MA-890 Charge DigitiserMA-890 Charge Digitiser INCORPORATING THE ATC SERIES 170 CURRENT-TO-FREQUENCY CONVERTER Microanalyical Research Centre School of Physics University of Melbourne

MA-890

Charge Digitiser

INCORPORATING THE ATC SERIES 170

CURRENT-TO-FREQUENCY CONVERTER

Microanalyical Research Centre

School of Physics University of Melbourne

Victoria, 3010 AUSTRALIA

Fax: + 61 (0)3 9347 4783 Ph: + 61 (0)3 8344 5376

Email: [email protected] Web: http://www.ph.unimelb.edu.au/marco

April 2004

ATC 170 – Current to Frequency Converter

Microanalytical Research Centre

Contains extracts from the

OPERATING MANUAL

for the

ATC SERIES 170 I-TO-F CONVERTER

And the MARC line driver unit

The ATC module is a product of: Analog Technology Corporation

1859 BusinessCenter Drive Duarte, California 91010

(626) 357-0098


Microanalytical Research Centre 3

CONTENTS

1. MARC LINE DRIVER/DISTRIBUTION DEVICE - OPERATING INSTRUCTIONS .................. 4 1.1. INTRODUCTION .................................................................................................................................... 4 1.2. SET-UP AND CONFIGURATION ............................................................................................................. 5 1.3. DEVICE CONTROLS AND CONNECTIONS ............................................................................................ 6

2. ATC SERIES 170 CURRENT-TO-FREQUENCY CONVERTERS................................................... 8 2.1 SCOPE ................................................................................................................................................... 8 2.2 APPLICATIONS...................................................................................................................................... 8 2.3 MEASURING ION CURRENTS ............................................................................................................. 9 2.3.1 COMPARISON OF I-TO-F CONVERTER WITH CONVENTIONAL ELECTROMETERS .............................. 9 2.3.2 COMPARISON OF I-TO-F CONVERTER WITH CONVENTIONAL ELECTROMETER PLUS A-TO-D CONVERTER..................................................................................................................................................... 10 2.3.3 THEORY OF OPERATION OF SERIES 170 CURRENT-TO-FREQUENCY CONVERTER.......................... 10 2.4 INSTALLATION.................................................................................................................................. 12 2.4.1 LOCATION....................................................................................................................................... 12 2.4.2 CURRENT INPUT CONNECTION........................................................................................................ 12 2.4.3 POWER, CONTROL, AND OUTPUT CONNECTIONS. .................................................................................. 12 2.5 SPECIFICATIONS SUMMARY ................................................................................................................... 14

3. BLOCK AND CIRCUIT DIAGRAMS ..................................................................................................... 15



Section 1: MARC line driver Section 2: ATC-170 unit 1. MARC LINE DRIVER/DISTRIBUTION DEVICE - OPERATING

INSTRUCTIONS

1.1. Introduction

The ATC 170 current to frequency converter unit (the “ATC unit”) from Analog Technology Corporation is a black box that contains a sensitive current digitiser circuit. Electrical inputs and outputs allow the device to be set to three different range sensitivities, provide positive and negative output pulses and +15, +5 and -15 volt power supply inputs.

The MARC MA890 system integrates an ATC 170 unit into a versatile and sensitive charge digitisation system.

Block diagram of the MA890 system (the bias battery to provide VB is optional).

The unit is designed to be connected to a specimen being subject to ion beam analysis, typically in a nuclear microprobe specimen chamber. The electrical connection to the specimen should be via an insulated feedthrough from the specimen chamber. The ATC unit current input (see block diagram) should be connected to the specimen chamber with the shortest possible cable. This cable should be firmly anchored so that it cannot move otherwise random currents will be generated by vibrations which will introduce noise on the input.

To suppress secondary electrons from the specimen, it is possible to insert a well insulated and shielded battery box to provide bias in series with the cable from the chamber. Typically the bias voltage is 250 V with the positive terminal connected to the specimen. The optimum bias voltage to suppress secondary electrons should be determined by experiment. The MARC interface line driver unit powers the ATC unit, performs range switching, provides a visual indication of output pulse rate and polarity, and provides floating line outputs driven by transformer output stages to allow connection to multiple devices with TTL input stages.

Mounting holes in the base plate of the ATC-170/line driver unit allow it to be securely bolted to a support located a close as practical to the specimen stage. Typically the distance from the specimen stage to the ATC-170 input should be less than 0.5 m.

i VB

ATC-170 Line driver Specimen

NIM module



The output of the line driver may be connected via long cables to the companion NUM module located remote from the specimen chamber.

