Overview

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 several models:

• The AH401 series (AH401B, AH401D) and AH501 series (AH501, AH501C, AH501D) picoammeters originally designed by Elettra. They are now sold commerically by CAENels. These devices communicate using TCP or UDP over 100 Mbit/s Ethernet or high-speed serial. They provide 4-channel current measurements at up to 6510 Hz (AH501 series) or 1000 Hz (AH401 series).

• The TetrAMM picoammeter sold by CAENels. This device communicates using TCP/IP over 1 Gbit/s Ethernet. It provides 4-channel current measurements at up to 20 kHz.

• The NSLS Quad Electrometer (called NSLS_EM in this document). This device consists of a 4-channel digital electrometer unit with Ethernet communication. The device provides 4-channel current measurements at up to 2500 Hz. The Sydor SI-EP-B4 is based on this design and is software comptabible.

• The NSLS2 Quad Electrometer (called NSLS2_EM in this document). This device consists of a 4-channel digital electrometer unit with Ethernet communication. The device provides 4-channel current measurements at up to 10,000 Hz. This design is based on a transconductance amplifier and can measure currents into the mA range, unlike the NSLS_EM which is based on a current integration chip and is limited to currents less than about 1 micro-amp. This unit is packaged with a Zynq processing and run this EPICS IOC software internally.

• The PCR4 picoammeter from SenSiC. This device communicates using TCP/IP over 1 Gbit/s Ethernet. It provides 4-channel current measurements at up to 53,000 Hz.

• The T4U electrometer from Sydor. It provides 4-channel current measurements at up to 2,500Hz.

The AH401 series, NSLS_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, TetrAMM, NSLS2_EM, PCR4, and T4U are based on a transimpedance input stage for current sensing, combined with analog signal conditioning and filtering stages. The AH501C, AH501D, and TetrAMM have an integrated programmable bias supply.

The quadEM software includes asyn drivers that provide support for the following:

• Analog input records using the NDPluginStats plugin from the synApps areaDetector ADCore module. This provides digitally averaged readings of the current, sum, difference and position at speeds that are determined by the AveragingTime record described below. The quadEM drivers do the callbacks on the asynGenericPointer interface for NDArray objects required to use the areaDetector plugins. The NDPluginStats module is used to compute the averaged readings. It also provides additional statistics including standard deviation, min, max, and a histogram of values. This averaging is termed the “primary averaging”.

• Analog input records using the asynFloat64Average device support from asyn. These records can either process at the standard EPICS scan rates, normally 10Hz to 0.1 Hz. Beginning with asyn R4-34 they can also use SCAN=I/O Intr, which allows them average and process at any integer divisor of the device sampling rate. This averaging is termed “fast averaging”, though it does not necessarily run faster than the primary averaging described above.

• The NDPluginStats provide time-series of the statistics. These can be used to for on-the-fly data acquisition applications, where the meter is triggered by an external pulse source, such as one derived from a motor motion.

• One could also use the standard asynFloat64 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 sampling times.

• Streaming data to disk at the full rate from the device. This is done using the file plugins from ADCore.

• Time series data (like a digital scope) of the current, sum, difference and position at speeds up to 20000 Hz (TetrAMM), 6510 Hz (AH501 series), 1000 Hz (AH401 series) 2500 Hz (NSLS_EM, T4U). The data is available in standard EPICS waveform records, using the NDPluginTimeSeries plugin from ADCore. 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. This uses the NDPluginFFT plugin from ADCore.

The following manuals provide detailed information on these devices:

• AH401B Users Manual

• AH401D Users Manual

• AH501C Users Manual

• AH501D Users Manual

• TetrAMM Users Manual

• T4U Operators Manual

The support is based on asynNDArrayDriver from ADCore, which in turn is based on asynPortDriver. It consists of a base class drvQuadEM, which is device-independent. There are device-dependent classes for the TetrAMM, AH401 and AH501 series, NSLS, NSLS2, PCR4, T4U, and softQuad electrometers.

The quadEM driver works as follows:

• The data from the device-dependent drivers are first placed into a ring buffer whose size is defined in the constructor and configuration function. The driver provides 4 current readings. 7 additional values are computed from these 4 values: SumX, SumY, SumAll, DiffX, DiffY, PositionX, PositionY. The meanings of X and Y are discussed in the geometry section below.

