# simulate the input/output impedance of opamp

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#### wonbef

##### Full Member level 3 output impedance op amp

hi,all,

how to simulate and measure the input/output impedance of operational amplifier. pls give me some methods. can you give me some papers about analysis and simulation of operational amplifiers. thanks in advance!

kind regards

#### renwl how to measure the input impedance of an op-amp

two port analysis. some more advanced technique you can refer to the book of paul r. gray.

#### yxo

##### Full Member level 4 op amp output impedance

Here Measuring Output Impedance of the Opamp with closed loop

• pavan5720

### pavan5720

Points: 2

#### wonbef

##### Full Member level 3 output impedance of an op amp

can you give me some papers about simulation of input/output impedance of opamp both open loop and closed loop. thanks

• Seetharaman

### Seetharaman

Points: 2

#### surianova input impedance opamp measurement

wonbef said:
can you give me some papers about simulation of input/output impedance of opamp both open loop and closed loop. thanks

To measure output impedance, regardless closed loop or open loop, u just put ideal current source Idc with ac=1 at the output, then measure the voltage at the output across frequency by running AC analysis. The voltage output is the output impedance

R=V/I where for I, ac=1, so V is output impedance.

if u driving capacitive load, jsut put Idc for current source =0

if u driving resistive load, then put Idc equal to the current output of the circuit source.

Thanks

• AMSA84 and Ramakrishna_444

Points: 2

### AMSA84

Points: 2

#### bharatsmile2007

##### Full Member level 3 opamp output impedance

hi

when measuring the output impedance of an opamp.what voltage levels do i need to connect the input pins(inverting and non-inverting teminals) to?

Thanks

#### erikl

##### Super Moderator
Staff member how to simulate output impedance

bharatsmile2007 said:
hi
when measuring the output impedance of an opamp.what voltage levels do i need to connect the input pins(inverting and non-inverting teminals) to?
Thanks
Whatever levels you like. Just provide a good operating point, i.e. stay within the valid common mode range.

#### bharatsmile2007

##### Full Member level 3 input output impedance simulation

Thanks erikl ...

hi erikl,

when i change the voltage levels for the inputs, i see different output impedance.
what does that mean?

iam doing any opamp,pmos diff pair and CS(output stage)....

Thanks again...

#### erikl

##### Super Moderator
Staff member output impedance of op amp

bharatsmile2007 said:
hi erikl,

when i change the voltage levels for the inputs, i see different output impedance.
what does that mean?

iam doing any opamp,pmos diff pair and CS(output stage)....

Thanks again...
The output impedance of a closed loop (CL) opAmp depends very much on its open loop (OL) gain (OLG) : Rout(CL) ≈ Rout(OL) * (CLG/OLG). Near the limits of the common mode range the OLG gets smaller, hence Rout(CL) gets larger.

#### bharatsmile2007

##### Full Member level 3 output impedance

Thankyou all...

#### anju420

##### Junior Member level 3 Re: input impedance opamp measurement

i have an application where i need to simulate the output of a sensor which has an output impedance of 50 ohms, 0.45-4.5v is the range of output voltage for various pressure values...how do i design a simulate a circuit to simulate this so that i can give the resulting signal to an ADC?

#### checkmate Re: input impedance opamp measurement

i have an application where i need to simulate the output of a sensor which has an output impedance of 50 ohms, 0.45-4.5v is the range of output voltage for various pressure values...how do i design a simulate a circuit to simulate this so that i can give the resulting signal to an ADC?
An ideal source with a series 50 ohm resistor?

#### anju420

##### Junior Member level 3 mmm...that doesnt make sense...i need to give it to an opamp/buffer right before giving to the adc....so how do i giving a varying voltage signal of that impedance to it..coz dont i need to match the impedance?

#### FvM

##### Super Moderator
Staff member mmm...that doesnt make sense...i need to give it to an opamp/buffer right before giving to the adc....
You didn't mention a buffer before, and I don't see a reason, why it would be needed with 50 ohm sensor impedance.

Try to ask a clear question.

#### anju420

##### Junior Member level 3 ok lets start frm square one...i have attached the datasheet of the sensor in mind....however, assume i'm not buying it, how do i simulate the characteristics of its output signal..do i use a signal generator? or if not a function gen, what is my other alternative circuit?

#### Attachments

• PS-A-1.pdf
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#### FvM

##### Super Moderator
Staff member how do i simulate the characteristics of its output signal..do i use a signal generator? or if not a function gen, what is my other alternative circuit?
The answer completely depends on the purpose of your simulation, that is still unclear. If you connect a load with sufficient high input resistance, you can simply model the sensor as a voltage source, as suggested by checkmate. Obviously, the simulation won't give much insights. You set a voltage and the ADC will measure it.

For a low impedance loads, also the maximum source and sink currents given in the datasheet have to be considered. But they are only limiting values, the exact behaviour can't be derived from the datasheet. Another point would be to model the effect of varying supply voltages. Again, the behaviour isn't specified completely. But you would be able to estimate model parameters from the datasheet.

#### anju420

##### Junior Member level 3 Well, ultimately the simulated signal needs to be given to the internal ADC of msp430. i need to vary the voltage of the signal to the ADC coz my msp is programmed to give an alarm if the voltage exceeds a particular threshold

#### jimito13 @surianova :

What do you exactly mean by this??

if u driving capacitive load, jsut put Idc for current source =0

if u driving resistive load, then put Idc equal to the current output of the circuit source.

Do you mean that for Iac / Isin source for the output impedance measurement,someone should set Idc at the properties of the source as you say above for each case?Why this?I suspect for the correct biasing of the output?
What happens when someone has a load with an impedance Z=(R+jΧ)??

@wonbef

My method for measuring the output impedance of a circuit is to connect a port at the output of my circuit (set the dc voltage of the port as your biasing voltage there) and then run SP analysis.Via the Zm results you get what you want(real,imaginary,magnitude etc...) concerning impedances.At the input side of my circuit i set the proper nominal biasing voltage and not some other potential of the ICMR (even if it is not wrong) since under this condition you expect a measurement.