quadEM

This page is the home of quadEM.

quadEM supports quad electrometers/picoammeters, typically used for photodiode-based x-ray beam position monitors, or split ion chambers. They can also be used for any low-current measurement that requires high speed digital input. There is support for two models:

1. The AH401B, AH401D, AH501, AH501C, and AH501D picoammeters originally designed by Synchrotron Trieste (elettra). They are now sold commercially by CAENels. These devices communicate using TCP/IP over 100 Mbit/s Ethernet or high-speed serial. They provide 4-channel current measurements at up to 1000 Hz (AH401 series) or 6510 Hz (AH501 series).
2. The Quad Electrometer built by Steve Ross from the APS (called APS_EM in this document). This device consists of a 4-channel digital electrometer unit and 2 VME boards. The device provides 2 readings per diode at up to 813 Hz.

The AH401 series and the APS_EM are based on the same principle of an op-amp run as a current amplifier with a large feedback capacitor, and a high resolution ADC. The AH501 series are based on a transimpedance input stage for current sensing, combined with analog signal conditioning and filtering stages. The AH501C/D have an integrated programmable bias supply.

The quadEM driver derives from the asynNDArrayDriver base class from the EPICS areaDetector module. The driver collects the current, sum, difference and position readings during a user-defined "averaging period" and puts the time series of readings into a number of NDArray objects. The NDArrays are passed via callbacks to any NDArray plugins. These plugins include the

• NDPluginStats statistics plugin, which computes the mean, min, max, historgram of values, etc. for this time series.
• NDPluginFile plugins (e.g. NDFileNetCDF, NDFileHDF5, etc.) which can stream all of the time-series data to disk.
• NDPluginStdArrays which puts the data into an EPICS waveform record for use by any Channel Access client.

The quadEM software includes asyn drivers and an SNL program that provide support for the following:

• Analog input records that provide digitally averaged readings of the current, sum, difference and position. These are computed using the NDPluginStats statistics plugin, which also provides additional statistics. One could also use the standard asynInt32 device support with ai records and scan=I/O Intr, in which case the record would update with every reading from the device. This is likely to overwhelm the EPICS IOC if the electrometer is run at very fast integration times.
• Time series data (like a digital scope) of the current, sum, difference and position at speeds up to 6510 Hz (AH501 series), 1000 Hz (AH401 series) or 813 Hz (APS_EM). The data is available in standard EPICS waveform records, using the drvFastSweep asyn driver from the synApps "mca" module. The time per point can be greater, in which case it does averaging.
• FFTs of the time series data, providing the power spectrum of each signal as another EPICS waveform record.
• epid (fast feedback). This provides fast feedback via an asyn D/A converter (e.g. dac128V) also at speeds up to 6510Hz, 1000Hz or 813Hz. If it is run slower it does signal averaging. This support is provided in the synApps "std" module. The quadEM drivers do the callbacks on the asynFloat64 interface required to use the epics fast feedback device support.

Please email any comments and bug reports to Mark Rivers who is responsible for coordinating development and releases.

Where to find it

You can download the software from the links in the table below:

Required Modules

The quadEM support typically uses the synApps modules mca (for digital scope mode), std (for the EPID fast feedback support), and ipUnidig for interrupt support .  But the quadEM module does not actually depend on these other modules to be built.

Installation and Building

After obtaining a copy of the distribution, it must be installed and built for use at your site. These steps only need to be performed once for the site (unless versions of the module running under different releases of EPICS and/or the other required modules are needed).

1. Create an installation directory for the module, usually this will end with
   
   .../support/
   
2. Place the distribution file in this directory. Then issue the commands (Unix style)

   gunzip quadEMRX-Y.tar.gz
   
   tar xvf quadEMRX-Y.tar
   

   where X-Y is the release.
3. This creates a <top> application.
   

   .../support/quadEMRX-Y
   

4. Edit the config[ure]/RELEASE file and set the paths to your installation of EPICS base and to your versions of other required modules.
5. Run gnumake in the top level directory and check for any compilation errors.
6. Please email Mark Rivers so that a record can be kept of which sites are using this software.

In Use

This software was originally developed by Mark Rivers.

• ANL/APS : In use at CARS and a number of other beamlines.