currently working on the ADC / DAC conversion using fmc151 card, I noticed that there is an offset input and output of this card.
In fact, when I put an input signal of the ADC fmc, I notice that my signal is shifted by an offset of 75mV, so that the output is also shifted 75 mv, and it bothers me when I make conversion.
Is it possible to tell me how to correct this offset.
Thank you.

As requested, we did several experiments.  We also made a copy of the oscilloscope screen
What is the email address I shoud send the data to?
Please find enclosed the requested data and scope scree copies :
0x000 0x0000 0x7fff 0x7fff
0xffff    0xfffff  0x7fff 0x7fff
0x000 0x0000 0x0000 0x0000
0xffff 0xffff 0xffff 0xffff
Hello,


I don't see any issue. There is a large offset when value 0x0000 is used and a large offset in the other direction if value 0xFFFF is used. So if you want an offset near 0mV its a matter of calibration and find the right value to program to compensate for the offset error in the whole signal chain.


Peter
First I don't agree with your analysis: whatever value between 0x0000 and 0xFFFF, the offset on ADC0 and ADC1 is always between 50 and 100 mW and doesn’t change sign as can be seen on channel 1 of the scope screen copies. (channel 1 is the ADC output and channel 2 is the DAC output).
Yes, offset does change sign for DAC0 and DAC1 and yes the accuracy will limit or impact how close we can be from 0 and this is quite acceptable. The DAC outputs are not the concerns. Once more the concern is about the CW voltage on ADC0 and ADC1 channels.
What I understand is that there is an issue with our card but you don’t have any solution for the ADC channels. However, your message seems to mean that I have to live with it and I am sorry to hear that from DSP support.
Having a voltage on the inputs channels is not acceptable as I will not connect the ADC to a scope but to a photodiode and the voltage will not only offset the signal but might well damage it or even reverse polarize it so that I got no/disturbed signal.
Could please transfer the issue to US: maybe they have some other ides that could help.
Hello,


I might have misunderstood the information given in earlier posts, but my conclusion based on what I have seen is that the card is working per design. It could be that the input specification does not match your requirements. Our input is 50 ohm matched and is expecting a 50 ohm source impedance, but might also work with other sources. A photodiode might require a different, or additional pre-amplifier of buffer. If you let me know what input characteristics you require we can look into the possibility of a specific build option.


Best Regards,
Peter
the offset on the ADC channels doesn’t come from an impedance problem. We did check this by changing the impedance on the scope (1Mohms or 50 Ohms) channels connected to ADC0 and ADC1.Correct me if I misunderstand you but you are writing me that the fact that the card delivers a voltage on the input channels is OK? If I indeed understood you correctly the FMC151 is the only acquisition device (regarding ADC0 and ADC1) that I known that delivers a voltage on the channels where external sources that are supposed to be digitalized are connected. And if this a normal feature, how come it is not mention on any document ?
Hello,


Sorry for the confusion. What I meant to say is that the input of the FMC151 is designed to be driven by a 50 Ohm source. The voltage you see comes from the output common mode of the input stage through the feedback. Please find attached the input circuit of the FMC151. Hopefully which this schematic you will be able to figure out whether this will work with your photodiode, or whether an additional opamp stage is required. I noticed this schematic is not in our User Manaul, we will update this.


Thank you,
Peter
Dear Sir,


Was the information enough to waive out your concerns and answer your questions?


Can I go ahead and close this topic?


Best Regards,
Arnaud
Not yet. I will tets this week a gain unity buffer amplifier and check whether it is OK. please keep the post open in the meantime
Dear Paul,


Okay, lets do that, keep us updated.


Best Regards,
Arnaud
I did try with an amplifier (Thurbly WA301). It is OK meaning that the DC offset is stopped by the amplifier and is not visible on the amplifier input side.
I can live with this even if I found very disturbing getting a voltage on the ADC channels (frankly speaking I did not completely understood your explanation!). Anyway, we bought a second FMC 151 and we will see whether it is a design characteristic feature.

Dear Paul,


I am sorry to hear that you are disappointed about the characteristics of one of our products. On the other side I am pleased to hear that you will be able to accommodate with that one way or another.


May I go ahead and close this topic? You are free to open any other topic any time you need!


Best Regards,
Arnaud
Yes you can close the topic and thank you again for your support.
Dear Paul,


Feel free to get in touch with us for anything more.


You are welcome.


Best Regards,
Arnaud
This topic is being closed because the issue is considered as resolved by 4DSP. Feel free to create a new topic for any further inquiries.