LMS7002M ADC Specification Clarification

Projects LimeSDR
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I’m using the LMS7002M in a custom application, but this question should also apply to the rest of the LimeSDR community: What is the load impedance of the LMS7 ADCs? If programmable (I believe this was programmable on the LMS6002D to external resistors), what are the programmable settings?

Specifically, I’m trying to calculate the full-scale input power to the ADC in dBm. While ambiguous with its label (it should be labeled “Full-Scale Input Amplitude”), the spec sheet v2.8 claims the “ADC Input Amplitude” is 0.8 Vpp. Please correct me if I’m wrong in assuming this is the full-scale amplitude corresponding to the maximum-value codeword from the ADC.

What’s interesting is that the dBFS value is used as a reference condition for the SFDR specification, but the dBFS value is not given anywhere else. I’ve also scoured the programming manual and other documentation before posting here.

I believe the last number to solve is the ADC impedance, then you can calculate everything from there. I’m following the procedure from this Analog Devices blog:

Also a great reference for the community for folks new to these calculations and why they matter: http://www.analog.com/media/en/training-seminars/tutorials/MT-003.pdf

Thank you very much for you help.

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Hello RyGuy,

What is the load impedance of the LMS7 ADCs?

The typical input impedance of the LMS7 ADC is quite high, and close to 50 kOhm. It is not programmable. But I do not believe you are interested in the input impedance of the ADC itself. I suspect, for your custom application, you want to know the input impedance of the interface the ADC via the external input pins (rbb(i/q)(p/n)pad(1/2)), since it has a fixed input buffer which in turn drives the ADC. The differential input on this buffer is 12 kOhm, and it is also fixed.

Please correct me if I’m wrong in assuming this is the full-scale amplitude corresponding to the maximum-value codeword from the ADC.”

You assumption is correct.

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Do I understand correctly that the ADC is 10 bit?

10_bit
10_bit.jpg1108×384 80.6 KB

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Why write that the ADC 12 bits, if only 10 are issued?

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Hi @Victor,

This register reading is just for test purposes only hence not all bits are provided.

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Hi Zack!
I noticed this before counting those registers on the DIQ bus, where the lower 2 bits were always 0. All devices in RxTSP were bypassed.
Thanks for the reply.
P.S.: If included in RxTSP TSG, on the bus DIQ change all 12 bits.