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LM15851EVM: NCO_x inputs connections

Part Number: LM15851EVM

Hi!


The LM15851 datasheet states that each NCO_x pair must be connected together and driven with a common 1.8V LVCMOS signal (but, as it's seen in the Pin Functions table, these inputs appear to have differential structure). In turn, in the LM15851EVM a single-ended connection to only one side of these differential inputs is made. A differential connection to these NCO_x inputs is made as well (exactly as it's made for DSx outputs). It's a bit confusing.

Can you give some explanations on why you connected to the NCO_x inputs in such a way? Why didn't you do it as the datasheet requires?

Thanks in advance.

Regards, Vic.

  • Hi Vic

    Those pins on the LM15851 are somewhat confusing. The reason for this is that the LM15851 is a related product to the ADC12J4000, ADC12J2700 and ADC12J1600. The ADC12Jxxxx products have one additional operating mode which utilizes 8 differential output data pairs. The additional 3 pairs of data in that mode use the same pins as the NCO_0, NCO_1 and NCO_2 inputs. To support the high speed nature of the differential outputs, the inputs had to be somewhat similar, and that is why each NCO_x input consists of 2 pins which must be tied together and driven with a common LVCMOS logic signal. The EQUIVALENT CIRCUIT symbol could probably be re-drawn to be somewhat less confusing, but since there isn't a standard logic equivalent to describe what is required at those inputs this is what we used.

    The EVM circuit board design is shared between the LM15851 and ADC12Jxx00 products listed above. For that reason we needed to support 8 pairs of high speed serial data, but with the ability to also drive the NCO_x input pairs with DC inputs to validate those operating modes of the devices. Because the outputs are AC-coupled near the ADC it does work OK as shown, with connections only to the NCO_x+ pins. It would be more correct to have 6 connections allowing either no DC connection, pull-up or pull-down of each NCO_x+/- pin. 

    I hope this helps explain the odd nature of those NCO_x pairs.

    Best regards,

    Jim B

  • Hi, Jim!

    Thank you very much for the comprehensive answer. Now it's all clear.

    Best regards, Vic.