input capacitance of LM318

[Amr Wael]

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]

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]

@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 Jun 13, 2021 ---

CIN is what i am talking about

 

[KlausST]

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]

Actual layout on PCB may govern C measured.

A Direct Method of Measuring Op Amp Input Differential Capacitance | Analog Devices

Input capacitance can be a key spec for high impedance and high frequency operational amplifiers (op amp) applications. the article describes a direct method to measure the OPAMP Input Differential Capacitance (CDM)

www.analog.com

Measure the Input Capacitance of an Op Am | Maxim Integrated

To measure an op amp's input capacitance, insert a large resistor in series with the op-amp input and plot the frequency response of the first-order lowpass RC filter on a network analyzer, like a Bode plot.

www.maximintegrated.com

Confusion with a measurement method suggested for an opamp input impedance

Before asking my question, I want to show the probe and scope model I use in the simulation: In the above schematics the section inside the rectangle represents a scope probe set for DC coupling a...

electronics.stackexchange.com

Regards, Dana.

 

[Amr Wael]

@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]

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]

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.

https://www.analog.com/media/en/training-seminars/design-handbooks/Practical-Analog-Design-Techniques/Section2.pdf

https://www.ti.com/lit/an/sloa051a/sloa051a.pdf

Regards, Dana.

 

[FvM]

Looking at other bipolar OPs, I'd expect a voltage follower input capacitance of 1 - 2pF.

 

[Easy peasy]

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.