THS4511: Problem using a fully differential amplifier

Hello Alessandro,

I am here to help!  For your digital background your design is not too far from what you had intended, I would describe your board revision as a good starting point for analog circuitry.  I have a few overall points to share, a tool to share, and some assistance I hope will help you start a revision.  

Overall points:

- For high speed layout, correct impedance and good PCB layout are important.  You may need to use 100-Ohm or 50-Ohm controlled impedance traces. The LVDS driver/receiver datasheet has a good short section about PCB line types and considerations.  You can use included tools in software such as Altium Designer or use standalone software such as Saturn PCB ToolKit to calculate your trace impedance(s).

- Your LVDS chip appears to need 100-Ohm termination, which would be the source input resistance to your FDA circuit

- Fully differential amplifiers (FDAs) are low-impedance on their input stage, which typically requires consideration of impedance matching at the input network.  FDAs are also very strict about matching the two feedback resistors and two gain-setting resistors on either side of the device, as balance and symmetry (of impedance and gain) are important here.  The gain on FDAs is similar to an inverting op-amp, however the resulting differential output enables a wider output voltage leveraging each output's peak-to-peak voltage.

- High-speed devices benefit from good layout techniques, including what I shared above.  Notes for FDAs include placement of Rf and Rg resistors near the FDA (your placement looks good above).  You may want to use smaller resistor packages (0402 or 0603) to get an even tighter layout.  You want to place decoupling and bypass capacitors on the supply pins of the amplifier; smaller nF caps should be near the supply pins and larger bulk capacitors (uF) should be near the supply circuitry.

- You may need to add two additional series resistors, one on each input path (between the outputs of the LVDS chip and the differential termination resistor.

Here is a link to our set of high-speed amplifier web calculators (taking the burden off of hand calculations from your shared FDA document): https://dev.ti.com/gallery/view/CircuitDesignStudio/Analog_Circuit_Design_Studio/ver/1.0.0/

You can select the FDA calculator and THS4511.  You can specify differential inputs, set your Rf and Rs (source impedance value), and see how your gain and output drive are affected.

A few notes from my review:

- Your Rg (gain setting) resistor should be doubled, you are using 249 Ohm which results in a gain of 3.16 V/V.  If you use 499 Ohm, you would more closely match your desired gain.  The calculator will help you handle Rf and Rg values to accommodate the 100-Ohm termination need on the input network.

- A good design tip when using new circuitry is to include additional passive component pads, such as at your FDA inputs and outputs, to allow for either more termination resistors or filtering choices.  A few FDA evaluation modules (EVMs) to examine are THS4511 EVM (in datasheet), THS4551RUNEVM; THS4541RGTEVM; DEM-FDA-DGK-EVM.

Overall you may be able to fix some performance issues with some component selection, but you may also need to check your overall PCB design.  I would start by debugging your LVDS driver/receiver circuit by putting correct output loading and sort of 'ignoring' the FDA to check proper operation.  Then you can introduce an input network on the FDA which properly loads the LVDS output signal.  This will also allow you to fix the correct gain setting.

I know this is a lot of information.  I want to equip you to start thinking about solutions and fixes.  I am here for any additional questions, but I do encourage you to read through the EVM user guides and get a good understanding for how to use FDAs (including THS4511) as well as the LVDS IC you selected.

I encourage you again: high speed design is challenging and analog high speed design is not forgiving.  Your design is a good starting point to grow from and improve.  Thank you for trusting TI to help you with your design.

Best,

Alec

Hello Alec, 

I greatly appreciate your help. 

I have designed the traces of the PCB with 50-Ohm at the input of the SN65LVDS049 and 100-Ohm at the differential input of the THS4511.

I will check the various Evaluation modules that you indicated to me. 

Thank you very much again for all your insightful advices. I wish you a good day, I will let you know if I have any news on the debug of this circuit. 

Best regards,

Alessandro

Hello Alessandro,

Great!  You may ask questions here or on a new thread if this one is closed between now and your next question.

You are welcome for the assistance.

Best,

Alec