1.2. Set-up and configuration

• Securely mount the ATC-170/line driver unit as close as possible to the source of current to be digitised.

• Ensure the current source is discharged before making connections!

• Connect the current source to the current input of the ATC-170 module (via a bias battery if desired). Ensure this cable cannot vibrate.

• The output signals from the NIM module may be set to “normally high” or “normally low” by changing the position of internal jumpers J1 (for the positive output) and J2 (for the negative output). For use with the MARC MicroDas unit the jumpers should be set to normally low. Jumper settings may be changed by removing the side of the NIM module to access J1 and J2. See figure below for how to change the jumper settings.

DB9 powerout

- in+ in

+ out- out

+ out

- out

Range ledindicatorsRange switch

J1

J2DB9 powerout

- in+ in

+ out- out

+ out

- out

Range ledindicatorsRange switch

J1

J2

The companion NIM module to the ATC-170 unit (internal view).

Currentinput

Mounting holesIndicatorsPower+ signal- signal

DB9 power

+ out- out Current

input

Mounting holesIndicatorsPower+ signal- signal

DB9 power

+ out- out

DB9 power

+ out- out

The ATC unit (right) and the MARC interface line driver (left). The signal outputs and

power input are connected to the companion NIM module.



• Insert the companion NIM module into a NIM crate with the power off.

• Connect the +out/-out/DB9 power sockets to the corresponding sockets on the companion NIM module by the cables supplied. Ensure the + output on the line driver is connected to the + input on the NIM module (likewise the – output).

• Connect either the front (BNC) or rear (leemo) outputs of the NIM module to the current integrator unit. For example, connect the positive BNC output to the “Charge” input of the MicroDas data acquisition system when using the “Dwell on charge” option.

• Ensure all cables are securely connected. It is advisable that the NIM module is retained in the NIM rack using the knurled knob provided on the front panel.

• Power on the NIM crate.

• Set the range switch to the desired sensitivity. Note that the highest output frequency (see specifications below) limits the highest output count rate and hence the highest input current for each range.

• The system is now ready for operation.

1.3. Device Controls and Connections

These instructions refer to the companion NIM module which must be connected to the line driver outputs as described in the previous section.

1: Range Switching.

The rotary switch on the front panel allows the selection of the 3 current digitisation ranges on the ATC 170.The selected value may range from

10-12 Coulombs/pulse Max input current 5000 nA



and is indicated by a labelled LED on the front panel.

2:The Count rate LEDs.

J1 J2J1 J2

The location of jumpers J1 and J2. Shown set to “normally high” output option on both the positive (J1) and negative (J2) outputs. These are the settings required for

MicroDas “Charge” input.



The red and green LEDs on the front panel are connected to monostable multivibrators in order to provide a visual indication of count rate and polarity of incoming signals. Approximate indication of count rate is provided by the LEDs , but at high count rates (above 30 counts/sec) the LEDs appear to be lit continuously. During switch on and switch off the LEDs will flicker due to their monostable circuitry.

[March 2004]



2. ATC SERIES 170 CURRENT-TO-FREQUENCY CONVERTERS

2.1 Scope This section of the manual describes the ATC Series 170 Current-to-Frequency

Converters and provides sufficient information to permit installation and operation by anyone who is generally familiar with the use of electronic instruments.

2.2 Applications The Series 170 Current-to-Frequency Converter is designed to accept input

currents of either polarity from low-level current sources, producing an output train of TTL-level logic pulses whose repetition rate is proportional to the input current. Some of the current sources for which the Series 170 is particularly useful are:

o Flame Ionization Detectors o Optical Photocell Detectors o Faraday Cup Detectors o Ionization Chambers o Electron Multipliers

Any other detector whose output is a current in the range of 10-14 to 10-6A will also

be a candidate for use with the Series 170 Current-to-Frequency Converter. Previously, these applications have been serviced by instruments described generically as “electrometer amplifiers

For most, if not all, applications, the Series 170 instruments provide the optimum

interface for coupling low-level current signals to instruments for data display, recording, and/or analysis.



2.3 Measuring Ion Currents This section discusses traditional techniques for measuring small currents and how

current-to-frequency conversion improves upon these techniques. Also included in this section is a functional description of the Series 170 Converter.

2.3.1Comparison of I-to-F Converter with Conventional Electrometers Current-to-frequency conversion provides numerous signal-processing results

superior to those obtained with conventional electrometers and associated hard-copy display devices.

