WO2011097368A1

WO2011097368A1 - Continuous ranging ammeter and method of use thereof

Info

Publication number: WO2011097368A1
Application number: PCT/US2011/023588
Authority: WO
Inventors: Brian Degnan
Original assignee: Ix Innovations, Llc.
Priority date: 2010-02-03
Filing date: 2011-02-03
Publication date: 2011-08-11

Abstract

The present invention relates a method and apparatus for implementing a continuously ranging instrument for measuring small currents using an amplifier with negative or differential feedback and using a logarithmic-compression conversion technique.

Description

CONTINUOUS RANGING AMMETER AND METHOD OF USE THEREOF

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001 ] This application claims priority to: provisional United States Patent Application Serial No. 61 /301 , 194, filed on February 3, 2010, entitled "Continuous Ranging Ammeter And Methods Of Use Thereof," which provisional patent application is each commonly assigned to the Assignee of the present invention and is hereby incorporated herein by reference in its entirety for all purposes.

BACKGROUND

Field of the Invention

[0002] The present invention relates a method and apparatus for implementing a continuously ranging instrument for measuring small currents using an amplifier with negative or differential feedback.

Background of the Invention

[0003] Ammeters are typically implemented using a linear operational amplifier in a positive gain mode to convert the magnitude of the current flow into a voltage value, which is then converted for readout into a digital value using an analog-to-digital converter. "Auto-ranging" ammeters employ an array of these conversion units selected by relays set to measure magnitude of the current in different ranges. This "auto-ranging" design uses numerous parts and, when the relays switch between ranges, introduces a surge of current due to parasitic inductance (fly-back current) that is dangerous to the circuit under observation and therefore requires additional protection devices or circuitry. For instance, when for an ammeter calibrated to indicate values of the current in picoamperes (e.g., a picoammeter), such a surge of current can be significant.

[0004] For instance, a Model 6485 5- 1 /2 digit Picoammeter with l OfA Resolution (manufacnired by eithley Instruments, Inc., Cleveland, Ohio) (" eithley 6485 Picoammeter") is a picoammeter can measure currents from 20fA to 20mA by utilizing eight current measurement ranges and high speed autoranging. The eithley 6485 picoammeter taking measurements at speeds up to 1000 readings per second.

[0005] The Keithley 6485 Picoammeter is an ammeter that sets a voltage across a feedback resistor to the current input to create a voltage that matches the voltage seen at the reference terminal. This relationship is governed by V = IR, and because the resistance "R" is fixed for a range, different resistors must be switched into and out of the feedback loop.

[0006] For the Keithley 6485 Picomammeter, when it is necessary to up-range during autoranging, multiple ranges may need to be crossed to find the correct range. Keithley Model 6485 Picoammeter Instruction Manual, Applications Guide (Chapter I), 2001 , ("Keithley 6485 Application Guide"), at 1- 12 to 1-14. Relays are used to switch between different feedback resistors. During a range change, the Keithley 6485 Picoammeter cannot perfectly maintain its voltage burden speci fication. The relays have an internal inductance, and inductance resists a change in current. Thus, when a range change occurs, the Keithley 6485 Picoammeter will momentarily become a current-limited source, as shown in FIGURE 1 . FIGURE 1 illustrates the type of transient voltage (shown in curve 101 ) that can be expected by the Keithley 6485 Picoammeter when up-ranging with a full input signal (200μΑ on 200μΑ, up range to 2mA range). The sudden current draw that occurs when a relay switches causes large spikes in voltage. These spikes in voltage can damage semiconductor devices, such as the destruction of gate oxides in MOS devices.

[0007] In the past, to minimize the risk of such surges, users of such ammeters and other devices had to: use a fixed range (thus negating the advantages of auto-ranging); minimize transients by only down-ranging (which complicates the testing set-up); use protection circuitry (at an additional instrumentation expense) and/or design the circuitry under review to compensate for the measurement by the ammeter or other device (at an additional per unit cost). [0008] Therefore, there is a need for a simpler device with fewer parts and that covers a range of input currents in a continuous manner without the danger of fly-back current. Such a device and method would be extremely useful. Indeed, eliminating electrostatic discharge is now a significant issue for chip design due to the reduced size of the chips (i.e., 45 nm and less). Electrostatic discharge can damage transistors, wires, and insulations therebetween.

