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Current clamp circuit with capacitance compensation

Posted: Fri Jul 28, 2017 1:04 pm
by finesniper
I am trying to create a current clamp circuit with bridge balance and capacitance neutralization. Luckily I was able to get a version of older demos, including init_r_comp and init_rc_comp3. Further on, my questions will be about init_rc_comp3, as I was unable to fully understand how this circuit works. First of all, when injecting a current to the cell, the current will come from two places, first, passing through the pipette RC, and second, passing through the ... which will create a compensatory waveform to compensate for the distortion through the pipette RC. However, I can not figure out where this current comes from. This would be interesting to know, as the capacitive artefact during perfect bridge balance compensation (Rs = 10 MOhm) has and opposite polarity as I would expect. Second, why does Af amplifier has a gain of 5? Woudn't that amplify the whole measured potential waveform by 5?

Thank you,

Re: Current clamp circuit with capacitance compensation

Posted: Tue Aug 01, 2017 12:38 pm
by ted
Where did you find the files to which you refer? Did they come with a readme.txt or notes.txt file, or do any of them contain informative comments?
passing through the ...
Passing through the "..."? What's a "..."?

Re: Current clamp circuit with capacitance compensation

Posted: Fri Aug 04, 2017 10:40 am
by ted
ted wrote:Where did you find the files to which you refer?
In order to answer your question, it will be necessary for me to execute the code that you are referring to. I can do that only if I know where to find it, or if you can email it to me
ted dot carnevale at yale dot edu

Re: Current clamp circuit with capacitance compensation

Posted: Mon Aug 14, 2017 10:58 am
by ted
After digging through old archives I found what may have prompted your question--a collection of models used to demonstrate features of the Linear Circuit Builder. Some of these date back to 2002. One of them was a circuit that compensated for electrode capacitance and series resistance, and sure enough its main file was called init_rc_comp3.hoc. Here is the source code for this particular model, updated slightly for better initialization, including explanatory nodes and a couple of figures ... and here is its equivalent circuit

The model cell it uses is a ball and stick approximation to the charging properties of the pyramidal cell that comes with NEURON's demo program; the specification of the model is

Code: Select all

soma { nseg=1  L=60.6576  Ra=160
        insert hh { gnabar_hh=0.12 gkbar_hh=0.036 gl_hh=0.0003 el_hh=-54.3}
dend { nseg=37  L=1632  Ra=160
  diam = 4.3
        soma connect dend (0), 1
        insert pas { g_pas=0.001 e_pas=-65}
The circuit uses an equivalent T approximation for the properties of the electrode. Adjust the gain of amplifier A2 to compensate for series resistance, and adjust the gain of Af to compensate for electrode capacitance. The time constant of the series combination of Cf and Rf must be the same as the electrode time constant (Ce Re / 4) to achieve good capacitance compensation.

The potentials at the ungrounded ends of Ce and Cf must be properly initialized, or spurious current will be injected at t = 0, causing large voltage transients and possibly a spike as well. The correct initializations are

Code: Select all

Vx    v_init
Ve    v_init
Vy    5*v_init (i.e. gain of A1 * gain of Af * Ve)
Vo    v_init
Assuming that this is the circuit that you're asking about, here are the answers to your questions.

The current that compensates for electrode capacitance is produced by the output of Af driving the series combination of Cf and Rf. Notice that the gain of A1 is 1, so Af must have gain Gf > 1 in order for positive feedback to occur.
why does Af amplifier has a gain of 5?
For this circuit's parameters, higher gains result in ringing or oscillations, and lower gains result in delay and attenuation of Vo compared to Vs. Ringing and oscillations occur if open loop gain G1 Gf > Rf/(Re/2) (G1 and Gf are the gains of A1 and Af, respectively); in this circuit, Rf is 25 megohms and Re/2 is 5 megohms.