# Differential amplifier simulation

#### engr_david_ee

Joined Mar 10, 2023
204
I got something working using the same differential amplifier configuration i.e, subtracting a voltage 0.9 V (this can be taken from voltage reference) and a gain of 7.8 times. The output voltage is plotted using smaller step between PT = 1000 Ohm and PT = 1600 Ohm. I guess this represents 0 degC (1000 Ohm) and 158 degC (1600 Ohm). he output voltage looks linear.

Is that circuit practically possible to make on a PCB with real components ?

How much the device characteristics of AD8397 changes itself with temperature ?

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#### ericgibbs

Joined Jan 29, 2010
19,120
Hi engr,
E

#### engr_david_ee

Joined Mar 10, 2023
204
The asc file is attached.

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#### ericgibbs

Joined Jan 29, 2010
19,120
Hi engr,
Got the asc file OK.
Where did you get the model for the AD8397?
E

BTW: you can run a temperature test by using. .step param temp 0 100 10

#### engr_david_ee

Joined Mar 10, 2023
204
I just have installed the latest version of LT Spice few days ago Version 24.0.12 released on Apr 23 2024. The latest version came already include the device in the library. I have not added as 3rd party component.

Can you please temperature test ? By running the command ".step param temp 0 100 10" will perform the thermal analysis ? I mean the calculating output at different temperatures ? I don't think we will have more than 100 degC even though the circuit simulations shows it can run at PT = 1600 Ohm (158 degC).

#### BobTPH

Joined Jun 5, 2013
9,288
even though the circuit simulations shows it can run at PT = 1600 Ohm (158 degC).
No. Changing the value of a single resistor in a simulation does not simulate operation at 158° C.

What does the datasheet list as the max operating temperature?

Edited: 85C

Last edited:

#### engr_david_ee

Joined Mar 10, 2023
204
I don't think we will have more than 100 degC. Let's consider the simulation up to 100 degC in the circuit shown in the first post.

The RTD PT1000 will change it's resistance between 0 degC to 100 degC. If I have that RTD located at PT in the circuit then the output voltage is linearly increasing with temperature between 0 degC to 100 degC, right ?

The question is how to run thermal in the simulation. I just added ".step param temp 0 100 10" and just found a single curve in the simulation. How do we run simulation with increasing device temperature.

#### BobTPH

Joined Jun 5, 2013
9,288
the output voltage is linearly increasing with temperature between 0 degC to 100 degC, right ?
Vx = 5 X / (3900 + X)

No, it is not linear in X. If X is small compared to 3900, it will be close to linear over a small range, but definitely not linear.

Compare, for example, the change in Vx if X changes from 600 to 610 Ohms, with same if X varies from 1600 to 1610 Ohms. If it is linear, the change in Vx should be the same.

Vx(600) = 5 x 600 / (3900 + 600) = 0.667V
Vx(610) = 5 x 610 / (3900 + 610) = .676V

difference = 0.009V

Vx(1600) = 5 x 1600 / (3900 + 1600) = 1.455V
Vx(1610) = 5 x 1610 / (3900 + 1610) = 1.461V

difference = 0.006V

#### engr_david_ee

Joined Mar 10, 2023
204
Correct. The red output curve is not linear. But if I export the data points of the red curve as the output voltage vs the resistance of RTD PT in a file. This kind of table can be used to find resistance of RTD PT from the measured output voltage of the OpAmp then I can get the actual temperature from the resistance of RTD PT, right ?

#### BobTPH

Joined Jun 5, 2013
9,288
A linear approximation may be good enough. It depends on what accuracy you need.

But that is not the big problem here. You cannot operate an opamp rated at 85C max at 100C and expect it to work.

#### engr_david_ee

Joined Mar 10, 2023
204

How do we run thermal simulation in LT Spice. I mean to calculate the device output as a function of device temperature.

#### BobTPH

Joined Jun 5, 2013
9,288