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THS4531: Datasheet value confusion

Vihan Shahu
Prodigy 50 points
Part Number: THS4531
Other Parts Discussed in Thread: ADS7945

Hi, we are using ADS7945 adc and thinking of using THS4531 fda with it. While going through datasheet I came across some things I didn't understand.

1. what is the range of input differential voltage of THS4531 considering Vs+ = 5V/ Vs- = 0V? What is the range of differential output voltage in that case?

2. If I keep the Vocm pin floating with 5V/0V supply, will the common mode voltage be 2.5V? What would happen if we add 0.22uF cap to Vocm instead of keeping it floating?

  • 0
    TI__Expert 6050 points

    Hi Vihan,

    1. The input differential voltage range is defined by the input common mode voltage range. More information on the input common mode voltage range can be found in section 9.2.4 of the datasheet. According to the datasheet, for a 5V single supply, the input common mode voltage can range from -0.2V to 3.9V. Another thing to note, as shown in the absolute maximum ratings section of the datasheet, the difference in voltage between Vin+ and Vin- should not exceed 1V.

    The differential output voltage range is dependent on the output load. The data sheet specifies for a 2kohm load for a 5V single supply configuration, the output voltage high on Vout+ or Vout- is 4.88V and the output voltage low on Vout+ or Vout- is 0.1V.

    2. If the Vocm pin is left floating, there is an internal circuit that will set the output common mode voltage to mid supply, which for a 5V single supply operation would set the output common mode voltage as 2.5V. Adding the 0.22uF capacitor to the Vocm pin is recommended as it will reduce noise behavior.

    Please let me know if you have any other questions.

    Thanks,

    Nick

  • 0 in reply to Nick Nauman
    Prodigy 50 points

    So, as I understand, the differential input voltage Vid should not be more than 1V at all cost and Vic input common mode voltage is between 3.9 to 0.1V for 5V single supply. Therefore on each input pin the maximum voltage that can appear is Vic + Vid * 0.5 = 4.4V  and minimum would be -0.4V?

  • 0 in reply to Vihan Shahu
    TI__Expert 6050 points

    Hi Vihan,

    The equation you used will not tell you what voltage can be applied to each pin. Can you provide what voltage you are inputting to the amplifier (i.e. what voltage is being applied to the Rg resistors)? Also, what gain are you configuring the amplifier in? Knowing this information, you can determine what the amplifier input common mode voltage is to make sure you are within the limits of that range.

    I have also included a link to a presentation on input, output, and common mode voltage for fully differential amplifiers below.

    Fully Differential Amplifiers - Input/Output Limitations and Common Mode Voltage

    Thanks,

    Nick