Model 1000C Current Integrator
History: Through the years Brookhaven Instruments Corporation (BIC) integrators have remained unchallenged as the most accurate and stable current integrators available. These qualities, combined with outstanding reliability, have established them as the instrument of choice in nuclear research, ion implantation and medical applications.
Unbiased tests have proven repeatedly that the .02% accuracy featured by the original Model 1000 in 1964 set a standard that has yet to be even remotely approached by any other commercially available current integrators.
Honors accorded to BIC integrators have ranged from their exclusive specification by an international medical committee for neutron therapy dosimetry to their employment as the standard for accuracy measurements in a government study of current integrators offered for ion implantation.
General Information
Today's Model 1000C Current Integrator represents a progressively seasoned and polished design combining the time-proven principles of its classic predecessors with the latest improvements permitted by the current level of solid-state technology.
An expanded selection of special optional features extends the utility of the Model 1000C to an ever widening range of applications.
The BIC Model 1000C Digitizing Current Integrator accepts pulsed or continuous currents of either polarity ranging from a small fraction of a nanoampere to 20 milliamperes. It provides a series of digital output pulses whose number is proportional to the accumulated quantity of charge. A digitizing rate of 1000 pps is achieved at full-scale input current. The current is displayed on a front panel meter and on any desired number of auxiliary meters (available from BIC) connected to the meter output terminals.
A built-in counter totals the digital output pulses with switchable 4 to 7 decade capacity, a 4-digit LED display and a thumb wheel selected preset for the 4 displayed decades. An LED decimal point in the display is shifted appropriately for the selected prescaling factor.
An external connector makes the digital pulses available for optional external counting as by a computer.
Internal logic enables the counter to be controlled by front-panel push buttons or by signals from a remote source. A flexible combination of TTL-compatible level and pulse outputs allows control of external functions by the integrator.
Wide Current Range: Fifteen ranges from 2 nA to 20 mA F.S. in a 2-6 sequence allow a total current span of 3 x 107 to be covered without operating below one-third of full scale current or exceeding a few hundredths of a percent error on any range.
The very low drift and inherent linearity of the instrument allow it to integrate as low as 20pa (1% of lowest range!) with a still respectable accuracy of one or two percent. An optional "Low Current" modification provides a switchable meter scale expansion to facilitate the reading of unusually small input currents.
Input Impedance: In any current measuring system the current to be measured will divide between the input terminals of the measuring instrument and any parallel leakage paths in inverse proportion to the impedance's presented. When measuring small currents, this effect can cause serious inaccuracies even with carefully insulated targets unless the input voltage drop of the measuring instrument is extremely low. A single-stage integrator having a 3 mV input voltage excursion, for example, would represent an impedance of 3 megohms to a 1 nA input current. To limit the resulting error to 0.1%, a minimum input circuit leakage resistance of 3 x 109 ohms would be required. At 0.1 nA the minimum leakage resistance requirement would increase to 3 x 1010 ohms for the same error limit. In many circumstances, values of this magnitude are virtually impossible to achieve.
In contrast, the BIC Model 1000C integrator with its 0.1 microvolt full-scale input drop is thousands of times more tolerant of input circuit leakage. With a BIC Model 1000C, the accuracy requirements of the foregoing examples could be satisfied with a leakage resistance as low as 50 k Ohms.
Pulse Integration: A 0.1 second input-charge storage feature enables the instrument to accept pulsed currents with peak values of up to 20 mA on any range high enough to accommodate the average current. Since the required charge storage is performed in the input amplifier feedback circuit rather than by an external filter, which would introduce a high series resistance, the advantages of extremely low input impedance are fully preserved for pulse integration.
A selector switch allows the charge storage network to be removed for special cases where a 0.1 second time lag between input current and the resulting digital output pulses would be undesirable.
Isolated Ground: In many applications the input cable sheath is grounded at the source. If the cable is also grounded at the integrator, a ground loop is created through which large noise and power frequency currents can flow. The resulting voltage drop in the cable sheath is capacitively coupled to the integrator input, impairing or even preventing, proper operation. The long input cables frequently used with accelerators aggravate this difficulty.
The BIC Model 1000C integrator has an isolated digitizing system with its own floating supplies. It can be grounded at the chassis, through the cable sheath, or at any other point required to avoid creating a ground loop. Note that NIM-powered instruments are perforce grounded at the chassis.
Offset and Test Supply: A highly regulated, variable offset current supply provides for the suppression of spurious input currents up to ±10% of the F.S. range in use. When the large offset range is not required, the offset supply is switched to the "normal balance" condition so that no adjustment is required when switching ranges.
Rapid, precise zero adjustment is facilitated by switching the instrument to a special balancing mode that magnifies the offset error by 20X.
The offset current supply serves also as a test current source to verify the proper operation on any selected current range.
Automatic Dead-Time Correction: Integration can be inhibited by dead-time signals from analyzers or other sources. A special gate which gives priority to the first signal to arrive (output or inhibit) with 20 nSec. resolution, allows accurate correction for dead-time periods as short as 1 microsecond.
Specifications
Input Amplfier: Chopper-stabilized, gain 2 x 108. Wire-wound feedback resistors of .01% accuracy, ±10 ppm/deg tempco maintain accuracy indefinitely.
