Request for help with Passive VLF front end filter design

[philwinder]

Hi, im designing an electrostatic reciever that works from approx. 0.1 - 15 Hz but im obviously getting loads of mains noise.
I have some simple active filters after the initial gain stages, but is there a way to filter the noise before the initial gain stage?
Normally this should be ok, but heres the snag: the receiver has to have a very high input impedance (trying to read fA) and the only way i can figure out how to do this is for the filter NOT to have a connection to ground, i.e. all the resistances/impedances add up in series and do not affect the overall Tohm input impedances. Is there a passive filter out there that does not need a ground connection?
It can either be a 50Hz notch, or a 20Hz low pass.

Many thanks,

Phil Winder

[audioguru]

Hi Phil,
Welcome to our forum.
There atre ECG circuits on the web that measure a patient's heatbeat at about the same frequencies you need and the patient picks up lots of mains hum.
The circuits use an instrumentation opamp that cancels the same signal at both its inputs much better than the differential inputs of ordinary opamps.

Here is one example of many ECG circuits in Google:

[philwinder]

Hi audioguru, thanks for the reply,
Thats a good circuit, but not quite what I want.  I am investigating the static field produced by a human body and this prevents a reference point.  The ECG example takes a reading from one arm, and references it against another.  Because these static fields I am investigating occur everywhere I have nowhere to take a reference from.  What does interest me though, is the filter on the output reference of the instrumentation amp.  I have seen this before, but only in a 1st order form.  This looks to be a second order filter.  I will look into this further.

Many thanks

Phil Winder

[audioguru]

A static field on somebody is a DC voltage with respect to the earth, isn't it? Then you need all the filtering you can get so that the probe doesn't pickup mains hum.

How about having another probe that picksup only mains hum and is used to cancel hum from the other probe?

[philwinder]

Your exactly right, you 'collect' static electricity from the environment and your movements within.
I did go down the route of having another probe, mainly either a connection to true earth and/or a probe to pick up the mains.
Unfortunately that would mean placing a probe within the proximity of the device, but not to receive the static field from the human.  This isnt very practical since the second probe would have to be 5m's away, and theres no guarantee that the mains noise would be the same 5m's away.
The only other way is to shield that probe from the human disturbances, but obviously, that would shield the mains hum too... You see my dilemma.
So the route im taking is to try and filter out as much mains noise as possible. And this is at the input stages.
Someone else on another forum mentioned a lattice network, but I have not heard of and cannot find any information on the subject.

Cheers,

Phil Winder