Charge is often measured as a quantity of particles on a physical surface or stored in a capacitor. Often, the human body, an electronic cable, or other electrical components may have a certain amount of inherent capacitance that allows charge to be stored on their surfaces. It can be important to know the quantity of this charge in order to understand the nature of an electrostatic discharge (ESD) waveform (voltage and current) when the charge is transferred to electronic circuitry. This allows appropriate circuit protective devices to be selected and installed to prevent damage.Sometimes, charge that varies with time is measured on a continuous basis, such as when using a coulombmeter to make very low current measurements. From basic physical principles, one way of looking at varying charge is as the time integral of current, I dt, between two points in time.An electrometer makes an ideal coulombmeter because it has very low input offset current and high input resistance. This means it wont allow the charge to bleed away and wont alter transferred charge during short time intervals. Most electrometers have a coulombmeter function that measures charge by integrating the input current. An integrating capacitor is used in the feedback loop of the electrometers input stage.As accurate as electrometers are, a variety of potential error sources can degrade charge measurement integrity if not taken into account. These error sources include input offset current, voltage burden, generated currents, and low source impedance.Input Offset Current Error. Input offset currents are background currents that are present in the measuring instrument when no signal current is applied to the instruments input terminals. An electrometers input offset current is very low. However, at low charge levels, even this small current may be a significant error factor. Over long time periods, the instrument will integrate the offset current, which will be seen as a long-term drift in the charge measurement. Typical offset current is four femto-amps, which will cause a change in the charge measurement of four femto-coulombs per second. If the offset current is known, its possible to compensate for this error simply by subtracting the charge drift due to offset current from the actual reading. However, determining the offset current of the entire system is likely to be much more complicated.Voltage Burden. The voltage burden of any ammeter type of instrument is the voltage drop across its input terminals. The voltage burden of a feedback coulombmeter is generally quite low (less than 100 microvolts). However, if the instantaneous peak current input is more than 10 micro-amps, the voltage burden can exceed this level momentarily. In an overload condition, the voltage burden can reach many volts, depending on the input value. If the source voltage is at least 10 millivolts, the typical electrometer operating in the coulombs mode will integrate the current accurately. However, if the source voltage is much lower, the voltage burden may become a problem, and the input stage noise will be amplified so much that making accurate measurements is impossible.Generated Currents. Generated currents from the input cable, or induced currents due to insufficient shielding, can cause errors in Wholesale Data Cables SIM charge measurements, especially with charge levels of 100 pico-coulombs or less. To minimize generated currents, always use low noise cable and electrostatically shield all connections and the DUT.Source Impedance. The magnitude of the source impedance can affect the noise performance of an electrometers feedback coulombmeter circuit. In a coulombmeter, the feedback impedance, ZF, is a capacitor. The noise gain of the coulombmeter can be calculated from this equation:Output Noise = Input Noise (1 + ZF/ZS),where ZS is the source impedance,ZF is the feedback impedance of the coulombmeter,Input Noise is the noise of the input stage of the electrometer.In general, as ZF becomes larger, the noise gain increases. The documentation or specifications for the particular electrometer being used will Violin Accessories generally provide the value of its feedback impedance.Other Important Charge Measurement Considerations. Unlike a voltage measurement, a charge measurement is a destructive process. That is, the process of making the measurement may remove the charge stored in the device under test. When measuring the charge on a device such as a capacitor, its important to disable the Zero Check of the electrometer first, and then connect the capacitor to the high impedance input terminal.Zero Check is a process where the input amplifier of the electrometer is reconfigured to shunt the input signal to low. Otherwise, some of the charge will be lost through the Zero Check impedance and wont be measured by the electrometer. Thats because when Zero Check is enabled, the input resistance of the electrometer is about 10 mega-ohms. Opening the Zero Check switch produces a sudden change in charge reading known as zero hop. To eliminate the effects of zero hop, take a reading just after the Zero Check is disabled, then subtract this value from all subsequent readings. An easy way to do this is to enable the REL function after Zero Check is disabled, which nulls out the charge reading caused by the hop.The charge measurement range of most electrometers can be extended through the use of the external feedback mode, which allows using an external device as the electrometers feedback element. Placing the electrometer in the volts mode and then enabling external feedback switches the feedback circuit from an internal network to a feedback circuit connected to the preamp output.To extend the coulomb measurement ranges, an external capacitor can be used as the feedback element. The external feedback capacitor is placed between the electrometers preamp output terminal and its HI input terminal. To prevent electrostatic interference, the capacitor should be placed in a shielded test fixture, which has the test leads running through this fixture from the device with the unknown charge to the electrometers HI input and Lo input terminals.In external feedback mode, the electrometer will display the voltage across the feedback element. The unknown charge can be Halloween Gifts Items calculated from the following formula:Q = CV,whereQ = charge (coulombs),C = capacitance of the external feedback capacitor (farads),V = voltage on