Instrument Ground: Isolated from chassis ground. Can be externally grounded at any point required to avoid ground loops.
Input Impedance: Virtual ground, less than 0.1 microvolt drop full-scale. Input impedance = 10-7 / current range in use.
Current Indication: Panel meter, 2% accuracy. Mirror scale, taut-band movement.
Input BIAS Current: Settable to 0 ± 2 x 10-13 A.
Maximum Input Loading Capacitance for Specified Performance: .25 microfarad.
Maximum Frequency of Pulsed or Fluctuating Input: No limit. Integrator responds only to true time integral (dc component) of input current regardless of wave shape or frequency of ac components.
Offset Adjustment: 10-turn potentiometer on front panel. Normal mode allows input bias and small external leakage currents to be set to 0 (±2 x 10-13 A.). Requires no readjustment when switching ranges. Special "±10%" mode provides stable, adjustable offset to approximately ±10% of current range in use. Useful for balancing out large external error or background currents. A non-integrating "zero" function provides a 20x meter scale expansion for rapid, precise adjustment of the zero point prior to integration.
Absolute Accuracy of Calibration: ±.02%, all ranges.
Linearity: No measurable nonlinearity.
Temperature Stability: Max. ±30 ppm/deg for ranges 1 through 12, ±60 ppm/deg for ranges 13 through 15.
Test Current: Adjustable to approximately 10% F.S. from internal offset supply. Selected automatically for each current range.
Peak Pulse Input Current: 20mA on any range. No duty factor limit. Inputs with peak-to-average ratios in the range of millions are integrated with full specified accuracy. No input filter is required.
Digitizing Rate: 1000 pps F.S.
Digital Output: 5V through 75 Ohm internal resistor, 5 microseconds.
External Meter Output: Will drive as many remote meters as required. Uses economical 0-1 mA meters (available from Brookhaven).
Analyzer Dead-Time Correction: Output inhibited by +2.5V to +10V signal applied to "Inhibit" input, 20 nSec resolution.
Ranges and Input Smoothing Time Constants (Average Times):*
*The averaging time specifies the effective time constant of the R-C feedback network in the input amplifier and, therefore, the time lag between the instantaneous value of accumulated charge and the digital count which represents it. It imposes no limitation on the ability to integrate currents with ac components of any frequency and has significance only when it is required to stop a process when a preset quantity of charge has been reached.
Method of Digitization: Gated-current charge-neutralization. Current pulses controlled by aged, stabilized, temperature-compensated reference diode and precision quartz oscillator. No drift, no dead-time, no linearity adjustments, no dielectric absorption errors. Maintains calibration accuracy for years without adjustment.
Mechanical: Standard 7" by 19" panel. Chassis is 10" deep.
Power: Wireable for 115 or 230 V AC ±10%, 50-60 Hz.
Weight: Approximately 14 lbs.
Internal Counter Specifications
Total Capacity: 7 decades.
Display: 4-digit, 7-segment LED.
Prescaling: Divides by 100, 101, 102, or 103, to provide operating period up to 104 seconds at F.S. input, LED decimal point automatically shifts as prescaling factor is selected.
Preset: Four-digit, decimal thumb wheel switch provides full preset of the 4 displayed decades. Preset may be disabled by front-panel preset selector switch.
External Function Control: "STOP" and "RUN" states are indicated by separate 5V logic level outputs. An additional 5V, 40 microsecond pulse marks the transition to the "STOP" state. All outputs are short-circuit proof.
Controls: Start and stop commands may be introduced by front panel push buttons or by momentary grounds applied by TTL, relays, switches, etc., to rear panel connectors. The counter is automatically reset to zero by a start command.
Optional Features
Current Range Identification: Provides binary coded readout of current range in use. TTL compatible, 4-line digital output.
Low Current Modification: Provides switch-selectable meter scale expansion to 60 pA, 200 pA, or 600 pA full-scale for indicating abnormally small currents when integrating on the 2 nA range.
Current Waveform Monitor: Provides an output voltage analog of the input current waveform. Sensitivity is 1 V output for full-scale current input. Useful for pulse currents with peak value up to 13 times full-scale current range. Rise and fall times are approximately 15 microseconds. Approximately 1 nA rms equivalent input current noise impairs usefulness progressively for input currents below about 10 nA peak amplitude.
Input Current Gate: Analog current gate allows input current to be transferred rapidly between the integrator input terminal and ground by application of a "Gate Control" signal. TTL "1" level turns gate ON. "0" level turns gate OFF and grounds the input current line. Switching delay: 60 nSec ON, 80 nSec OFF. Useful for integrating pulses from sources with high average background noise (e.g., photo multipliers, etc.) and other cases requiring synchronous selection of input live time.
Computer Control: Provides parallel BCD TTL inputs for external programming of counter preset from computer or other remote source. Includes internal storage register with strobed input.
BCD Counter Output: Provides parallel BCD output of the content of the four displayed decades and of the selected prescaling factor. The outputs are open-collector with 510Ohm pull-up to +5V.
Additional Prescaling Decade: Additional blind decade divider, switch selectable on front panel. Extends maximum integrating period tenfold, to 105 seconds at full-scale input current.
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