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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 4 2IMPLEMENTATION OF TRANSIMPEDANCE ANALOGFRONT – END CARD FOR LOS ALAMOS NEUTRON SCIENCE CENTERACCELERATOR WIRE SCANNERS *D. Rai†, H. Leffler, S.A. Baily, D. Zimmermann, L. Montoya,
L. Kennel, D. Martinez, J. Duran, A. Braido
Los Alamos National Laboratory, Los Alamos, NM, USA
Abstract
The Los Alamos Neutron Science Center’s (LANSCE)
Accelerator Operations and Technology division - Instrumentation and Controls (AOT-IC) team executed a projectthat implemented a new Analog Front-End (AFE) card fortheir wire scanner’s Data Acquisition (DAQ) system. The
AFE accommodates the signal amplification and noise reduction needed to acquire essential measurement data required for beam diagnostics for LANSCE accelerator. Wire
Scanners are electro-mechanical beam interceptive devices
that provide cross-sectional beam profile measurement
data fitted to a Gaussian distribution that is then extrapolated to provide beam shape and position. The beam shapeand position information allow the operator to adjust parameters such as acceleration, steering, and focus to provide optimized beam delivery to targets. The project included software and hardware implementation that eliminated the dependency on legacy systems and consolidated
various AFE designs for diagnostics systems into a single
design with 11 gain settings ranging from 100 nA to 40 mA
at 10 V full scale to accommodate its future applications onother diagnostics systems.

When the beam impinges on the wires, current resulting
from the emitted secondary emission yield is deposited on
the wire; this current is proportional to the beam current [1]. Since the wire’s position is scanned in the beampipe, the wire scanner provides the transverse vertical andhorizontal profile measurements with a Gaussian fit, asshown in Fig. 2.
The data from the wire scanner is obtained by hardware
and software implementation of the QAC (Quad ActuatorController) DAQ, National Instruments – CompactRIO(cRIO) Systems. The QAC DAQ cRIO system controls
movement and acquires data. The user interface JAVA program analyzes and displays the result of the received scandata [2]. The AFE card is the component of the DAQ thatacquires beam-impinged secondary electron current flowfrom the wires. The AFE is a compact Peripheral Component Interconnect (cPCI) form factor card supporting 16channels, assuming one channel for each axis, supportingeight wire scanners per card.

BACKGROUND
The LANSCE facility utilizes various beam diagnostics
devices for operators to adjust beam parameters such as acceleration, steering, and focus to provide efficient and effective beams to all five experimental areas. Wire scanners
are interceptive beam diagnostics devices with a mechanical actuator assembly and a single or pair of Silicon Carbide (SiC) or Tungsten (W) wires (sensor) mounted vertically and/or horizontally to the sensor fork mount as shownin Fig. 1.

Figure 2: LBEG (Long Bunch Enabled Gate) Beam scan ofTRWS07 Wire Scanners.

Figure 1: Wire Scanner and Sensor Fork Assembly.
___________________________________________* This work was supported by the U.S. Department of Energy throughthe Los Alamos National Laboratory. Los Alamos National Laboratoryis operated by Triad National Security, LLC, for the National Nuclear
Security Administration of U.S. Department of Energy (Contract No.
89233218CNA000001). † drai@lanl.gov, LA-UR-23-30149

During the implementation of the QACDAQ system forwire scanner systems in the transition region and downstream areas, the LANSCE Harp AFE was used. TheLANSCE Harp AFE full scale range is from 3 µA to270 µA and was not sensitive enough for typical Wire scanner application [3]. As shown in Table 1, LBEG is a higher
current beam with 50 times greater macro pulse average
current and 100 times greater charge per pulse than MPEG
(Micro Pulse Enabled Gate) [4]. Therefore, the LANSCEHarp AFE was adequate in performing wire scanner scansof the LBEG beam but not for MPEG.

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09 Data Acquisition and Processing Platforms12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3

Table 1: LBEG and MPEG Comparison
Descriptions

LBEG

MPEG

Pulse Spacing

358 ns1800 ns

Pulse Duration

290 ns

20 ns

Pulses Per Macro
Pulse (625us)

~1746

~347

Average Current

100 µA

2.0 µA

Charge/Pulse

5.0 µC.05 µC

Protons/Pulse

13

3.1x1011

3.1x10successful use of the Thorlabs, PDA200C - Benchtop Photodiode Amplifier during testing under nominal beam
delivery conditions, we gained assurance of the effectiveness of the TIA design approach.
The TIA circuit in Fig. 3 presents near 0 voltage and impedance to the current source, maximizing signal transferfrom a high-impedance current signal source. Due to thischaracteristic and with a known voltage output requirementof +/- 10 V and the desired current range for each gain stage
shown in Table 3, the values of feedback resistors, Rf, canbe calculated using Eq. (1).

