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active electrode and bootstrapping
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alireza7:
Hi
I am designing an active electrode circuit in order to achieve high input impedance and this is my circuit:





My op amp is OPA333 and diode is 1N4148.
OPA333 is a RRIO with 10 μV offset voltage.

The current source inject 1ma and bias the diode with 0.55v.
Vdd=7v;
I'm sure current source is not saturated and diode is biased correctly.
The problem is that i expect Vo=Vi but when i apply 3.3v to the Vi , Vo is 3.55 volt and output of op amp is 3 volt.
I spent a lot of time to figure out why it is so and why i don't get Vo=Vi but i had no progress |O. Do you have any idea?




 
duak:
Alireza, does the output voltage change when input voltage changes?

The OPA333 has a 5.5 V recomended maximum Vdd.  7 V is the absolute maximum Vdd so there is a small chance the OPA333 is damaged.

Your diagram shows a current source.  Does it actually source current, that is, does it have its own power supply or does it limit current?

I would try reducing the the Vdd supply to 5.5 V and confirm that only 1 mA can actually flow.

I have seen circuits like this work.  I would never recommend one.

Cheers,
spec:
Hi Alireza7

Not sure exactly what you are trying to achieve. Care to tell us more about your application?

In addition to Duak's comments, there are a few other issues with your circuit. Rather than go into a long description of the issues, attached below is a schematic for a circuit that should work OK.

Afraid to say though that the OP333 is not optimum for this application.
alireza7:

--- Quote from: duak on December 08, 2018, 11:31:44 pm ---Alireza, does the output voltage change when input voltage changes?

The OPA333 has a 5.5 V recomended maximum Vdd.  7 V is the absolute maximum Vdd so there is a small chance the OPA333 is damaged.

Your diagram shows a current source.  Does it actually source current, that is, does it have its own power supply or does it limit current?

I would try reducing the the Vdd supply to 5.5 V and confirm that only 1 mA can actually flow.

I have seen circuits like this work.  I would never recommend one.

Cheers,

--- End quote ---

My current source is lm334 and doesn't have individual power supply ,it limits current.

I worked on my circuit and replaced the op amp with a new one and applied VDD=5V . when i apply Vi=3.3v  i get Vo=4.1v
when i apply Vi=3v i still get Vo=4.1v
(Vo is positive supply pin of op amp)
I set the current source to 0.7ma but now it just gives 0.2ma.
actually 4.1v is saturation voltage of my current source.

Then i placed a 1uF capacitor between Vo and GND and then Vo became equal to Vi and circuit works as expected !!!  :o
do you have any idea why it is so?

 Why don't you recommend this circuit?
alireza7:

--- Quote from: spec on December 09, 2018, 01:37:31 pm ---Hi Alireza7

Not sure exactly what you are trying to achieve. Care to tell us more about your application?

In addition to Duak's comments, there are a few other issues with your circuit. Rather than go into a long description of the issues, attached below is a schematic for a circuit that should work OK.

Afraid to say though that the OP333 is not optimum for this application.

--- End quote ---


This article explains my circuit and my application :
https://www.eevblog.com/forum/projects/active-electrode-and-bootstrapping/?action=dlattach;attach=593206
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