• The PV called AveragingTime determines the time period over which to average the readings. The AveragingTime divided by the SampleTime determines the number of samples to average, NumAverage_RBV. When this number of samples have been accumulated in the ring buffer a separate thread copies them to a set of NDArrays and calls any registered plugins.

• There is a separate NDPluginStats plugin loaded for each of the 11 data values. This plugin receives an array of dimensions [NumAverage_RBV]. This plugin computes not only the mean, but also the standard deviation, histogram of values, etc.

• One of the NDArrays contains all of the data values, and has dimensions [11,NumAverage_RBV]. This array can be passed to any of the file writing plugins, which can thus stream all of the data to disk for arbitrarily long time periods.

• The [11,NumAverage_RBV] NDArray is also passed to the NDPluginTimeSeries plugin which is used to generate time-series arrays.

• The time-series plugin output is used by the NDPluginFFT plugin to compute FFTs, producing arrays containing the frequency power-spectrum of the time series data.

• The commonPlugins.cmd from ADCore is also loaded, which provides ROIs, file plugins and many others. The ROI plugin can be used, for example, to extract just the 4 currents from the [11, NumAverage_RBV] array and pass the [4, NumAverage_RBV] NDArrays to the HDF5 file writer.

• The NDPluginStdArrays, and NDPluginPVA plugin to pass all the data [11,NumAverage_RBV] or any of the individual data arrays to any channel access or pvAccess client.

• The computationally intensive work of calculating the statistics is done in plugins, so can be done in different threads (if CallbacksBlock is set to No), each potentially running in a separate core on modern CPUs.

• In addition to placing the data from each time point into the ring buffer, the driver does callbacks on the asynFloat64 interface for each data value. This is used for fast averaging support with ai records. These records can be processed periodically with SCAN=1 second, 0.1 second, etc., or they can have SCAN=I/O Intr. In this case the ai record device support sums the callback readings until NumAverage readings have been received, at which point the average is computed and the record is processed. he quadEM database computes NumAverage from the FastAveragingTime record and writes NumAverage to the .SVAL field in each ai record.

Note: This version of the driver requires a minimum firmware version on some models

• AH501D Firmware version 2.0.

• TetrAMM Firmware version 2.9.11

Prior to R5-0 the quadEM driver assumed the following geometry for the 4 currents:

   4

1     2

   3

The PV for the computed quantities were called Sum12, Sum34, Sum1234, Diff12, Diff34, Position12, and Position34. The differences, and hence positions, were computed as 2-1 and 4-3, which would correspond to Position12 being positive to the right and Position34 being positive up in the above diagram.

For R5-0 and later two geometries are supported, and the names of the Sum, Diff and Position PVs were changed. The computed quantities are called SumX, SumY, SumAll, DiffX, DiffY, PositionX, and PositionY.

The first geometry is the same as that illustrated above, and is called Diamond. For this geometry only 2 diodes are used for the position calculation in each direction. SumX=(1+2), SumY=(3+4), DiffX=(2-1), DiffY=(4-3). This geometry is identical to the geometry assumed prior to R5-0. The X diodes are 1&2 rather than 1&3 so that it is possible to use just the first 2 inputs on the AH501 series to increase readout speed in cases where all 4 diodes are not used. This would not be possible if diodes 1 and 3 were used for the X calculations.

The second geometry is:

1     2


4     3

This geometry is called Square. For this geometry all 4 diodes are used for the position calculation in each direction. SumX=SumY=(1+2+3+4), DiffX=(2+3)-(1+4), DiffY=(1+2)-(3+4).

R9-5 added a variant of the square geometry, SquareCC:

1     4


2     3

• It is similar to Square, but the 4 diodes are arranged counterclockwise. SumX=SumY=(1+2+3+4), DiffX=(3+4)-(1+2), DiffY=(1+4)-(2+3).

For all geometries SumAll=(1+2+3+4), PositionX=DiffX/SumX, and PositionY=DiffY/SumY. X positive is to the right and Y positive is up for both geometries.