THS4509: Impedance match for 100 OHM ADC analog input

Hi Nan,

the datasheet of AD9083 says:

"This differential resistor can be programmed to 100 Ω, 200 Ω, or can be left open."

So why not leaving it open? Then the output impedance of THS4509 will play a way less role.

Kai

Hi Kai,

Yes I know this point, but I also want to make the THS4509 suit for "100 OHM impedance" because the analog inputs are not only all bypass THS4509, but also transformers, such as the test board design of TI "ADS52J90".

kai klaas69 said:

So why not leaving it open? Then the output impedance of THS4509 will play a way less role.

So if the THS4509 output impedance is nothing, the impedance is not match for ADC, maybe more noise appears. 

kai klaas69 said:

Then the output impedance of THS4509 will play a way less role.

So the impedance match is important for me.

Best Regards,

Nan

Hello Nan, 

  Are the inputs to the ADC close in the layout to the outputs of the THS4509? The schematic you have shown is correct for a simple 100 ohm match load. In the original thread, the LCR DNP components help attenuate high frequency noise and harmonics. This network changes the characteristic impedance of the load, but its impedance can be matched at a certain frequency.

Thank you,
Sima 

Hi Sima,

Sima Jalaleddine said:

Are the inputs to the ADC close in the layout to the outputs of the THS4509

It's not close from THS4509 to ADC, the route length is about 3000 mil most.

Sima Jalaleddine said:

The schematic you have shown is correct for a simple 100 ohm match

You mean that I can use this design as my analog input, right?

Sima Jalaleddine said:

This network changes the characteristic impedance of the load, but its impedance can be matched at a certain frequency.

I don't understand this point, do you mean that only the certain frequency is suitable for 100 OHM impedance? Why?

Best Regards,

Nan

Hi Nan,

I don't see why you can't set your output impedance with Ro?

Best,
Sam

Hi Sam,

I think the output impedance is not only relative to "Ro" but also "Rot" etc.

Best Regards,

Nan

So can't you set the ROT value used with baluns to the input impedance setting of the ADC?

Best,

Sam

Hi Sam,

Do you mean use one good transformer after ROT to make the impedance match?  

We don't use the transformer after THS4509 because it takes too much space.

Best Regards,

Nan

Hi Nan,

Nan Li said:

Do you mean use one good transformer after ROT to make the impedance match?  

We don't use the transformer after THS4509 because it takes too much space.

Why do you show figure 77 with the transformer, if you don't want to use a transformer?

If you are not sure whether figure 76 works in your application, why not running a TINA-TI simulation?

You will find the TINA-TI reference design here:

https://www.ti.com/product/THS4509#design-tools-simulation

TINA-TI is free and can be downloaded here:

https://www.ti.com/tool/TINA-TI

Kai