ADS54J66: ADC analog input schematic

RajK

Part Number: ADS54J66
Other Parts Discussed in Thread: DAC37J82, DAC37J82EVM, LMK04828, , , DAC38J84EVM

Hi Jim,

We have used TC1-1T-152X+ (https://www.minicircuits.com/pdfs/TC1-1T-152X+.pdf ) 50 Ohm transformer on the analog input. We also have a digital attenuator on the input chain. Is it possible for you to review the schematic and provide constructive feedback on it ? Thank you

over 4 years ago

0 jim s over 4 years ago

TI__Guru* 91947 points

RajK,

I would suggest the following:

1. Remove or do not install R8 and R9.

2. Replace R7 and R10 with 24.9 Ohm resistors. These two in parallel will provide the required 50 Ohm termination.

3. Suggest adding a series resistor on either side of C15 to provide an option for a filter. See attached schematic for an example.

Regards,

Jim

3125.ADS54J66_ADS58J63_ADS58J66-SCH_A.pdf

0 RajK over 4 years ago in reply to jim s

Intellectual 740 points

Thank you for your helpful response, Jim. If we decide to not install or remove C15, what else will we have to change ?

0 jim s over 4 years ago in reply to RajK

TI__Guru* 91947 points

Just the other suggestions I mentioned.

0 RajK over 4 years ago in reply to jim s

Intellectual 740 points

Thank you, I have a similar question for DAC37J82 analog output schematic. It has same TC1-1T-152X+ 50 Ohm transformer on the output chain. Is this schematic okay ?

0 jim s over 4 years ago in reply to RajK

TI__Guru* 91947 points

RajK,

This is incorrect. The output needs to be referenced to GND. See attached DAC37J82EVM schematic for reference.

Regards,

Jim

2235.DAC3XJ8XEVM-SCH_D.pdf

0 RajK over 4 years ago in reply to jim s

Intellectual 740 points

Hi Jim,

I have a question regarding LMK04828 ferrite beads for power pins. I noticed a difference in the schematics of DAC37J82EVM and ADS54J66EVM for LMK04828. (see attached photos). Could you please explain the difference ? Thank you

0 David Chaparro over 4 years ago in reply to RajK

TI__Genius 14901 points

RajK,

The reason for the extra ferrite beads and decoupling capacitors is that we found that the power pins are a little more sensitive and required more isolation, so they were added to the schematic for the ADS54J66EVM. The DAC37J82EVM does not include them because this EVM was made before the changes to the schematic were recommended.

Regards,

David Chaparro

0 RajK over 4 years ago in reply to David Chaparro

Intellectual 740 points

Thank you, David for your quick and helpful response. I have a similar question regarding CLK and Sysref to ADC/DAC from LMK. Which one is better ? (see attached photos)

0 Rob Reeder over 4 years ago in reply to RajK

TI__Mastermind 27210 points

Hi Rajk,

a single pair of 240ohm resistors to ground near the clock outputs is fine if using the LVPECL style clocking from the LMK.

No need for the other pair of 240ohm resistors.

If your clocking trace lengths are long, say more than 2-3inches, use a far-end termination of a single differential 100ohms located near the ADC, either before or after the AC coupling caps.

Regards,

Rob

0 RajK over 4 years ago in reply to Rob Reeder

Intellectual 740 points

Thank you, Rob for your helpful response. Is it safe to use this schematic for clk and sysref to DAC37J82 as well?

The trace lengths are as follows -

LMK to ADS54J66 = 2.1 inches (Clk and Sysref)

LMK to DAC37J82 = 8.6 inches (Clk and Sysref)

LMK to FPGA = 13 inches (Clk and Sysref)

Could you please recommend changes to the schematic, if any, for these trace lengths. Attached is my custom schematic.

0 David Chaparro over 4 years ago in reply to RajK

TI__Genius 14901 points

Hi RajK,

You it is safe to use the DAC38J84EVM schematic as a reference for your design. For the length of the traces it looks like you are terminating correctly. There is one thing that you can change, for your LVDS differential lines you do not need the 240ohm resistors to ground, they are only required for your LVPECL lines.

Regards,

David Chaparro

0 RajK over 4 years ago in reply to David Chaparro

Intellectual 740 points

Thank you for your response. I have another question regarding using 4:1 transformer at the input of the ADC. https://www.minicircuits.com/pdfs/TC4-25X+.pdf This is the part we're looking at. Could you please recommend any modifications to the schematic. Thank you

0 jim s over 4 years ago in reply to RajK

TI__Guru* 91947 points

8838.DAC3XJ8XEVM-SCH_D.pdfRajk,

Remove the 100 termination resistors R60 and R65, and route SYSREF as DC coupled per the attached schematic. ADC looks fine.

Trace lengths are fine. Just make sure SYSREF and CLK are matched as close as possible to each device.

Regards,

Jim

0 RajK over 4 years ago in reply to jim s

Intellectual 740 points

Thank you, Jim for the recommendation. We currently have a 1:1 transformer Tc1-1T-152X+ at the input of ADS54J66 and we would like to replace that with a 1:4 transformer TC4-25+ (https://www.minicircuits.com/pdfs/TC4-25+.pdf). Do you have a recommendation on any modifications to the below schematic. Thank you

0 jim s over 4 years ago in reply to RajK

TI__Guru* 91947 points

Suggest changing R33 and R37 to 100 Ohms. The 50 Ohm transformer input will need to match the output impedance of the transformer. Since this is now a 1:4, the output impedance should be 200 Ohms.

Regards,

Jim

0 RajK over 4 years ago in reply to jim s

Intellectual 740 points

Thank you for the suggestion. We are changing both T5 and T6 to 1:4 transformer and connecting them back to back such that T5 is 1:4 and T6 is 4:1. The input at T5 is 50 Ohm, output at T5 is 200 Ohm and input at T6 is 200 Ohm, output at T6 is 50 Ohm. Will R33 and R37 still need to be changed to 100 Ohm ?

0 jim s over 4 years ago in reply to RajK

TI__Guru* 91947 points

Since there will be 2 transformers back to back, no, you do not need to change the resistors.

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ADS54J66: ADC analog input schematic