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XTR111-2EVM: What is the purpose of the 10k and 1k ohm resistors from the Vin pin on the XTR111 EVM BOARD?

Electronx Electronx
Intellectual 275 points
Part Number: XTR111-2EVM
Other Parts Discussed in Thread: TIDA-01471, , XTR111

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ı think its related about this section 

if there is no 1k ohm resistor is not connected the and Vin source lower than 5V  Vin source will sinking .

and when we add 1k res The voltage at the point we want to create will remain constant and our source will flow over 1kOhm, not sinking.

please correct me if ı wrong

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in my design (0-5 Vin 0-20mA ouput )ı have used as this (When input is floating  pull down to gnd ,When the input is 5V, 5mA current is drawn, I realized that it might be better to set a higher value.)

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Likewise, we see that only 10k is used in Tida-01471. What is the purpose of this? 

When I look at the table, I see that the bias current is quite low.

  • 0
    TI__Expert 7485 points

    Hi Electronx,

    I believe the designer of the XTR111-2EVM included the 1kΩ resistor so that the input will be weakly pulled to ground in the case that Vreg is disconnected (J3 open) and there is no input connected at the Vin terminal. This ensures that the input voltage is always in a known valid state and prevents the input from floating.

    Although, you are correct that if your input voltage source is unable to sink current, the 1kΩ resistor forces your input voltage to source current depending on the input range. Keep in mind that if the input is designed to go all the way to 0V, the input voltage source will need to sink a small amount of current, ~100μA. Most voltage sources are designed to both source and sink current so this is typically not an issue.

    For a 4-20mA output, only R6 (40.2kΩ) and R8 (10kΩ) are required to create a voltage divider that level shifts your input from 0-5V to 1-5V at the XTR input. When Rset = 2.49kΩ, the 1-5V level-shifted input produces a 4-20mA output. As your design requires 0-20mA output, this voltage divider is not required.

    Electronx Electronx said:
    in my design (0-5 Vin 0-20mA ouput )ı have used as this (When input is floating  pull down to gnd ,When the input is 5V, 5mA current is drawn, I realized that it might be better to set a higher value.)

    To reduce the power dissipated in the pull-down resistor, you may increase the value to 10kΩ.

    Electronx Electronx said:
    Likewise, we see that only 10k is used in Tida-01471. What is the purpose of this? 

    The series 10kΩ resistor between the DAC output and XTR input is a current-limiting resistor that prevents the XTR input from being damaged in a fault condition. Notice that the DAC is on the AVDD supply whereas the XTR is on the DVDD supply. If the DAC power supply is brought up before the XTR power supply, it is possible that the DAC output can violate the absolute maximum ratings of the XTR by driving the input voltage above the supply voltage. According to footnote (3) of the abs max table, the current flowing into the input pin during the fault condition must be limited.

    Regards,

    Zach

  • 0 in reply to Zach Olson
    Intellectual 275 points

    thanks Zach , in xtr111 datasheet  its saying about 

    If a voltage exceeds the voltage limited by the clamp diodes, current will flow through the clamp diodes and we add a resistor to limit the current to prevent damage to the clamp diodes. I understand. "Place a small resistor" "Consider a resistor value equal to RSET for bias current cancellation." Did it mention here that we should put a resistor equal to or approximately equal to the Rset resistor on the Vin pin? (Maybe I don't understand because of my bad English) In the reference design, the rset resistor is 1.47k ohm and the resistor connected to the vine is 10k ohm, am I wrong?

    So, is the current limiting resistor we put here the same as the set resistor (I use 7.5k as the set resistor in my new design) a good option for canceling the bias current? So in this case, is it the right choice to use 7.5k ohms for the current limiting resistor in the VIN?

  • +1 in reply to Electronx Electronx
    TI__Expert 7485 points

    Hi Electronx,

    You are correct, using a current-limiting resistor that is equal to Rset can reduce the offset voltage that is generated due to the input bias current flowing across these resistors.

    If your design requires a 0-5V input voltage and a 0-20mA output current, the Rset resistor should be 2.5kΩ. An Rset resistor of 7.5kΩ will result in a maximum output current of 6.7mA for a 5V input signal. You may use 2.5kΩ for the current-limiting resistor as well.

    Regards,

    Zach