Dear Support,
I plan to use an OPA140 as voltage follower to buffer the output of a low noise voltage amplifier. The OPA140 non inverting input is referred to ground by means of a 10k resistor.(Rld).
The OPA input will be driven by the voltage amplifier (100ohm output impedence) through a 1m-long 50ohm coax cable.
In a couple of old app notes, it is suggested to insert a 100ohm-to-300ohm resistor (Rin) in series to the non inverting pin, to "decouple" the coax cable capacitance.
Please, note that the coax cable can not be terminated on its impedance (the voltage amplifier can't drive loads lower than 10k).
The suggested schemes are the following:
(1)
Vin -----Rin------------ OPA non inverting input
|
Rld
!
GND
(2)
Vin ---------------------Rin----- OPA non inverting input
|
Rld
!
GND
Is Rin useful in my application ? If so, is (1) better than (2) ? How can I calculate the optimal value for Rin ?
Thank you for your support.
Best Regards,
Alberto
