A Tale of Two Grounds

It was the best of rigs, it was the worst of rigs,

It was the age of 0.07 pA RMS, it was the age of 0.5 pA RMS, ±5 pA p-p at 60 Hz,

It was the epoch of separating signal ground from power ground, it was the epoch of connecting all power grounds to the signal ground,

It was one really annoying run on sentence written by some old white dude, it was a kick ass blog post written by a middle aged white d00d.

There's one thing I've learned here as postdoc that I've actually never seen discussed in other places. When I first heard it, I did a "Watch you talkin' about" head turn, thinking it was crazy.

That thing is, to get really good noise on your patch clamp rig (good meaning low here), you can make 2 grounds. One ground connects everything that is physically close to the headstage. This gets connected to the signal ground plug. The second ground connects everything else. That gets connected to power ground.

Figure 1: Nat's newly refurbished set up (which is tight if I do say so myself). Axopatch 200B and Sutter MP-285. Green As indicate things attached to signal ground. Magenta Bs are connected to power ground.

So what goes on the signal ground?

• Microscope
• chamber
• condenser
• bent piece of metal that can be put right in front of headstage/chamber to further shield (not shown)

That all gets connected to the gold pin at the back of the headstage (or equivalently at the signal ground input on the back of the amplifier). Note that you'll need to break the power ground connection (with a 3 to 2 adaptor). Doing this will get you most of the way there.

What goes on power ground?

• Faraday cage
• air table surface
• manipulator

This is connected to power ground (either attach it to an exposed copper pipe or to power ground through the case of the manipulator). Doing this will get you to the super low RMS values specified in teh Axon manual.

Now I no longer fear denoiseing the setup.

The only tricky things I've run into are that most of the BNC inputs on the Digidata are connected to power ground. So usually I have to break the ground connection to connect the gain value output from the amplifier to the digitizer input (I've never had the amplifier signal output to digitizer analog in cause this).

I just turned on my amplifier, and with a pipette holder on, the PATCH mode RMS noise is 0.099, and WHOLE-CELL mode is 0.48. SHU-WEET!