display of electrometer (volts).For example, using an external feedback capacitor of 10 microfarads and measuring 5 volts on the display of the electrometer, the calculated charge is 50 micro-coulombs. The capacitance of the feedback element should be at least 10 pico-farads to avoid errors due to stray capacitance and noise gain. To ensure low leakage current and low dielectric absorption, the feedback capacitor should be made of a suitable dielectric material such as polystyrene, polypropylene, or Teflon.Additional elements of measurement hygiene are critical to making accurate charge measurements with electrometers. These include making proper connections, minimizing electrostatic interference, and minimizing the impact of environmental factors. Connections. To avoid measurement errors, its critical to make proper connections from the electrometer to the device under test. Always connect the high resistance terminal of the meter to the highest resistance point of the circuit under test. Electrostatic Interference and Shielding. Electrostatic coupling or interference occurs when an electrically charged object approaches the input circuit under test. At low impedance levels, the effects of the interference arent noticeable because the charge dissipates rapidly. However, high resistance materials dont allow the charge to decay quickly, which may result in unstable measurements. The erroneous readings may be due to either DC or AC electrostatic fields, so electrostatic shielding will help minimize the effects of these fields.DC fields can produce noisy readings or undetected errors. These fields can be detected when movement near a test setup (such as the movement of the person operating the instrument or others in the immediate vicinity) causes fluctuations on the electrometers display. To perform a quick check for interference, place a piece of charged plastic, such as a comb, near the circuit. A large change in the meter reading indicates insufficient shielding.AC fields can be equally troublesome. These are caused most often by power lines and RF fields. If the AC voltage at the input is large, part of this signal is rectified, producing an error in the DC signal being measured. This can be checked by observing the analog output of the electrometer with an oscilloscope. A clipped waveform indicates a need to improve electrostatic shielding.AC electrostatic coupling occurs when an electrostatic voltage source in the vicinity of a conductor, such as a cable or trace on a PC board, generates a current proportional to the rate of change of the voltage and of the coupling capacitance. This current can be calculated with the following equation:i = C dV/dt + V dC/dtFor example, two conductors, each with 1cm2 area and spaced 1cm apart by air, will have almost 0.1 pico-farad of capacitance. With a voltage difference of 100 volts between the two conductors and a vibration causing a change of capacitance of 0.01 pico-farad/second (a 10% fluctuation between them), a current of 1pA AC will be generated.To reduce the effects of the fields, a shield can be built to enclose the circuit being measured. The easiest type of shield to make is a simple metal box or meshed screen that encloses the test circuit. Shielded boxes are also available commercially. Made from a conductive material, the shield is always connected to the low impedance input of the electrometer.The cabling between the HI input terminal of the meter and the device under test also requires shielding. Capacitive coupling between an electrostatic noise source and the signal conductors or cables can be greatly reduced by surrounding those conductors with a metal shield connected to the LO input terminal. With this shield in place, the noise current generated by the electrostatic voltage source and the coupling capacitance flows through the shield to ground rather than through the signal conductors.To summarize, follow these guidelines to minimize error currents due to electrostatic coupling: Keep all charged objects (including people) and conductors away from sensitive areas of the test circuit. Avoid movement and vibration near the test area. When measuring currents of less than 1 nano-amp, shield the device under test by surrounding it with a metal enclosure and connect the enclosure electrically to the test circuit common terminal.The word shielding usually implies the use of a metallic enclosure to prevent electrostatic interference from affecting a high impedance circuit. Guarding implies the use of an added low impedance conductor, maintained at the same potential as the high impedance circuit, which will intercept any interfering voltage or current. A guard doesnt necessarily provide shielding.Environmental Factors. A stable test environment is essential for making accurate low level measurements of all types. This makes control of the following factors important.Temperature and Temperature Stability. Varying temperatures can affect low level measurements in several ways, including causing thermal expansion or contraction of insulators and producing noise currents. Also, a temperature rise can cause an increase in the input bias current of the meter. As a general rule, JFET gate leakage current doubles for every 10C increase in temperature, but most electrometers are temperature compensated to minimize input current variations over a wide temperature range.To minimize errors due to temperature variations, operate the entire charge measurement system in a thermally stable environment. Keep sensitive instruments away from hot locations (such as the top of a rack) and allow the complete system to achieve thermal stability before making measurements. Use the instruments zero or suppress feature to null offsets once the system has achieved thermal stability. Repeat the zeroing process whenever the ambient temperature changes. To ensure optimum accuracy, zero the instrument on the same range as that to be used for the measurement.Humidity. Excess humidity can reduce insulation resistance on PC boards and in test connection insulators. A reduction in insulation resistance can, of course, have a serious effect on high impedance measurements. In addition, humidity or moisture can combine with any contaminants present to create electrochemical effects that can produce offset currents.To minimize the effects of moisture, reduce the humidity in the environment (ideally
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