ADS4249EVM: Analog Input termination?

Hi Nicholas,

Just to make sure we understand, are you changing the termination impedance because you are changing the balun/input network from the one that comes in the EVM? 

R25/R26 and R71/R60 are part of an RCR filter used to improve the filtering of sampling glitches in combination with the R-C filter that is inside the device itself. For more information please see sections 9.1.2, 9.1.2.1 and 9.1.2.2 of the ADS4249 datasheet. There are different circuit configurations shown in section 9.1.2.2 for different bandwidths.

https://www.ti.com/lit/ds/symlink/ads4249.pdf?ts=1701127814930&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FADS4249 

Best,

Camilo

Thanks Camilo. Yes my intention is to change T2 and T4 from 1:1 to transformers with other impedance ratios like 1:2, 1:4, etc.. For each case I expect to change the values of R5/R6 and R11/R12 each to 1/2 the target termination impedance. The signal of interest frequency (~60MHz), bandwidth (30 MHz) and sampling rate (250 MSPS) will be constant.

As previously stated my concerns are the R-C-R circuit component values as the termination impedance changes. Since my input frequencies are less than 80 MHz is this R-C-R more of a don't care in this situation? Also, If I change R5/R6 and R11/R12 to accommodate the target input impedance does this create an issue for common-mode switching currents (per para. 3, section 9.1.2.1)? For example if I use a 4:1 transformer at T2 and T4 (with a 50-ohm source) this will result in a nominal 200 ohm across the analog inputs. I would plan to change R5/R6 and R11/R12 to 100-ohm, respectively. This appears to violate the <50 ohm requirement in section 9.1.2.1. I'll stop here for now. Appreciate your assistance.

Nick

Hi Nicholas,

Thank you for the additional info.

What 1:4 Z ratio balun PN do you plan to use? And do you plan to use two of them to improve the phase imbalance/HD2? That is the only requirement for two baluns.

My assumption from the additional information you gave us above, is that the HD2 won't really play into your frequency planning since you are only sampling 30MHz of BW at 60MHz. Please correct me if I am in error here.

As for the termination, I would recommend starting out with the 200ohm termination and use the default RCR filter. This will give you a good bench mark to start from, then you can tweak it as needed to meet your performance.

Regards,

Rob

Thanks, Rob!

I plan to use transformers from the Coilcraft WBC family. I'll keep the 1:1 in T1 and T3 since my source impedance will remain singled-ended and 50-ohms.  Maybe none of this will matter since my signal of interest is centered at 60 MHz across 30 MHz, using 250 MSPS.