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THS4522 common mode voltage questions

Madhuri Akkenepalli
Expert 1215 points
Other Parts Discussed in Thread: THS4522

We need some clarification on the THS4522IPWR part in terms of setting the common mode voltage.

 

We would like to set the voltage by adding a pulldown resistor of 150k to the VOCM pin. This assumes the part has an internal resistor divider network as described on the datasheet. We tried it in the lab and it appears to work but I think that process and tolerance inside the part will make the voltage we set vary a bit by device.

 

For our present application we just need the common mode voltage to be stable (DC) and in the fairly large range of the ADC. Right now the default value of the part is outside that range however.

 

Please advise.

  • 0
    TI__Genius 15975 points

    Hello Madhuri,

    The THS4522 has the following table specifications for the VOCM pin at 5V supply with the limits set by characterization and simulation.

    You could add an external pull-down resistor to the VOCM pin to set the output CM voltage, but I would recommend the 150k resistor tolerance to be less than 1%. If the default value of the part outside the range, can you also use a pull-up resistor to bring it within the output CM range? Also, is there a specific reason not to directly use the VOCM pin?

    Best Regards,

    Rohit

  • 0
    TI__Genius 13935 points

    I cannot give a specific value of external resistor to use in a “pulldown” manner on the Vcm pin without knowing the voltage (and range desired).

     

    Trying to make a voltage reference using external and internal resistors in a divider configuration is not a good idea. This sets up a voltage divider between poorly matched devices this is not a good idea. See below for the method to use an external voltage divider. An external voltage divider uses only one more resistor and a slight amount of power.

     

    The datasheet specifies that the amplifier will have an output voltage of (Vs+ - Vs-)/2 within +-5mV guaranteed. This is very good and is in the center of the common mode range. The internal voltage divider circuit is set up with 275 k Ohm resistors which match very well, but can have a process variation of +-15%. This tolerance range is +-41k Ohms. If you use an external pull down resistor of 150k Ohms this 150k Ohms in parallel with the internal resistor turns into about 97k Ohms. This will set the input common mode range at the lower end of the common mode range (~1.3V).  But this setting will be inaccurate.

     

    If you want to set the Vcm pins with external resistors you need to use two resistors in a voltage divider configuration. If you do this the external resistors need to be 1/10 of the internal resistors so that the internal resistor tolerances do not interfere. Since the sum of the internal resistors is 550k, the sum of your external divider should be 55k or less. For example, to set the external voltage at 2.0V use 22k Ohms and 33k Ohms in a voltage divider. With these external components the internal resistors will not interfere much.

     

    The best way to mitigate the pop suppression feature is to use the PD pin to enable or disable the device and leave the power supply turned on. Do you need to have a fast turn/off time for the amplifier?