SUMMARY OF THE INVENTION

[0009] The present invention relates a method and apparatus for implementing a continuously ranging instrument for measuring small currents using an amplifier with negative or differential feedback and using a logarithmic-compression conversion technique. The basic theory of an ammeter assumes a linear feedback loop with V = IR across a resistor that supplies a resistance of "R." By rendering "R" nonlinear, a device can be made that has larger dynamic range for current measurement that do not require relays, or multiplexors to cover the range. A nonlinear resistance can be generated by a feedback loop with a transistor. The transistor may have logarithmic behavior with a linear amplifier, or the transistor may have linear behavior with a logarithmic amplifier.

[0010] In general, in one aspect, the invention features an ammeter. The ammeter includes an amplifier and a nonlinear resistance circuit, which are operatively connected. The amplifier and nonlinear resistance circuit are operable for measuring current using a logarithmic-compression conversion technique.

[001 1] Implementations of the invention can include one or more of the following features:

[0012] The nonlinear resistance circuitry can include a feedback loop and a first transistor, which are operatively connected.

[0013] The amplifier/first transistor combination can be a linear amplifier and a first transistor that has logarithmic behavior. [0014] The amplifier/first transistor combination can be a logarithmic amplifier and a first transistor that has linear behavior.

[0015] The ammeter can be a continuously ranging instalment for measuring small currents.

[0016] The feedback loop can be a differential feedback loop. The first transistor can be operatively connected to the amplifier in the differential feedback loop. The amplifier and the first transistor combination can be (a) an amplifier that is a logarithmic amplifier and a first transistor that has linear behavior or (b) an amplifier that is a linear amplifier and a first transistor that has logarithmic behavior. The amplifier and the first transistor can be operable for performing the logarithmic-compression conversion technique.

[0017] The ammeter can further include a second transistor. The nonlinear resistance circuit can include the second transistor. The second transistor can be operatively connected to the feedback loop and ihe first transistor. The amplifier, the first transistor, and second transistor can be operable for performing the logarithmic-compression conversion technique.

[0018] The first transistor can be a PNP bipolar junction transistor or a positively doped field- effect transistor. The second transistor can be a NPN bipolar junction transistor or a negatively doped field-effect transistor.

[0019] The amplifier, the first transistor, and the second transistor combination can be (a) the amplifier is a logarithmic amplifier, the first transistor has linear behavior, and the second transistor has linear behavior, or (b) the amplifier is a linear amplifier, the first transistor has logarithmic behavior, and the second transistor has logarithmic behavior. The amplifier, the first transistor, and the second transistor can be operable for performing the logarithmic-compression conversion technique. The ammeter can be operable for use as a bidirectional continuously ranging ammeter.

[0020]. The first transistor and second transition combination can be a first transistor that is a PNP bipolar junction transistor and a second transistor that is a NPN bipolar junction transistor. [0021 ] The first transistor can be a bipolar junction transistor or a field-effect transistor. The ammeter can be operable for use as a unidirectional continuously ranging ammeter.

[0022] The first transistor can be a NPN bipolar junction transistor.

[0023] The first transistor can be a PNP bipolar junction transistor.

[0024] The ammeter can be a picoammeter.

[0025] The ammeter can be operable to measure a current in picoamps.

[0026] The ammeter can be operable to measure a current in milliamps.

[0027] The ammeter can be operable to measure at a rate between about 5,000 and about 15,000 samples per second.

[0028] The ammeter cannot be operated to perform auto-ranging.

[0029] The ammeter may exclude relays operable to switch between current ranges.

[0030] The amplifier and nonlinear resistance circuit can be operable for performing a temperature compensation technique.

[0031] The ammeter can further include an analog-to-digital converter, which is operatively connected to the amplifier and the nonlinear resistance circuit. The ammeter can further include a microprocessor, which is operatively connected to the analog-to-digital converter. The ammeter can further include an input, which is operatively connected to the microprocessor. The ammeter can further include an output, which is operatively connected to the microprocessor. The ammeter can further include power circuitry.