NEW DESIGN
Design Requirements
Table 2: Design Requirements
ID
1Requirement
Same Form and Fit as the LANSCE Harp
AFE with 16 input channels

2

LBEG and MPEG beam scans capable

3+/-50 VDC Bias Input Capability4Gain selection options for user optimization
which cover expected beam currents

5Protected Input

6Greater than +/-5 VDC Full Scale Output

Dynamic Range

Figure 3: TIA Circuit
Table 3: New AFE Gain Stages
Gain#IMin

IMax

VMin

VMax

fp00100 nA

-10 V

+10 V

1.6 kHz

10500 nA

-10 V

+10 V

1.6 kHz20

2.5 µA-10 V+10 V

1.6 kHz30

12.5 µA

-10 V

+10 V

1.6 kHz

40

10 µA

-10 V

+10 V

6.6 kHz

5050 µA-10 V+10 V

6.6 kHz60250 µA

-10 V

+10 V

6.6 kHz701.25 mA

-10 V

+10 V

6.6 kHzThe dynamic range for expected induced current had tobe determined to design a new AFE. Induced sensor current
measurements of strategic wire scanner locations were performed for LBEG and MPEG beam during beam development time using an off-the-shelf Thorlabs, PDA200C Benchtop Photodiode Amplifier [5]. We determined therange of induced current for the wire scanners to be between 300 nA and 10 µA, giving us a better understandingof potential gain ranges for the new AFE design.

805 mA-10 V+10 V6.6 kHz

9020 mA-10 V+10 V

6.6 kHz

Bandwidth

10040 mA

-10 V+10 V

6.6 kHzAt LANSCE, two-beam pulse widths are used: a pulse
width of 625 µs (1.6 kHz) is typically used for the production beam with repetition rate of 120 Hz, and 150 µs(6.6 kHz) is generally used during beam tuning efforts withrepetition rate of 4 Hz.

Design Approach
It was necessary to use an amplifier scheme that accepts
current to amplify low current signal accurately and stably
from the wire scanner wires to a usable voltage level. Transimpedance amplifier (TIA) circuit design is typically used
to convert and amplify low current output of sensors suchas Photodiodes to a proportional output voltage. With the𝑉= −𝐼 𝑅 or 𝑅 =(1)At higher input frequencies, fp, it is important to ensure
the feedback network doesn’t interact with the open loop
gain of the op-amp to cause instability. Without compensation, a frequency causes a gain of 0 dB and complete phasereversal to the input, causing positive feedback. Even a tiny
input capacitance results in stability issues. Parasitic capacitance in the circuit is unavoidable; therefore, adding afeedback capacitor, Cf, in parallel with a feedback resistor,Rf, as shown in Fig. 4, is crucial for compensating these
undesired effects [6].
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12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 4 2

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Design Challenges
The design's main challenge was finding an operational
amplifier (op-amp) with low bias current and high GBWP
to meet the gain and bandwidth requirements of all the gainstages listed in Table 3. We narrowed our selection to theTexas Instruments, THS4631- High-Speed FET-Input Operational Amplifier based on performance, cost, and availability. It was the only op-amp available with GBWP over
200 MHz that supported +/-12 VDC dual input supply railthat was available at the time of the development.
Figure 4: TIA Circuit Layout with Feedback Capacitor.
Figure 5 shows the transient response of how an uncompensated TIA circuit is marginally stable with substantial
ringing, causing the output to overshoot. On the other hand,Fig. 6 represents a compensated TIA transient response,
which is stable with a clean step response.

Final Design Discussion
Figure 5 shows the simulation result for Gain 0, which
required the highest gain of 100 M, and the transient response is stable and has a clean step response when excitedwith 100 nA. Figure 7 shows the final fabricated assemblyand Fig. 8 shows the final single-channel layout of the TIA
circuit of the new AFE design. The design includes 11 selectable gains divided between seven high and four low
bandwidths, shown in Table 3.

Figure 5: Transient Response of Uncompensated TIA.

Figure 7: New TIA AFE Assembly.

