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input capacitance of LM318

Amr Wael

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Hello ,
I wonder if there is a method to determine the input capacitance of an opamp whose value isn't mentioned in the data sheet. Could i simply use an LCR meter to measure the value of the capacitance ? Second question for the Oscillscopes I know that their input capacitance which is usually 16 pF is taken into consideration when designing the oscilloscope probes in order to avoid signal attenuation while increasing the frequency. Do I need to take the input capacitance of opAmp also in consideration (through adding capacitive dividers for example) if I am going to attenuate high voltage signals before it using resistive divider ? or is the resistive divider enough in case I am using voltage follower stages first between the voltage divider and between the Difference amplifier?
 

KlausST

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Hi,

What capacitance are you talking about?

There are two inputs. Each of them may and will have a capacitance against:
* the other input
* the output
* positive supply
* negative supply

Klaus
 

Amr Wael

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@KlausST I am actually talking about the capacitance that has a configuration same as the oscilloscope's input capacitance (Parallel to the input votlage) between an input and ground for example if exists?
--- Updated ---

CIN is what i am talking about
 

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KlausST

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Hi,

A usual Opamp has no GND pin, thus there can be no capacitance to GND. The given picture is not valid for a typical Opamp.

The given picture rather shows a draft of a "Opamp_circuit". Where the capacitance is caused by connectors, wiring, traces .... (not Opamp internally)

****
In detail there may be some opamp internal capacitance that acts like the shown Cin. But the "active capacitance" depends on the external circuitry.
Example:
with an inverting Opamp, the voltage at -In, +In, +VSupp, -VSupp are constant. With constant voltage any capacitance has no effect, no capacitance current...

Klaus
 

danadakk

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Actual layout on PCB may govern C measured.





Regards, Dana.
 

Amr Wael

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@KlausST So let's say I would like to do voltage division followed by an opamp in a voltage follower configuration , in this case I can only use resistive divider without the need of Capacitive dividers because as long as I am within the opamp's bandwidth/slew rate , there is no input capacitance I should worry about?
 

KlausST

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Hi,

As said: it depends on circuit and PCB layout.
A voltage follower will have some "active" stray capacitance. Maybe in the range 5...20pF.
So how much this influences your signal depends on source impedance

For details: post the schematic (with all part values and part names) and the PCB layout.

Don't use wire wound resistors.

Klaus
 

danadakk

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You can also do a s plane analysis of the LM318 to examine if Zin( f ) and Zout ( f ).
its an interesting analysis as you will see Zout starts looking inductive. And you also
can see the virtual ground at input starts degrading. Doing the LM318 a tad comp-
licated as its two pole Aol response, and with stray C you quickly wind up with 4 pole
system (Cload adds a 4'th pole). But using PFE you can quickly get at the lumped
equivalent Zin and Zout.




Regards, Dana.
 
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Easy peasy

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any input capacitance is determined by the Vbe & Vbc capacitance of the xtors in the op-amp - there may be a few in series - cap to gnd may only be of import if one of the op-amp input pins is connected to gnd - else the input cap will mainly consist of strays - e.g. 1 - 2 pF as suggested above.
 

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