THS4509: How to design the relative circuit as an 100OHM input-impedance ADC driver

Nan Li

Part Number: THS4509
Other Parts Discussed in Thread: ADS52J90, ADS5500

Hi TI,

I want to use THS4509 as my ADC input driver.

I find one example from the EVM design of "ADS52J90"(website: ADS52J90EVM Evaluation board | TI.com). But this ADC chip's input impedance is 50 OHM.

The ideal input impedance of my ADC is 100 OHM. So how should I design the front end circuit for my ADC input.

Best Regards,

Nan

over 3 years ago

0 Alec Saebeler over 3 years ago

TI__Mastermind 19146 points

Hello Nan,

If possible, could you share the part number or relevant specifications for the ADC you are considering? The ADS52J90 is a rather complex ADC and it's EVM documentation takes a while to read through. I found that within the THS4509 datasheet another ADC is referenced, Section 9.2.1.2.4. This ADC, the ADS5500, has a bit easier datasheet and parameters to investigate for your comparison.

I will investigate calculating a proper output impedance network for the THS4509 for a presumed 100 Ohm source impedance ADC, but I look forward to getting more details (and the name) of the ADC so I can observe the full specifications.

Best,

Alec

0 Nan Li over 3 years ago in reply to Alec Saebeler

Intellectual 260 points

Hi Alec,

Thanks for your reply. I don't show the ADC part because it's "AD9083" which is designed by ADI.

You can find the ADC details from: AD9083 (Rev. 0) (analog.com)

Alec Saebeler said:
Section 9.2.1.2.4. This ADC, the ADS5500,

For "ADS550", the input impedance is about 6.6 OHM. It doesn't suit me

Alec Saebeler said:
I will investigate calculating a proper output impedance network for the THS4509 for a presumed 100 Ohm source impedance ADC

I am waiting your result, thank you very much!

Best Regards,

Nan

0 Nan Li over 3 years ago in reply to Alec Saebeler

Intellectual 260 points

Hi Alec,

Any results for the 100 OHM impedance?

Alec Saebeler said:
I will investigate calculating a proper output impedance network for the THS4509 for a presumed 100 Ohm source impedance ADC,

Best Regards,

Nan

0 Nan Li over 3 years ago in reply to Alec Saebeler

Intellectual 260 points

Hi Alec,

May you answer my question?

I really want to solve this matter now.

Best Regards,

Nan

0 kai klaas69 over 3 years ago in reply to Nan Li

Guru 140200 points

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?

Kai

0 Nan Li over 3 years ago in reply to kai klaas69

Intellectual 260 points

Hi Kal,

Yes you are right, but it's not my expected answer.

I need 100 OHM impedance match circuit because the analog input channels are not only bypass THS4509 but also transformers like"Test board of ADS52J90".

Thanks for your reply.

Best Regards,

Nan

0 Nan Li over 3 years ago in reply to kai klaas69

Intellectual 260 points

Hi Kal,

kai klaas69 said:
So why not leaving it open?

Another question, if leave it open, the impedance match problem is solved? Why?

Best Regards,

Nan

0 kai klaas69 over 3 years ago in reply to Nan Li

Guru 140200 points

Hi Nan,

because the non-ideal output impedance of THS4509 is no longer forming a frequency dependant voltage divider in combination with the input resistance of ADC.

Kai

0 Nan Li over 3 years ago in reply to kai klaas69

Intellectual 260 points

Hi Kai,

I don't understand this point, may you give me one more detailed explanation?

Thanks for your reply!

Best Regards,

Nan

0 kai klaas69 over 3 years ago in reply to Nan Li

Guru 140200 points

Hi Nan,

this is exactly what happens when you start two different threads about the same issue -> total confusion!

I'm sorry but I don't understand your question. Please explain in detail how the two threads are related to each other and what exactly is your problem. I'm talking about this twin thread:

https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1128492/ths4509-impedance-match-for-100-ohm-adc-analog-input

Why do you think impedance matching is a problem when the ADC has no longer any input impedance but is high ohmic?? Then the output impedance of driving OPAmp is totally irrelevant.

Kai

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THS4509: How to design the relative circuit as an 100OHM input-impedance ADC driver