Figure 6: Transient Response of Compensated TIA.
Equation 2 can be used to calculate Cf, using the computed value for Rf and fp given in Table 3 for each gainstage. Lastly, to ensure that the Gain Bandwidth Product(GBWP) of the op-amp is sufficient for each gain stage
configuration, Eq. (3) can be used. Cin in Eq. (3) is the total
input capacitance, which includes the capacitance for theinput cable and sensor along with amplifier common modeand differential mode capacitance. The calculated GBWP
(right-side) must be less than the GBWP of the op-amp;
otherwise, it will lead to instability and a non-linear response of the TIA output [7].
𝐶𝐺𝐵𝑊𝑃

(2)
∗∗

(3)

The gain stages 0 to 3 for low bandwidth and gain stages
5 to 10 for high bandwidth are designed to have 20% overlap higher to lower gain stage to ensure continuity between
gain stages. This overlap helps in the case of a user experiencing saturated voltage output at selected scan gain selection, then lowering the gain still provides overlap to better
capture the signal within an acceptable range.
During beam development, we had issues with unknown
transient voltage damaging the TIA inside the Thorlabs,
PDA200C - Benchtop Photodiode Amplifier. Hence, tomake the design robust, we added a dual diode, D1, asshown in Fig. 8, to protect the amplifier from overvoltage.
The current limiting resistor, R26, is also placed at the noninverting input of the amplifier, as shown in Fig. 8.
A low pass filter with a 6.6 kHz corner is added to improve noise attenuation. Gain stages 8, 9, and 10 wereadded later to accommodate other beam diagnostics systems using the same QAC DAQ system.
Even though the dynamic range for the sensor current
was between 300 nA and 10 µA, we added gain 0 covering
the 0-100 nA current range to have the capability to measure lower current for additional diagnostics purposes.

WEP042444

09 Data Acquisition and Processing Platforms12th Int. Beam Instrum. Conf.
ISBN: 9 7 8 - 3 - 9 5 4 5 0 - 2 3 6 - 3Figure 8: Single Channel Layout of New Transimpedance AFE.
Also, with additional gain stages out to 40 mA full scale,
future efforts can be made to convert all diagnostics systems to the new Transimpedance AFE, streamlining and reducing the life-cycle management cost of maintaining configuration. Additional opportunities exist to use the same
design scheme for other DAQ systems but in a different
form and fit.

REFERENCES[1] E. Sternglass, “Theory of Secondary Electron Emission byHigh-Speed Ions,” Phys. Rev., vol. 108, no. 1, Oct. 1957.
doi:10.1103/PhysRev.108.1

Figure 9: MPEG Beam scan of LDWS3X and LDWS3Y
Wire Scanners.

CONCLUSION
During the V&V (Verification and Validation) of the newAFE, hardware and software implementation it showedthat the new Transimpedance AFE design for the DAQ system meets all the design requirements listed in Table 1. Figure 2 and Fig. 9 show successful wire scanner scans withLBEG and MPEG beam taken during beam development
time, respectively. Both scans show a good Gaussian fit
with a clear beam center and size for horizontal and verticalaxes providing essential data for beam tuning. The new design has adequately addressed design and functional limitations of the LANSCE Harp AFE providing a dynamic
gain range to measure the induced sensor current with accuracy and stability.

[2] L. Montoya, “LANSCE QAC/DAQ Wire Scanner Instrumentation Upgrade,” presented at the 12th Int. Beam Instrumentation Conf. (IBIC’23), Saskatoon, Canada, September 2023,
paper WEP029, this conference.
[3] J. Sedillo et al., “LANSCE Isotope Production Facility Emittance Measurement System,” in Proc. IBIC’16, Barcelona,Spain, Feb. 2017. pp. 548-550.
doi:10.18429/JACOW-IBIC2016-TUPG79[4] B. White, “AOT-OPS Accelerator Pulse Structure,” unpublished.
https://www.thorlabs.com/thorprod[5] Thorlabs,
uct.cfm?partnumber=PDA200C[6] UltimateElectronics, Op-Amp Transimpedance Amplifier
https://ultimateelectronicsbook.com/op-amptransimpedance-amplifier/[7] Texas Instruments, Transimpedance Amplifiers (TIA):
Choosing the Best Amplifier for the Job
https://www.tij.co.jp/jp/lit/an/snoa942a/snoa
942a.pdf

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IBIC2023, Saskatoon, Canada
JACoW Publishing
ISSN: 2 6 7 3 - 5 3 5 0
d o i : 1 0 . 1 8 4 2 9 / J A C o W - I B I C 2 0 2 3 - W E P 0 4 2