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DC coupled input to ADS548XEVM

Jack Hunt
Prodigy 10 points
Other Parts Discussed in Thread: THS4509

Hi,

We would like to use the ASD548XEVM to sample a 10MHz single-ended psuedo square-wave composed of stepped amplitudes from 0 to 3V. We care about the absolute amplitude of each 100ns step, so we need to configure the ADC buffer to be DC-coupled. I found this post which seems like it should be what we need to do,

http://e2e.ti.com/support/data_converters/high_speed_data_converters/f/68/t/53552

but I'd like some instructions on how to implement this circuit  on the ASD548XEVM (if it is an appropriate buffer circuit).

Thanks,

Jack

  • 0
    TI__Mastermind 29220 points

    Hi,

    Take a look at the User Guide for the ADS548x EVM.  It is on the TI web with a link to it on the page for the EVM http://www.ti.com/tool/ads5484evm?keyMatch=ads5484evm&tisearch=Search-EN 

    The EVM is set up by default for transformer coupled operation, but there is an optional THS4509 amp on the EVM that could be put into service with some soldering.  There is a section in the User Guide that describes the use of the THS4509, but the schematics for the EVM are indispensible and the schematics are shown in Figure 9 of the User Guide.  (Newer EVMs would have a separate zip file on the web for the physical design files but when the ADS548x was released this info was just pu into the User Guide.) 

    From Figure 9 you can see that there are some zero ohm jumper resistors that would have to be unsoldered and resoldered on different pads of the surface mount jumper locations in order to steer the input signal through the ampifier and then back to the ADC.  The amp path *still* includes transformer coupling twice, so each transformer would have to be removed and have zero ohm resistors soldered in to jumper the signal path across from the primary side to secondary side of the transformers locations.   Following the amplifier there are pads on the EVM to include a filter design of your own chooing such as a lowpass or bandpass filter.  For DC coupled operation you would choose to design a lowpass or simply solder down 0ohm resistors on all the series elements to have no filtering at all.    Dont forget AC coupling caps C1 and C4 in that figure 9 would also have to be replaced with 0 ohm resistors.

    When you design you circuit around the THS4509 you would also design it to do the single ended to differential conversion for you.

    The other option would be to install the other SMA J2 to bring in a differential signal of your own choosing and shunt all the elements on the EVM between the SMAs and the ADC with series 0 ohm resistors such as transformer T1 and the AC coupling caps.  Then you could do your own signal conditioning off the EVM.  But using the amp that is on the EVM makes it easier to get the differential signal biased up to the proper level since the THS4509 already has the CM common mode input voltage set to the desired VCM from the ADC.  (again - Figure 9 of the User Guide.)

    Regards,

    Richard P.