• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Subscribers 48 subscribers
  • Views 609 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

THS4521: Driver to ADC connection

Miguel Angel Alfonso
Prodigy 20 points
Part Number: THS4521
Other Parts Discussed in Thread: ADS4245, , THS4551

Hello,

In my project, I am using two ADC drivers THS4521IDR and an ADC ADS4245 and I would like to know how to connect them properly.

The input signal of the drivers comes from an Operational Amplifier and it is single ended.

  1. Should I connect the Operational Amplifier output to the positive input of the Driver?
  2. If I connect the Operational Amplifier output to the negative driver input, will I have the ADC output inversed or will I get a completely wrong signal?

How should I connect the positive and negative driver outputs to the ADC (ADS4245 )?

  1. Positive output to positive input and Negative output to negative input? I ask this because in the datasheet of another driver datasheet (ADA4940) the connection is positive to negative and negative to positive. Please, see next picture:

Could you tell me if my circuit is correct (please, note that the positive driver output is connected to the negative ADC input and the negative driver output to the positive ADC input)?

 If you need further information, do not hesitate to consult me.

Thank you very much in advance.

Have a nice day,

  • 0
    TI__Genius 17156 points

    Hello Miguel,

    You have asked a really good question.  I think the best way to answer it is to refer you to a portion of TI's Precision Labs training series.  These short videos can really help with understanding important characteristics and methods for using TI devices.  Each video also includes a pdf of the slides with notes and quiz questions to assess what you learned.

    For your question, I recommend the final video in Section 5: Analog-to-digital converter (ADC) drive topologies -> https://training.ti.com/ti-precision-labs-adcs-driving-sar-adc-fully-differential-amplifier

    Slide #3 already begins to provide insight into how the drive amplifier handles the signal input to an ADC.

    Your referenced image does show an implementation where FDA outputs are aligned with opposite-labelled inputs to the ADC.  A TI reference design on improving THD by careful drive circuit design also shows FDA 'plus' output (OUTP) going into ADC 'minus' input (INM), and vice-versa. 

    www.ti.com/.../tiduck3a.pdf

    However, TI documentation also shows the validity of a matched-label implementation, such as the example shown in Section 9.2.2 of the THS4521 FDA datasheet.

    So what is the answer?

    The choice of how to drive an ADC depends on the drive amplifier, ADC, and the circuit application.  Rather than share a 'one-time' rule that breaks in other contexts, I hope you can look over the pdf (or better, watch the video!) on driving ADCs with FDAs.

    For additional information on this topic, Section 8: Successive-approximation-register (SAR) analog-to-digital converter (ADC) input driver design of the ADC Data Converter series goes into detail about designing ADC drive circuitry.  

    Please continue to ask questions and seek clarification.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Guru 48395 points

    And, you should switch to the THS4551 which was intended to improve on the THS4521

  • 0 in reply to Alec Saebeler
    Prodigy 20 points

    Hello, Alec,

    Thank you very much for all the information.

    I watched the video you recommend me but I still don't know when it is needed to connect the positive driver output to the positive ADC input or to the negative ADC input. During the whole video, the positive driver output is connected to the positive ADC input, but you said that this depends on the driver model and the ADC model. 

    With my Driver model THS4521IDR and my ADC model ADS4245 how should I connect them? What would happened if I connect them backwards?

    Thank you very much in advance.

    Have a nice day,

  • 0 in reply to Miguel Angel Alfonso
    TI__Genius 17156 points

    Hello Miguel,

    I second Michael's point on the THS4551; the THS4551 would be an upgrade for your application.  For your case, I think matching the polarity of the outputs to the ADC inputs is a wise choice.  In short, if you use an 'inverting' output to drive a 'noninverting' input, your converted signal will be the inverse sign of the signal you input into the system.  The idea is the polarity you choose affects the signal 'seen' at the ADC.  So a signal injected into the noninverting node of the FDA, which then is properly routed into the noninverting/positive node of the ADC, would result in the original input signal being converted into digital form, with whatever respective gain and filtering occurring in the system as well.

    Follow the guide and images in the THS4551 datasheet and you will be in good hands.  If you must keep the THS4521, you should follow the datasheet convention shown in the THS4521 document.  If you are not trying to do something fancy or complex with your signal, it is probably fair advice to match the polarity inputs to each other, as a general rule.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 20 points

    Hello Alec,

    Thank you very much for the support and the advices. 

    I will match the polarity inputs/outputs then.

    Have a nice day,