resistance terminology

Thread Starter

PeteHL

Joined Dec 17, 2014
363
In the data sheets for an op amp, units of input resistance are given as T Ohms. What does "T" stand for, please?

Thank you if you know,
Pete
 

Thread Starter

PeteHL

Joined Dec 17, 2014
363
Thank you all for your replies. If interested, the given input resistance of 1.5 T Ohm is for the 3130 op amp (obsolete). If I use this op amp or similar as a comparator, and connect a voltage source directly to one of the input pins of the opamp, then the source "sees" an input resistance of 1.5 T Ohm, is that correct?

-Pete
 

DickCappels

Joined Aug 21, 2008
7,746
The gates of JEFTs and MOSFETs on the inputs of opamps are basically little capacitors. They are metal sitting on an insulator -Its been a while but I think its Silicon Dioxide that insulates the gate from the rest of the FET. That's how they can get such a high input resistance. Now, be careful, there is input leakage current to consider.
 

MrSoftware

Joined Oct 29, 2013
2,015
Only theoretically. It is likely/certain that the opamp packaging, atmospheric moisture and surface contaminants on the pcb will drastically reduce the resistance seen.
This is an important point. When impedances start getting that high, little things that are normally insignificant start becoming significant. Maybe not significant for your design, but significant relative the 1.5T specification.
 

MrSalts

Joined Apr 2, 2020
551
It can be easily measured
- by finding the “typical” or max gate capacitance from the datasheet (or measure it if you have a meter)
- make a voltage divider on (-) input equal to 37% of Vcc
- connect +input to Vcc
- start a timer when you disconnect (+) input from Vcc
- when the output goes LOW, that is one time constant.

Divide the time constant by the published input capacitance.
The answer is your resistance.

Assuming the Input capacitance of your chip is the same as published on the datasheet = 4.3pF,
And assuming the 1.5T ohms is correct, the output will go LOW in about 6.5 seconds

Note: do not use a pull up/pull down resistor on the input
 
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