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THS4531A: TIDU187...Extending Rail-to-Rail Output Range for Fully Differential Amplifiers to Include True Zero Volts

Part Number: THS4531A

In figure 11. what is the reason for connecting the THS4531A negative input to the Vref ? 

The schematic note mentions "bias unused input". 

Why not connect this input directly to ground for a single-ended input ?

Thanks.

  • Hello Tony,

      You could connect THS4531A input directly to ground for a single-ended input. But, be careful with avoiding violating the input and output voltage range of the amplifier for single-supply configurations: 

    For Vs = 5V 

      

    Below VCM is set to mid-supply internal to the device with no bias at the input: 

      In the TI Design, TIDU187, it explains that you can adjust the common-mode voltage of FDAs in two different ways: using the REF/VCM pin or adding a bias to the input of the amplifier. For standard single-ended output amplifiers, adding a bias would be the only method. However, this TI Design shows that even with FDAs this method can be used if interfacing with an ADC with a lower common-mode requirement or when needing to match the full-scale range of the ADC while maintaining the SNR and linearity which may come with decreasing the gain and Vref. 

    Thanks,

    Sima 

  • Thank you Sima.