Limitations Imposed by Display Medium - The dynamic range of conventional

linear electrometer amplifiers, as determined by the ratio of maximum voltage output

to noise or drift levels, is often as high as 106. Because the outputs of such amplifiers

are generally viewed on output devices such as chart recorders and meters that have

substantially less dynamic range, it is necessary for truly wide-range linear

measurements to be able to selectively modify the sensitivity of the electrometer in

such a way as to make most of the prominent data observable on the output device.

Range-Switched Electrometers - A traditional method of producing such sensitivity

modifications is the manual or automatic changing of feedback resistors or resistor networks. The most significant disadvantage of range changing during an analysis is the production of discontinuities and artifacts in the output data. In addition to complicating the visual analysis of the data, the changing of signal-processing sensitivity in mid-analysis also makes the automatic processing and manipulation of the data considerably more difficult.

Logarithmic Electrometers - The discontinuities of range switching can be avoided

through the use of an electrometer with logarithmic or other non-linear compression. However, such compression makes the output data less recognizable and impractically difficult to manipulate.

Current-to-Frequency Converters - One of the widest ranging and most convenient-

to-handle analog signals is frequency. As an example, ATC’s Series 170 Current-to-Frequency Converter transforms detector currents to variable-frequency pulse trains over a linear dynamic frequency range of 5 x 106 and this entire range of frequencies can be accommodated with a single sampled-data digital accumulator. Among the



advantages of such a converter are the following: (1) the current-to-frequency conversion is performed with an equivalent speed of response superior to the speed with which current-to voltage conversions are performed by conventional electrometers; (2) the pulse-train output is one of the most convenient types of output signals for data-processing systems to handle; and (3) data from the widest-range detectors is preserved without distortion.

2.3.2 Comparison of I-to-F Converter with Conventional Electrometer Plus A-to-D Converter

Standard A-to-D Conversion - If the electrometer amplifier were assumed to have a

dynamic voltage range comparable to the frequency range of the Series 170 Converter (which is not likely with standard low-voltage integrated circuits), it would be necessary to provide about 22 bits of analog-to-digital conversion to preserve the same dynamic range. In practice, A-to-D conversion would be limited to a resolution of about 16 bits (or a range of 65,536) because of performance and price limitations.

Voltage to Frequency Conversion - A voltage-to-frequency converter that

processes the output of a single-range electrometer represents the closest performance competitor to a wide range current-to-frequency converter. Used in commercial voltage integrators, the voltage-to-frequency converter falls somewhat short in dynamic frequency range and speed of response - - response often being determined by the electrometer. Finally, without auxiliary electronics to provide an offset or other means of accommodating signals of both polarities, this two-stage configuration is intrinsically a unipolar one, unlike the one-stage bipolar current-to-frequency converter.

2.3.3 Theory of Operation of Series 170 Current-to-Frequency Converter Designed for use with most current-source detectors, the all-solid-state Series 170

is a fast, stable circuit that converts input currents of either polarity to an output pulse train with a frequency proportional to current over a continuous dynamic range of 5 x 106 .

Separate variable-frequency pulse-train outputs are provided for positive and negative currents.

In this I-to-F conversion technique, discrete, fixed amplitude current pulses are fed back to the input of the converter in such a way that the feedback current (charge per pulse times pulse rate) is equal in amplitude to the input current and opposite in polarity. The output signals from the converter are the current-pulse feedback rates, which are proportional to the input currents and may vary between 0 and 5 MHz. A programmable



feedback charge generator permits the manual selection of feedback pulses of either polarity in values of 10-14 , 10-13, or 10-12 C/pulse.

An important characteristic of the current-to-frequency converter is that the high-impedance integrating input amplifier operates as a perpetually rezeroed circuit, thus eliminating the need for a wide output-voltage range. A positive input current charges the amplifier negatively until a fixed threshold has been exceeded.

This condition is detected by a comparator or discriminator that changes state when its threshold is exceeded. At that time, the comparator output enables a clock-pulse gating circuit to transmit a clock pulse to a precision negative charge-pulse generator. The charge pulse thus produced is fed back to the integrating amplifier upon which the integrator output is discharged to its initial condition. When that occurs, the comparator output is immediately restored to its initial condition, thus inhibiting or closing the clock-pulse gating circuit. Following this, the closed-loop sequence repeats, each time restoring the integrator to its initial condition. Under equilibrium conditions, the feedback current becomes equal to the input current just as in any standard operational amplifier. Expressed in terms of pulse frequency, f, and feedback charge, Q, IFB = IIN = (Q/pulse) f and therefore f = IIN / (Q/pulse)

