I have made a mistake, and perhaps led you all astray.
It is true that in the Axopatch, the signal ground is not connected to the power ground.
But, it turns out that it's very difficult to isolate power ground from signal ground, without essentially breaking every BNC connection between the amplifier and the digitizer. And when I did that, I got a good amount of hum pickup from those now unshielded wires. At times you could find a position where the pickup was minimal, but it wasn't easy. And figure that you've got at least 3 connections at a minimum (signal output, analog input, and gain), that all equals one big pain in the butt.
This advice originated when the digitizers were the old Digidata 1200. That digitizer is specifically designed to isolate the signal ground from the power ground (most important to isolate it from the computer, whose fast switching power supply is a huge noisemaker). That all changed in the 1300 line of Digidatas.
However, it does seem that the simple act of using the grounded to non grounded plug adapter on the amplifier, does dramatically reduce the RMS noise level. Why it exactly does this, when there's still a connection between signal ground and power ground, I am not clear on.
Nevertheless, I am available to come and lay my hands on your setups, to reduce the noise. Cause it seems I'm back to that level of rationality for this whole process.
Gotta bring this up with the big guy soon.
Thursday, November 19, 2009
Wednesday, November 18, 2009
Tuesday, November 17, 2009
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?
- bent piece of metal that can be put right in front of headstage/chamber to further shield (not shown)
What goes on power ground?
- Faraday cage
- air table surface
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!