Thursday, November 19, 2009

Strike that....substitute silence.

Dear all,

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.

5 comments:

Dr. Jekyll and Mrs. Hyde said...

level of rationality

Yeah, "rig noise" and rationality don't seem to go hand-in-hand....

Nat Blair said...

*sigh* What a pain.

In other news, checking the RMS noise with the 200B and headstage cooling on requires some time. Even though the TEMP setting on the meter might settle, I think leaving it on longer can lead to reduced noise.

Checking it right after turning on the amp gave something like 0.131 pA RMS. That was 0.075 after about 20 minutes. THen after leaving the amp on all weekend, I come in today and it's 0.055.

Though, it could be the goat I sacrified to Electrophysiologicus this weekend. Hard to tell with n=1

DSK Samways said...

Was it a black goat with one blue eye and one black eye, and born under a waxing moon?

Because when I burn the entrails of one of those bad boys, the noise on my single channel currents are tight. At least, they are until the new moon wanes and Taurus is rising in Saturn.

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