Leakage/bias current cancelation techniques

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In summary, there is a thread on physicsforums discussing the possibility of electrometers having bias currents in the range of 1-3 femtoamps. The conversation delves into the use of active leakage cancelation circuits and simulation experiments to achieve this level of performance. Some suggest using operational amplifiers with extremely small bias currents, such as the LMC6041IN, which has a bias current of only 0.002 picoamps. Others mention the use of DIFETs and dielectric isolation techniques to reduce leakage current. However, it is unclear if there is any circuitry or signal processing involved in canceling the bias current or if it is solely achieved through process and material selection. Further research is needed to fully understand
  • #1
DragonPetter
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I became curious as to how this is possible after seeing this thread:
https://www.physicsforums.com/showthread.php?t=562370

How are these electrometers able to have bias currents of 1-3 femtoamps? What kind of active leakage cancelation circuits are capable of this? Can this be used on other components (diodes, capacitors, Vds channels, etc.)?

I tried to see if it was possible, so I did a simulation experiment. The simulations are attached below. I reverse biased a diode into a capacitor to collect the charge, and I see that in my simulation that there is a reverse bias of 2.5nA. I try to cancel the leakage current with a KCL at the diode node, and I do this with a current mirror with a leakage reference current using another diode. I had to adjust the biasing just right to eliminate the leakage current. I admit that this circuit is impractical and I just made it to see if the idea would work.

Does anyone have any insight as to how this is done in these electrometers? Am I in the right direction?
 

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  • #2
Some operational amplifiers have extremely small bias current.
You may want to research the LMC6041IN. It has only 0.002 picoamp bias current typical and is low cost.
0.002 picoamp = 2 femtoamp=0.000,000,000,000,002 amp.
There are operational amplifiers with lower bias current that cost more.
 
  • #3
maybe this will help..


http://www2.cambr.uidaho.edu/papers/dielectrically_isolated.pdf

hmmm apparently the DIFET process make them radiation tolerant, they're talking about 200KRAD devices...

a google on : dielectrically isolated integrated circuit

led to several IEEE articles i can't access...

and TI OPA128 folder has a link to spice model.

but I'm getting in over my head. maybe you'll educate me on this one.

Have fun!
 
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  • #4
Looks like they did a little more than use DIFETs, but I am sure that is a big part of it.

http://www.ti.com/lit/an/sboa061/sboa061.pdf
"Replacing the reverse-biased gate-to-substrate isolation diode structure of BlFETs with dielectric isolation removes this large leakage current component which, together with a noise-free cascode circuit, special FET geometry, and advanced wafer processing, allows far higher Difet ® performance compared to BIFETs."

I don't think using a current source to cancel input bias leakage would work at this range as it would need to hold to fA accuracy over voltage and temp. I suspect process would be the only way to go.
 
  • #5
I'm sorry, but all of those documents including the difet isolation technique are what i'd call passive techniques. Basically they are reducing leakage current by the materials and process rather than canceling existing leakage currents.

From the original thread, I was under the impression that there is some kind of circuitry, signal processing, or mathematical cancelation of the bias current. Is there such a thing, or is it only possible through process and material selection?

I still am not sure how electrometer opamps can have input bias currents within a couple femto amps or less. The most interesting paper I've read is called "Modern electrometer techniques" but it is dated to 1979, and I'm sure there must be more information on it since then.

p.s. that TI appnote was interesting still, and I was surprised that a larger resistance reduces noise.
 

Related to Leakage/bias current cancelation techniques

1. What is leakage/bias current cancelation?

Leakage/bias current cancellation refers to a variety of techniques used to minimize or eliminate the effects of leakage and bias currents in electronic systems. These currents can cause errors and inaccuracies in the operation of electronic circuits and devices, and so it is important to find ways to cancel them out.

2. What are leakage and bias currents?

Leakage current is the small amount of current that flows through a component or circuit when it is supposed to be off. This can happen due to imperfections in the materials or manufacturing process. Bias current, on the other hand, is a small current that is intentionally applied to a circuit to keep it in a certain state. Both types of current can cause problems in electronic systems if not properly managed.

3. What are some common techniques for leakage/bias current cancelation?

Some common techniques for leakage/bias current cancelation include offset cancellation, auto-zeroing, chopper stabilization, and bootstrapping. These techniques involve using additional circuitry and clever design strategies to minimize or eliminate the effects of leakage and bias currents.

4. Why is it important to cancel out leakage/bias currents?

Leakage and bias currents can cause errors and inaccuracies in electronic systems, which can lead to malfunctioning devices and incorrect measurements or data. In some applications, such as medical devices or avionics, these errors can have serious consequences. Canceling out these currents helps to ensure the proper and accurate operation of electronic systems.

5. Are there any drawbacks to leakage/bias current cancelation techniques?

While leakage/bias current cancelation techniques can be very effective, they can also add complexity and cost to electronic systems. Additionally, some cancellation techniques may introduce their own sources of error or noise. It is important for scientists and engineers to carefully evaluate the trade-offs and choose the most appropriate technique for their specific application.

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