THS4541

Hello Alain,

The THS4541 datasheet provides Vocm range on page no. 7 for 5V supply and on page no.10 for 3V supply. Yes, the Vocm range is dependent upon the supply voltage.

The below table provides Vocm range as a headroom specification from both the positive and negative supply voltages. This means the worst case minimum output common mode voltage of THS4541 can be controlled from (Vs+) - 1.2V to (Vs-) - 0.94V. So, for a single 5V supply, the min worst case output common mode voltage is supported from 3.8V to 0.94V.

Looking at the above table, you can match the ADS4245 Vocm with the output common mode of THS4541 since it is within the output common mode loop range. Also, as the full-scale input swing of the ADS4245 is 2Vpp differntial, the THS4541 should be able to swing 0.5V below 0.95V output common mode voltage due to the rail-to-rail output structure of THS4541. 

However, it might also be worthwhile looking at the distortion performance of THS4541 across output common mode voltage. The distortion degrades at the extremes of the output common mode voltage as the output signal swings close to the supplies. Below is the plot of harmonic distortion performance vs Vocm for 5V supply. As you can see, the performance holds good even at the extreme 0.95V Vocm with 2Vpp 10MHz output. But adding any loss at the output (for example anti-aliasing filter loss), could make the output swing higher than 2Vpp at the THS4541 to maintain the same full scale input at the ADC, and thus increase the distortion products. In such a scenario, I would recommend using split supplies at the THS4541 centered around 0.95V common mode of the ADC in-order to improve the dynamic range. For example, using Vs+ = 3.45V and Vs- = -1.55V as supplies for the THS4541 such that the output common mode is centered at mid-supply or 0.95V. 

Best Regards,

Rohit