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.

  • Resolved

AFE5818: VCNTL Input Common Mode - Driven from Single Supply Fully Differential Amplifier

Prodigy 30 points

Replies: 3

Views: 217

Part Number: AFE5818

In a previous design of our ultrasound system we used the AFE5807 as our front end. To control the VCAT, we were using a differential output DAC with FDA (OPA1632) to drive the VCNTLP and M pins between 0V and 1.5V difference. In order to drive the VCNTLM pin to 0V, we created a negative supply for the FDA.

For our next system, we are looking at using the AFE5818, which appears to have similar specifications with double the channel count, meaning we can use a single chip rather than two. I am investigating using this in our design.

On the old system we had other devices which required a negative rail, so driving the FDA from a dual supply was not really an issue. However on the new system we no longer have any other sections that require a negative rail, so creating a negative supply just for a single amplifier isn't ideal.

I've been looking for rail-to-rail FDAs to see if we can use a single supply, however the closest I have found would be something like THS4551 or THS4521. Both have good noise performance and plenty of bandwidth. However although nominally rail-to-rail FDAs, they can on drive as low as 0.2V or 0.08V respectively.

I note that the VCAT gain is relative to the difference between VCNTLM and VCNTLP, and that no minimum or maximum specification is given for the common mode voltage - only the typical value of 0.75V. I wonder therefore if it is possible to adjust this common mode voltage to something like 0.95V? (essentially keep the 0V to 1.5V differential swing, but drive the VCNTL pins between 0.2V and 1.7V).

Alternatively, if that is not possible, could you recommend an FDA that is capable of driving to 0V from a single supply?

  • Hi Thomas,

    Welcome to TI E2E forum.
    Let me check with the team and will get back to you by Friday.

    Regards

    Praveen

    TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

  • Hi Thomas,

    As far as we are aware, there is no FDA that is capable of driving to 0V from a single supply.
    It seems like the common mode voltage cannot be adjusted by 200mV. You could probably change by 50mV like 0.8V.
    In which case you drive the VCNTL pins between 0.05V to 1.55V.
    THS4521 seems to be a better option than THS4551 as you can drive as low as 80mV. This would obviously limit the VCAT gain.
    But be wary of the noise at the VCNTL pins for the system performance. Refer Figure 69 for the allowed noise on the VCNTL pin depending on the total number of channels in the system.

    Regards

    Praveen

    TI makes no warranties and assumes no liability for applications assistance or customer product design. You are fully responsible for all design decisions and engineering with regard to your products, including decisions relating to application of TI products. By providing technical information, TI does not intend to offer or provide engineering services or advice concerning your designs.

  • In reply to Praveen Aroul:

    Dear Praveen,

    Thank you for checking this for me. It seems there isn't going to be enough headroom then to avoid a negative supply rail for the amp.

    In the mean time, I've come across TI app note TIDU187 which recommends using something like LM7705 to generate a small negative bias (-0.27V in that case) from a positive supply rail in order to allow the output of an FDA to reach 0V. We'll probably investigate this further to check noise performance.

    Kind Regards,

    Tom

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.