Need suggestions on applying 5-10uA@ 10hz-20kHz through a 10MOhm-20MOhm circuit.

[middlehein]

Hey All,

So I am trying to measure some impedance though for the measurement a micropippette is used and that adds 10-30Mohs of resistance to the circuit. Obviously this is a bit tough to try and drive current through. Currently using a howland source I can drive a couple uA through a 1M resistor though I need some thing more. I was considering a push pull howland or some in parallel though I am looking for other options that can do a better job. I have seen some stuff on current mirrors though I don't know much about them and if they would be able to handle the kind of output voltage I am talking about here. any suggestion would be much appreciated as I am new to this type of stuff and dont know what options are out there that can handle these conditions.



Thanks
Matt

 

[FvM]

10 uA * 20 Meg = 200 V.

 

[middlehein]

10 uA * 20 Meg = 200 V.

I am glad to see that our calculations agree thanks for the double check. That is the issue so I am not sure what option are out there that might get me close to being able to doing that. Maybe a high output voltage op amp that may get me closer? I have not found one yet though I am looking.
Thanks

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or how about this if there is a good way to measure tens to hundreds of nV that may alleviate some pressure from the power source, though I think that is at the noise limits assuming that I have very little noise. One option I am considering though am not sure that I want to do is to put the circuit into a Wheatstone bridge as I know measuring across one helps to increase the signal though that will complicate things quite a bit and I still will need to drive a substantial voltage.

 

[crutschow]

Perhaps one of these high voltage amps would work for you.

 

[FvM]

I presume that this is an electrophysiology application. I'm not familiar with present state-of-the-art, I did some support for the electrophysiology guys as a student. Electrode impedance measurement is very likely a standard problem discussed in literature?

I guess, you'll have serious problems to determine a cell membrane impedance with a single 20 Mohm electrode? So what's the standard method in literature?

Using low current and voltage levels for the impedance measurement shouldn't be a problem referring to sensitive lock-in amplifiers, it's more a question of measurement time. I also guess you won't want to apply current levels that are at risc to affect membrane potentials or even cause electrolytic effects?

 

[middlehein]

crutschow, Good find. I checked the page out and unfortunatly I could not look at data sheets for some reason the links were all dead. I am curious to see what the output looks like over my frequency range.

I presume that this is an electrophysiology application. I'm not familiar with present state-of-the-art, I did some support for the electrophysiology guys as a student. Electrode impedance measurement is very likely a standard problem discussed in literature?

I guess, you'll have serious problems to determine a cell membrane impedance with a single 20 Mohm electrode? So what's the standard method in literature?

Using low current and voltage levels for the impedance measurement shouldn't be a problem referring to sensitive lock-in amplifiers, it's more a question of measurement time. I also guess you won't want to apply current levels that are at risc to affect membrane potentials or even cause electrolytic effects?

You are correct this is electrophysiology, though it is cellular elecrophysiology and looking through the literature from what I have seen so far it is vague on this topic. A lot of papers do this just using gold electrodes, though some do use micropippettes such as I will be using but the papers I have read have been quite vague on the equipment. The papers that use just a skin probe or gold probe often uses a howland current source and a inamp. I was trying to do the same though to no luck. I will have to do some more literature digging thats for sure.

Do you have any thoughts on the use of current mirrors?

Thanks

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I should also mention I do have a lockin thats supposed to do nV, I have never tried using it at that level before, but I guess I may soon. I was hoping to use a InAmp to as both a imput buffer(my lockin has a 10M imput resistance), initial gain implementation and CMMR.