ADS54J60: ADS54J60 input interface

Samaneh Afshar

Part Number: ADS54J60
Other Parts Discussed in Thread: TSW54J60EVM

Hi all,

Hope all is well with you.

Per TI datasheet for ADS54J60 on page 75, Fig. 136. and also in the design provided by the evaluation board, ADS54JXX EVM, no bias resistors to VCM-ADC =2.1 V is required when interfacing the ADS54J60 with a transformer. In this configuration the ADC inout interface circuitry is ac-coupled.

In another design provided by the evaluation board , TSW54J60EVM, bias resistors, R5 and R9 are used to provide the VCM-ADC for the differential signals driving the ADC. This configuration is also in ac-coupling config.

Could you please explain why biasing resistors are used for one and not the other while both are ac-coupled interface?

Regards

Samaneh

over 3 years ago

0 jim s over 3 years ago

TI__Guru* 91947 points

Samaneh,

On the TSW54J60EVM, the VCM going to the analog inputs should only be used if the input was DC coupled. R7 and R21 should be DNI for the AC coupled setup.

Regards,

Jim

0 Samaneh Afshar over 3 years ago in reply to jim s

Intellectual 330 points

Hi Jim,

Thank you very much.

Samaneh

0 Samaneh Afshar over 3 years ago in reply to jim s

Intellectual 330 points

Hi Jim,

Thank you again for your help. I appreciate it if you also help me with the following question.

In the design provided by the evaluation board , TSW54J60EVM, if I configure the differential amplifier with VS+= 2.5V and VS- = -2.5 V which leads to

VOCM-FDA = 0 (assume VICM-FDA = 0) and also use DC coupled interface at the input of the ADC, do I still need to use biasing resistors to provide the VCM_ADC? (should be R7 and R21 mounted)?

Regards

Samaneh

0 jim s over 3 years ago in reply to jim s

TI__Guru* 91947 points

Samaneh,

Will you still have the VCM connection from the ADC connected to the amplifier? If so, and the common mode is at 2V on both analog input legs, then you should not be installed R7 and R21. Other wise, you will need them in. The ADC must have this common mode level on the inputs or you will loose resolution.

Regards,

Jim

0 Samaneh Afshar over 3 years ago in reply to jim s

Intellectual 330 points

Hi Jim,

Thank you very much for your explanation. No, I do not have VCM-ADC connected to the VCM-FDA. Since I configured the FDA for VS+=2.5V and VS-=-2.5V, I have connected the VCM_FDA to GND to have the common mode voltage of the FDA output at 0 V (to achieve FDAs best performance ). I suppose since VOCM-FDA = 0 V, I will not need to install R7 and R21 or I am wrong? ( this is like AC coupling case, there is no DC voltage on the differential signals driving the ADC).

Regards

Samaneh

0 jim s over 3 years ago in reply to Samaneh Afshar

TI__Guru* 91947 points

Samaneh,

This is wrong. You have to have 2V DC voltage on the input. If you have 0V (GND), the ADC internal common mode source will be pulled to GND.

Regards,

Jim

0 Samaneh Afshar over 3 years ago in reply to jim s

Intellectual 330 points

Great, I got. Thank you.

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ADS54J60: ADS54J60 input interface