Because of the Thanksgiving holiday in the U.S., TI E2E™ design support forum responses may be delayed from November 25 through December 2. Thank you for your patience.

  • State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 47 subscribers
  • Views 3047 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LM2902 input impedance

Mike, Jung
Prodigy 230 points
Other Parts Discussed in Thread: LM2902

Hello

 

I wonder the input impedance LM2902.

Now, Vin(+)/(-) = 0.45V, Vout = 4.7V, Vcc = 6.8V

LED is in front of FET drain.

Inverting input is 750 resistor, but non inverting input isn't.

I test this circuit inserting the inverting 1K resistor.

The result is Vin(+)/(-) = 0.46V, Vout = 4.7V.

when 100K resistor insert the circuit, the result is

Vin(+) = 0.48V, Vin(-) = 1V, Vout =5.8V

 

I have any question.

Is it need the input impedance matching or how to calculation input impedance?

 in order words, I want to know that 750 resistor must be inserted in non inverting input.

 

Thank you

Mike.

  • 0
    TI__Guru*** 149776 points

    Mike,

    The key data sheet parameter will be "Input Bias Current". This current (typically 20nA) flows out of each input pin. To minimise DC offset, the resistance seen by each input should be equal. However, the offset is usually small enough to ignore unless the input resistance diffrence is large.

    If R10 is the inverting resistor and R7,R8 are the non-inverting divider resistors, then set R10 = 1 / ( 1/R7 + 1/R8 )

    Regards,
    Ron Michallick