instrumentation amplifier - Rectification of Micro DC voltage levels?

I am building a payload for a high-altitude Balloon launch in the club I started which will be measuring DC voltages (electric field) at different altitudes using a wire dipole or two metal plate formation.

I obviously need a low noise amplifier to even register any data in the logger, and we are not worried about polarity, only magnitude.

To do this, I am thinking an instrumentation amplifier fed from the metal plates to the data-logging device, but I am facing a problem, how can I assure the DC amplifier is only measuring positive values? Even some good diodes have a few hundred millivolt drop, which would not be suitable for the experiment. A transformer would not work because this is only DC we are working with (even if it was AC, the high input impedance requirement would not be fulfilled).

Any ideas on rectification or ways to amplify the small DC signals which we can't be 100 percent sure the polarity is constant.

Answer

That won't work. It may detect ion flows and AC fields above a certain frequency, but it can't detect DC fields. After all, the plates constitute a coupling capacitor in series with the fields you want to detect. It's a high-pass filter.

The usual instrument for this is called a "field mill," where the detector plate is placed close behind a grounded metal propeller or rotating "sector disk." The rotating disk chops any DC e-fields into low-freq AC, which is easily amplified/rectified/A-toD'd etc. Perhaps even use low-pass filtering and a synchronous detector to reject any unwanted AC.

DIY e-field sensor field mill from Scientific American:

Antenna without the field mill: if your detector plate is 10 picofarad, and the amplifier's input is MOSFET with 100 giga-ohm Z-inp, then you've formed a high-pass filter which rejects all DC below ~1Hz (since RC is 1e11ohm * 1e-11farad) It's possible to use a much higher-Z amplifier, plus a periodic shorting relay that "resets" the detector's input before taking data. But then usually you'll run into unknown humidity leakage across plastic insulators, and a (drifting) high-pass period of ?tens? of seconds. Better to just add a field mill, and measure true DC fields.

(edit) PS. Here's something I've not tried: just build a non-fieldmill e-field antenna with well-known RC time constant of perhaps a few seconds, use high-res A/D input, then massage the numbers to cancel out the RC and restore the DC value. Of course this includes a stage of integration, so any slight errors in zero-adjust would produce a constantly increasing drift. The amplifier's DC-zero would need to be extremely stable across the environmental temperature changes involved.