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Subject: Re: analog circuit design for ion mobility spectrometer

Date: 02/16/03 at 10:00 PM
Posted by: Ravindra
E-mail: puthumbaka@hotmail.com
Message Posted:

In Reply to: Re: analog circuit design for ion mobility spectrometer posted by Russ Kincaid on 02/16/03 at 8:54 PM:

Hello Russ,
IMS tube has a homogeneous electric field, about 500V/cm. Ions travel down the voltage gradient towards the detector at the end of the tube. IMS operates at atmospheric pressure and elevated temperature (anywhere from 100-250 C)

The IMS tube is made up of a stack of evenly spaced ring electrodes that are connected by resistors in series and physically insulated from one another with a ceramic ring in between. A high voltage is place on the first ring electrode. Subsequent ring electrodes will receive fewer voltages because of the resistors. There is a faraday plate at the end that picks up current signal as ions slam into it.

A gate is used in IMS to deliver a short pulse of ions through. Gate is located a third down the tube approximately. An Aperture Grid is the last electrode in IMS tube, the one before the
detector. It's a ring electrode with parallel wires (same as gate electrode). The aperture grid is grounded through a capacitor with resistor, this effectively decouples the current detected from the approaching ions. The Detector used is faraday cup. It's a metal plate. Analog signal from this needs to be amplified and converted to digital signal.
I tried searching for an appropriate amplifier circuit, but couldn't find one. The current signal from the detector would be of the order of pA-nA. This should be amplified to 0-5 V. I guess that a log amplifier would be appropriate but I couldn't find one. I hope someone would help me with the design.
thanks,
ravindra




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