Experimental Physics and
Industrial Control System

"Mark Davis" <[email protected]> · Tue, 27 Nov 2007 07:28:11 -0500

We have an in-house design we use extensively for a 2-channel Beam Current
Monitor (BCM) that has 8 ranges from +/- 100 picoamps to +/- 1 milliamp
(those are full-scale values - 12-bit ADC's are used, so divide by 4096 for
a rough idea of sensitivity).

The accuracy at the most sensitive ranges is not great, but at least it
gives you a good idea how much change has occured.
The units contain a ZWorld/Rabbit BL2105 Single Board Computer with a 10
Mbps ethernet. The interface to EPICS is via an Open ModBus/TCP connection.
Kelly Davidson ([email protected]) is the engineer who has done most the
design work on these if you are interested in more details (I did the
programming).
Not sure what the university or the lab's general stance is on our in-house
work or how much we can share without major hassles (or if they might be
interested in selling the device to others).
Some contacts if you are interested:
John Vincent ([email protected]) - my boss

Kelly Davidson ([email protected]) - The engineer responsible for most
of the design

Mark Davis (me) ([email protected]) - I wrote the software and the latest
requirements/specs document
Mark Davis
Control Systems Software Engineeer
National Superconducting Cyclotron Laboratory
Michigan State University
----- Original Message -----
From: "David Dudley" <[email protected]>

To: <[email protected]>

Sent: Monday, November 26, 2007 12:40 PM

Subject: Re: current measurement - hardware
Might look into something like a Measurement Computing PCI2517 card,
which has 8 differential analog inputs.
Looking at the spec sheet, it says it has a 40pA bias current, and
20Meg input impedance, while measuring a 100mv signal.  With proper load
resistors (if you even need them at these low currents), you could
probably measure currents in the Ua range with no problem.
David Dudley
Heinrich du Toit <[email protected]> 11/26/2007 8:25 AM >>>

Hi guys
I'm looking for some information on how you solve this problem...
We need to measure currents.. from harps and slits and faraday cups.
All of this gives of currents that is important.
Some of the currents we measure with keithleys but it is way to
expensive to buy keithleys for harps as you will need 100's of them :)
We need to measure currents between nA and uA range for harps and
slits.
For faraday cups this sometimes goes to 100+ uA but our main problem is
harps/slits.
I hope that we are not the first to have this problem, is there some
cost effective method to solve this problem? That can give descent
results.
The problem can basically be given as:
How do I measure 50+ current inputs (harp = 96, 4 per slit-set) across
a
wide range (2nA fullscale = 50pA sensitive up to 100uA full scale)
still
have resonable accuracy especially between the lines and have a descent
update rate and not 2 high noise levels.
(I guess noise can to some extend be solved with a software filter if
updates are high enough)
Thanks for your support :)
-Heinrich

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System