Low Noise Charge Ramp Electrometer

Electrometers are instruments for measuring electric charge and thus, can be configured to measure either small voltages or small currents. The present invention is an instrument for measuring small currents.

The measurement of a small electrical current in the range of 1.0 E-15 amps is usually accomplished through the use of an electrometer amplifier configured as a current to voltage converter. This configuration makes use of a high megohm feedback resistor, of the order of 1. 0 E+12 ohms, to develop a useful voltage from the small currents being measured. However, the resistor also acts as a noise source and generates a noise voltage defined by either the Johnson or the thermal noise equation. The thermal noise equation can be expressed as follows:

where SQRT=square root

E=volts RMS noise

K=Boltzman's constant

T=Temperature in degrees Celsius

f=noise bandwidth for the measurement in hertz

R=resistance in ohms

The subject invention eliminates the high megohm resistor and its associated noise, E, by using a small, typically 10 to 20 picofarad, capacitor as the sole feedback element in the electrometer configuration. The electrometer output voltage using this latter configuration is proportional to the input current multiplied by the time the input current is applied divided by the value of the feedback capacitance. An equation depicting this relationship can be expressed as follows:

where E=output voltage of the electrometer in volts

I=input current in amps

t=length of time input current is applied in seconds

C=feedback capacitor value in farads

Thus, for this configuration, the electrometer output voltage increases linearly for a constant input current; however, when employing this system, the capacitor must be reset or zeroed before the electrometer amplifier reaches its saturation voltage. To accomplish this, the subject invention automatically resets or zeros the accumulated charge on the feedback capacitor when the voltage level of the electrometer amplifier approaches saturation.

The subject invention employs a data collection system which assimilates voltage data over fixed segments of time and then uses this data to determine the current from the above cited relationship E=(I)(t)/(C).

Accordingly, it is an object of this invention to provide an electrometer which is capable of measuring small currents without experiencing the noise generated by a feedback resistor.

It is still a further object of this invention to provide a means to automatically reset or zero the accumulated charge on the feedback capacitor as required.

A further object of this invention is to provide a means to automatically compile the data from the electrometer and to determine the input current based on the accumulated data.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the foregoing and other objectives and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides for a means to measure small currents without the use of a high megohm feedback resistor. A feedback capacitor is used in lieu of the feedback resistor to eliminate the noise inherent in using a feedback resistor. The apparatus is capable of automatically zeroing or resetting the feedback capacitor. The input current to the electrometer is determined through the use of a time controlled data collection unit and a data processor where the timing function is responsive to a signal from a crystal oscillator.

FIG. 1 depicts a schematic of the low noise charge ramp electrometer.

FIG. 2 represents the linearly increasing frequency from the voltage-to-frequency converter as a result of a constant input current being applied to the electrometer.

The present invention, a small-current measuring system, provides a means for measuring small currents through the use of an electrometer amplifier in conjunction with a feedback capacitor and a means for data collection and circuit control.

The input current is conducted by means of a source connector, 1, to the electrometer amplifier, 2. A feedback capacitor, 3, is the sole feedback element in the electrometer configuration. During operation of the small current measurement system, the feedback capacitor, 3, must be zeroed or reset prior to the electrometer amplifier, 1, reaching its saturation voltage. The resetting or zeroing of the feedback capacitor is accomplished by momentarily closing a set of normally open magnetic relay contacts, 4, that are electrically connected across the feedback capacitor, 3. The electrometer amplifier, 2, the feedback capacitor, 3, and the magnetic relay contacts, 4, are electrostatically shielded through the use of an electrostatic shield, 5. The magnetic relay contacts, 4, are closed through the application of an external magnetic field to the electrostatic shield, 5, and subsequently to the magnetic relay contacts, 4. The magnetic field is developed by energizing the magnetic reset coil, 6. The use of an external magnetic field to actuate the magnetic relay contacts, 4, eliminates the potential of electrostatic disturbances which could interfere with the sensitive electrometer input current. The magnetic relay contacts, 4, are encapsulated in glass to maintain an extremely high leakage resistance across the feedback capacitor, 3, when the relay contacts, 4, are not magnetically actuated.

The output from the electrometer amplifier, 2, is transmitted to the electrometer buffer amplifier, 7, which in turn transmits a voltage signal to a reset discriminator, 8, and a reset complete discriminator, 9. The reset discriminator, 8, detects when the electrometer buffer amplifier, 7, is producing a 10.0 volt output. When this condition occurs, the magnetic reset coil, 5, is energized creating a magnetic field which closes the magnetic relay contacts, 4. Thus, whenever the electrometer amplifier, 2, output voltage reaches 10.0 volts as reflected by a 10.0 volt output of the electrometer buffer amplifier, 7, the feedback capacitor, 3, is automatically reset or zeroed. With an input current of 1.0 E-10 amps and with a 20 picofarad feedback capacitor, 3, the feedback capacitor, 3, will be automatically reset or zeroed every 2 seconds. For smaller input currents, fewer resets will occur. However, typically, the operational electrometer amplifier, 4, has its own leakage current and results in about one automatic reset per 2 hours with no external input.

The reset complete discriminator, 9, senses when the electrometer buffer amplifier, 7, output returns to zero, thus, completing the resetting or zeroing process. When the reset is completed, a signal is transmitted to the control and sequence unit, 10, which responds by sending a signal through the I/O bus, 11, and on to the polarity section of the electrometer buffer amplifier, 7, which responds by allowing the measurement process to continue. To determine the input current to the electrometer amplifier, 2, the output voltage ramp from the electrometer amplifier, 2, must be measured two or more times with each measurement occurring for a predetermined period of time. The subject invention has the capability of automatically making the voltage measurements for the predetermined time periods.

