active electrode and bootstrapping - Page 1
Forum section: electronics / projects

author: alireza7
author_rank: Regular Contributor
timestamp: December 08, 2018, 08:59:30 pm
content: HiI 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. Do you have any idea?

author: duak
author_rank: Super Contributor
timestamp: December 08, 2018, 11:31:44 pm
content: 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,

author: spec
author_rank: Frequent Contributor
timestamp: December 09, 2018, 01:37:31 pm
content: Hi Alireza7Not 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.

author: alireza7
author_rank: Regular Contributor
timestamp: December 10, 2018, 04:09:49 pm
content: Quote from: duak on December 08, 2018, 11:31:44 pmMy 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=4.1vwhen 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 !!!do you have any idea why it is so?Why don't you recommend this circuit?

author: alireza7
author_rank: Regular Contributor
timestamp: December 10, 2018, 04:14:05 pm
content: Quote from: spec on December 09, 2018, 01:37:31 pmThis article explains my circuit and my application :https://www.eevblog.com/forum/projects/active-electrode-and-bootstrapping/?action=dlattach;attach=593206

author: duak
author_rank: Super Contributor
timestamp: December 10, 2018, 06:08:19 pm
content: Thank you Alireza, the circuit makes more sense now. It's a clever design. I think the opamp is oscillating and the capacitor stops it. If you have an oscilloscope available it should show you what is happening with and without the capacitor. You may find that you can use a smaller value capacitor, eg. 100n.I was thinking of a circuit where the designer used a quad opamp with a maximum Vcc of 36 V but the actual voltage could be as much as 40 V. One of the opamps was used to hold its supply voltage below the maximum. It eliminated the need for a regulator and it was working in a product, it is not worst case design.My concern is that if the operating voltage of the opamp is exceeded, the opamp might fail and inject current into the electrode and thus become a hazard. I would work out a receiver circuit that limited the maximum possible voltage and current that could be applied. I see this is for an EEG and so could have serious consequences.

author: alireza7
author_rank: Regular Contributor
timestamp: December 11, 2018, 09:02:12 am
content: This is Vo while i connected a constant voltage to Vi and there is no capacitor between Vo and GND.Why does it oscillate like this? Have you ever seen this kind of oscillation?I don't know if the oscillation relates to current source or op-amp. but i guess it relates to the op amp.When i connect a 20uF capacitor between Vo and GND it became stable. But i'm not sure if it's the right solution.How can i eliminate this oscillation and make my system stable?

author: duak
author_rank: Super Contributor
timestamp: December 11, 2018, 07:10:44 pm
content: Alireza, p.22 of the OPA333 data sheet shows the power supply recomendations. The last paragraph discusses the need for a 100n capacitor between the power supply pins close to the opaamp itself. If one analyzes the complete circuit including the power connections as a number of interconnected current loops one sees that the complete circuit is far more complex than it appears. If you are testing this circuit on the bench with wires in 3D space you have an inductance in series with the power connections. This inductance increases impedance at higher frequencies and allows the currents in the output stage of the OP333 to couple into the input stage and cause it to oscillate. The capacitor compensates for the impedance and reduces the coupling and should eliminate the oscillation.How long do you expect the electrode leads to be? I would guess 50 to 100 cm or so. If the two conductors are placed close together, as in a cable or opposite faces of a flex circuit, the enclosed area they surround is minimized which reduces inductance. The two conductors will also form a capacitor which can help.This circuit is complicated because there is a diode between the output and Vdd. The diode introduces a non-linearity into the output current loop. However, it seems to work so it may be a matter of experimenting with different values for the capacitor.Cheers,

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 12, 2018, 06:14:46 pm
content: Very simple bootstrapped opamp designs such as this are hard to deal with. The opamp is basically servoing its own power supply, so it's easy to get it to oscillation. The internal structure of the opamp will play a big role in the overall behavior.You should also consider the resulting bandwidth. Adding a pole with an output capacitor may "tame" it, but what kind of bandwidth are you going to end up with? What's the expected frequency response?I managed to get something relatively stable (in simulation - that may not work as well in the real world) with an AD8505 in LTSpice and no output capacitor. (Tried the official Spice model of the OPA333 and we can see the conditions to make it work with even a DC input signal are very tough to find. But again, that's just simulation.) Then I took a look at the behavior with a sine input signal. The max usable frequency is something in the order of a few tens of Hz, beyond which the output signal gets heavily distorted. Attached is the LTSpice file.