Some special features of this configuration are as follows:

a. Bipolar Operation - A parallel opposite-polarity feedback loop permits identical measurements to be made on negative-polarity input currents.

b. Programmable Charge Pulser - A proprietary circuit for the selection of three decade-spaced feedback charge pulses enables the converter to operate in three overlapping decade-spaced sensitivity ranges, each with a dynamic range of 6.7 decades (5 x 106 :1).

c. High-Frequency Operation - Successful operation to pulse rates as high as 5

MHz requires that the entire 1oop be restored to its initial condition in less than 200 ns after a clock pulse has been generated. This ensures that the operating clock-pulse gating circuit is closed before the occurrence of a successive clock pulse. To accomplish this requires an integrating circuit capable of having the unusual combination of properties of low current thresholds and high bandwidth.



2.4 Installation

2.4.1 Location The I-to-F Converter should be located as close as possible to the current source

being measured in order to minimize input capacitance, pickup of external noise signals, and thermal effects. The converter may be operated in any desired position within a normal laboratory environment.

2.4.2 Current Input Connection The current input connector is a grounded BNC coaxial receptacle labeled Jl. The

input current should be applied through this connector, using a mating BNC plug and a suitable coaxial cable. The input end to this cable may be terminated in a manner appro-priate to the signal source. The cable’s outer conductor (shield) should be grounded at the current source.

Since the converter is intended to measure very small currents its input may be sensitive to electrical noise generated by motion or flexing of the input cable and to thermo-electric effects produced by the junctions of dissimilar conductors. For stable measurements at the lowest currents, these effects must be minimized.

The input is a virtual ground with a 100 kΩ resistor in series. This resistor protects the input, but is not so large that the input voltage becomes excessive for currents within the range of the converter. Consideration should be given to the amount of capacitance connected across the input so that the speed of measurement is unaffected.

2.4.3 Power, Control, and Output Connections. The power, control, and output interfaces are on the card edge at the opposite end

of the module from the input. For these connections, see the interface list, Table I. The output frequency is available separately for positive and negative polarity

inputs. This arrangement facilitates driving an up-down counter such as a 74192 directly. Many other schemes can make use of different combinations of the two outputs. For single-polarity applications, only one output may be used, but consideration should be given to the offset current at the converter input of up to 10-13 A, which may show up in the unused output. A zeroing circuit for the offset current is available as an option at extra cost.



The case is connected to ground.

TABLE I - MODULE INTERFACE SERIES 170 CURRENT-TO-FREQUENCY CONVERTER Cinch Connector 50-l5A-20 or equivalent

PIN # Description

1 10-13 scale active low1



8 Ground

9 Ground

10 -15V Power ± .25V @ 15 mA

11 +15V Power ± .25V @ 35 mA

12 +5V Power ± .25V @ 5 mA2

14 +Frequency Output3

15 -Frequency Output3

1 One input only must be low. TTL levels should be used. Input may go to –l5V.

Unselected inputs may be left open. 2 +5V current is dependent on length of cables connected to frequency outputs. 5 mA is

maximum for 3 ft. of 50Ω cable driving one standard TTL gate load at the maximum frequency.

3 50Ω series terminated outputs. TTL levels. Normally high with 100 ns wide pulse to low

level. Will drive 10 ft. of cable safely.



2.5 Specifications Summary

Input Current Polarity: Positive or Negative, no switching required.

Input CurrentRange: 10-14 A (at 1 sec sampling interval) to 5 x 10-6A

Input Offset Current: <1 x 10-13 A

Input Voltage Reference: Chassis ground Input Noise: <10-14 A rms @ 1 sec sampling interval. Pulse-Period Transient Response τ<30 µs with 200-pF source capacitance.

Accuracy: 1%

Integral Linearity: Less than 0.5% nonlinearity over more than 6 decades.

Maximum Charge Impulse: 10-10C

Operating Temperature Range: -10 to +45°C Pulse-Train Outputs: Separate outputs for positive and negative input

currents. Output Pulse Shape: Negative TTL pulses, 100 nsec wide.

Current-to-Frequency Conversion Gain: 1 Hz per 10-12, l0-13, or 10-14 A, selectable.

Output Frequency Range: 0 to 5 MHz.


Microanalytical Research Centre

3. Block and Circuit Diagrams









Notes:

Documents

MA-890 Charge DigitiserMA-890 Charge Digitiser INCORPORATING THE ATC SERIES 170 CURRENT-TO-FREQUENCY CONVERTER Microanalyical Research Centre School of Physics University of Melbourne