In addition, the output voltage of the electrometer amplifier, 2, as applied to the electrometer buffer amplifier, 7, generates a plus or minus voltage gain of 1.0; this is required to configure the positive or negative input currents for use by the high precision voltage-to-frequency converter, 12. The buffer amplifier, 7, inputs a voltage signal representative of the input current to the conventional high precision voltage-to-frequency converter, 12. The voltage-to-frequency converter, 12, generates an output frequency proportional to the input voltage supplied by the buffer amplifier, 7, and thus, outputs a frequency signal in response to the ramping electrometer output voltage. The linear frequency range of the voltage-to-frequency converter, 12, is from about 300 Hz to 1 MHz. A crystal controlled oscillator, 13, operating at 1 MHz provides a precise time base for the subject small current measuring system. The data collection system for the frequency output of the small current measuring system consists of two identical frequency counters, a first frequency counter, 14, and a second frequency counter, 16. The data collection system also contains two crystal oscillator counters each of which is driven by a crystal controlled oscillator and each having a thirty-two bit count capacity which is formed by employing two concatenated sixteen bit binary counters to form a thirty-two bit counter for a first crystal oscillator counter, 15, and a second crystal oscillator counter 17. Using the crystal controlled oscillator, 13, as the input to the thirty-two bit programmable crystal oscillator counters, 15 and 17, provides a precise 1.0 second timing period for the small current measuring system. Each of the frequency counters, 14 and 16, is also configured with a thirty-two bit count capacity by using 16 bit concatenated binary counter pairs. The control and sequencing unit, 10, provides control logic for starting, sequencing, status, and stopping the measurement process. The measurement process begins with the Data Processor, 18, sending a start pulse to the start Control and Sequence Unit, 10. The start pulse causes the Control and Sequence Unit, 10, to select the A set of counters consisting of frequency counter, 14, and crystal oscillator counter, 15, which await the triggering of the sequence operation by the next arriving pulse from the voltage-to-frequency converter, 12. This sequence ensures the time synchronization of operation with the voltage-to-frequency output pulse. With the first voltage-to-frequency pulse, the crystal oscillator, 13, and the frequency counter, 14, each begin the accumulation of their respective input counts.

The count sequence continues for precisely 1.0 second, as dictated by the crystal oscillator capability, at which time the control and sequence unit, 10, is signaled by the crystal oscillator counter, 15, that it has reached the preset count time value of 1,000,000 pulses or 1.0 second. With the control and sequence unit, 10, enabled the next pulse from the voltage-to-frequency converter, 12, will terminate the A unit crystal oscillator, 14, and the frequency counter, 15, and initiate the B unit consisting of crystal oscillator, 16, and frequency counter, 17. The termination of the count accumulation in the A unit and the starting of the count accumulation in the B unit is synchronized with the voltage-to-frequency output pulses; in this sequencing, no voltage-to-frequency periods are by-passed in changing from one counter unit to the other counter unit. At this time the A unit frequency counter, 14, will contain the exact number of voltage-to-frequency pulses having occurred in the count interval. The count contained in the A unit crystal oscillator counter, 15, equals the number of 1.0 megahertz crystal oscillator pulses accumulated during this same count time interval. The number of 1.0 megahertz pulses accumulated defines the time interval precisely to plus or minus 1.0 E-6 seconds where the time interval selected is equal to the number of 1.0 megahertz pulses accumulated, a minimum of 1.0 E6, multiplied by 1.0 E-6 seconds, the period for the crystal oscillator output. This provides a precise measure of the voltage-to-frequency time interval accumulated in the crystal counters, 15 and 17. With the determination of the voltage-to-frequency from the above process, the value of the average electrometer output voltage is directly related through the voltage-to-frequency conversion constant. For this system, the constant is 1.0 microvolt per hertz for the voltage-to-frequency converter, 12.

FIG. 2 represents the linearly increasing frequency from the voltage-to-frequency converter, 12, as a result of a constant input current being applied to the electrometer amplifier, 2. The frequency is proportional to the voltage at the output of the electrometer amplifier, 2. The horizontal axis is equivalent to the voltage. The precise measurement time periods are shown as P1, P2, and P3. The contents of the crystal oscillator counter, 15 or 17, give the value for this time period. The contents of the frequency counter, 14 or 16, provide the frequency value and therefore, the equivalent voltage value. Dividing the count value in the frequency counter, 14 or 16, by the time interval, contained in the crystal counter, 15 or 17, gives the average of the frequency in that interval. Since no voltage-to-frequency periods are by-passed in changing from one counter to the other counter and since two contiguous periods ar used for a single measurement, the difference between the two average values is equal to the frequency change in the measurement interval. With the voltage being proportional to the frequency, the change in voltage is obtained.

The data processor, 18, a single board computer system running a BASIC interpreter operating system, is interrupted at the completion of each measurement interval by the signal line output from the control and sequence unit, 10. The data processor, 18, transfers the data contained in the counters a well as the status of the control sequence unit, 10. The data processor, 18, then calculates the input current resulting from the average of the slope values while the next count interval is in progress. The data processor, 18, has the capability of resetting the electrometer amplifier, 2, via the PSTOP command line, 19. This command line gives the processor the capability to confine measurement to the most linear region of the measurement process operation. A new value of input current is available every 1.0 second from the data processor, 18.