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non Inverting OP-AMP configuration do not work as expected

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Nikos A.:
Hi everyone,

I have a circuit with a non inverting Op-Amp configuration as shown below. I apply a 0.4V ref. voltage on the non inverting input.
I am using this Op-Amp http://ww1.microchip.com/downloads/en/DeviceDoc/20005367B.pdf



I supply the Op-Amp with 1.8V

In my case Rs is a variable resistor.


This is the formula I am using to calculate the output voltage:
Vout = (Vref x Rf) / Rs +Vref

The circuit works fine for high resistor values. For example when

If Rf = 90Ω and Rs 680Ω I measure around 0.456V which is correct
If Rf = 90Ω and Rs 180Ω I measure around 0.616V which is correct

My problem is that when Rs < Rf I do not have any gain and the output voltage decrease instead of increases... For example

If Rf = 90Ω and Rs 56Ω I measure around 0.736V which is incorrect (it should be around 1.05V)
If Rf = 90Ω and Rs 33Ω I measure around 0.624V which is incorrect (it should be around 1.50V)

Rs (Ω)Vout (V)680 0.4562700.5361800.6161000.768820.808560.736330.624100.512
As you can see for resistor values less than 80Ω (that is close to Rf) the Vout stop increasing and instead decreases

I do not have this problem for high Rf and Rs values ( in the order of kΩ).

Any advice?
Nick

Vovk_Z:

--- Quote from: Nikos A. on May 14, 2021, 10:18:40 am ---I do not have this problem for high Rf and Rs values ( in the order of kΩ).
--- End quote ---
Possibly, it isn't intended to work with such a low Rload. Datasheet data contains typical load "RL = 20 kΩ" - quite a large value, which may confirm my guess.
Typical opamp minimal Rload usually has to be more than 600 R (if it isn't clearly stated).

tooki:

--- Quote from: Nikos A. on May 14, 2021, 10:18:40 am ---
As you can see for resistor values less than 80Ω (that is close to Rf) the Vout stop increasing and instead decreases

I do not have this problem for high Rf and Rs values ( in the order of kΩ).

Any advice?
Nick

--- End quote ---
Calculate the current that flows from the output through the resistors and compare it to the datasheet.

Zero999:
Quickly looking at the graphs of the output voltage headroom vs current, gives an output resistance of roughly 100Ω, when the output stage is saturated hard on and the power supply voltage is 1.8V. This output resistance forms a potential divider with the resistors in the feedback network, limiting the maximum output voltage.

Alti:
You cannot expect the output to drive just any load.
It has some limitations, especially when output is close to the rails (sinking near Vss and sourcing near Vdd).

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