ADS6445: DC input for ADS6445 and ADS5482

Hirotaka,

Regarding the ADS6445 EVM, Though DC coupled analog inputs are not the default. It is possible. For DC- coupling, replace C46 and C47 with 0 ohm resistors and remove R9 and R18. The ADC VCM should be used to set the CM input of the amplifier by making sure R84 is populated with a 0 ohm resistor. Because the ADC has a common-mode voltage of 1.5 V, and because the THS4509 is not a rail-to-rail amplifier, users should adjust VCC to 4 V and –VCC to –1 V, which can be done by applying the respective voltages to J21 and J22.

Please reference the following user guide for more information.

www.ti.com/.../slau196.pdf

Yusuf

Hirotaka-san,

Sorry for the delay. As far as your questions goes. In order to modifiy the EVm to allow DC coupling you will have to remove all of the transformers (T1) and install 0 ohm resistors in their place. (use 0-ohm to bridge 1-6 & 3-4). If your signal is single-ended you will need to use the on boar THS4509 to convert it to differential. If using the THS4509 you will need to change the jumper SJP4 from 1-2 to 2-3 and also remove T2 by using 0-ohm resistors just like T1. You will need to remove C1, C4, C11, & C15 install 0-ohm resistors in their place. You may also need to adjust the filter on the output of the THS4509 to make sure it passes your input signal. More on this in section 2.3.2 in the user guide. You may have to use tina spice to experiment with componenets to find the right response for your application. Also, if you are using single ended you will need to terminate the un-used differential input to the amplifier. I am not completely sure how this is done for the THS4509, but that is a question for the high speed op amp team you can post on their page. Lastly you will need to verify that the common mode voltage at the output of the op amp matches the recommended common mode voltage of the ADC.

Please let me know if you have any more questions

Userguide : http://www.ti.com/lit/ug/slau207/slau207.pdf

Thanks

Yusuf