[0032] In general, in another aspect, the invention features a method that includes the step of selecting an ammeter having a amplifier and a nonlinear resistance circuit operable for performing a logarithmic-compression conversion technique when measuring current. The method further includes using the ammeter to measure current.

[0033] Implementations of the invention can include one or more of the following features: [0034] The step of selection can include selecting an ammeter of one or more of the above embodiments.

[0035] The method can further include using the ammeter in one or more of the following uses: (a) a fixed current reference, (b) detecting hidden signals, (c) chaotic oscillator calibration, (d) heart beat monitoring, (e) component, sensor, device I-V and material characterization, (f) leakage current testing, (g) photodiode and dark current measurements, (h) ion beam alignment, (i) spectroscopy and fluorometry, (j) insulation resistance measurement, (k) analog and mixed circuit analysis, and (1) nanomaterial testing.

[0036] The method can further include using the ammeter to measure between about 5,000 and about 15,000 samples per second.

DESCRIPTION OF DRAWINGS

[0037] FIGURE 1 illustrates the type of transient voltage that can be expected by the Keithley 6485 Picoammeter when up-ranging with a full input signal (200μΑ on 200μΑ, up range to 2mA range).

[0038] FIGURE 2 illustrates a virtual short between input terminals to convert a current to voltage.

[0039] FIGURE 3 is a schematic of an embodiment of the present invention in which a log compression ammeter is used to measure current using a current sink circuit.

[0040] FIGURE 4 depicts a differential feedback loop that can be used to implement an embodiment of the present invention.

[0041] FIGURE 5 depicts a negative feedback loop that can be used to implement an embodiment of the present invention.

[0042] FIGURE 6 depicts a positive feedback loop that can be used lo implement an embodiment of the present invention. [0043] FIGURE 7 illustrates a comparison between the type of transient voltage that can be expected by the eithley 6485 Picoammeter and an embodiment of the present invention when up-ranging with a full input signal (200μΑ on 200μΑ, up range to 2mA range).

DETAI LED DESCRIPTION ₍

[0044] The fundamental voltage to current relationship is Ohm's Law,

V = IR,

where V is the amount of force (voltage) required to push mass over time (current) through a medium (resistance). If " V" and " ?" are known, " (the current) can be calculated from a circuit. One possible way to do the current to voltage conversion is shown in FIGURE 2. FIGURE 2 illustrates a virtual short between input terminals to convert a current to voltage. A common analysis of amplifiers involves the concept of a "virtual short," meaning that the amplifier 201 will try to match the voltage seen at the positive input terminal 202 and the negative input terminal 203. The amplifier 201 has a feedback through a single resistor 204 from the output 205 to the negative input terminal 203. The output voltage 206 of the output 205 can be set so that the current generated through the resistor 204 will cause the summing node 207 to be the same voltage as the voltage reference input 208.

[0045] By way of example, assume that the input current 209 (I_/.v) is 1 mA, and the resistor has a value of I kH with the value of voltage reference input 208 at GND. In the arrangement of FIGU RE 2, the amplifier 201 will set an output voltage 206 on the output 205 to match the voltage reference input 208 seen at the positive input terminal 202. In order to match a 1 mA input across a 1 &Ω resistor, the amplifier must produce - I V at the output 205.

[0046] In embodiments of the present invention, a "current sink" circuit concept is utilized to measure current, which can result in a bidirectional or unidirectional current measurement when the current flows through an ammeter device. FIGURE 3 is a schematic of an embodiment of the present invention in which a log compression ammeter is used to measure current using a current sink circuit. This design is different from a resistor-based ammeter, including because the feedback controls a MOSFET that is effectively a voltage-controlled current source. In the same way as shown in FIGURE 2, the current calculation can be achieved by current summing at a CMOS amplifier input. The voltage required at the output 302 of the amplifier 301 to achieve a node sum of zero at the summing node 303 is the voltage representation of the current. (As shown in FIGURE 3), output 302 is connected to an analog-to-digital converter 305 to provide a digital reading of the magnitude of the measured current.

[0047] As the current increases, the voltage at the output 302 increases so that the current at the summing node 303 cancels for voltage reference input 304. The equation that governs this behavior is

which combines all regions into a single equation. In this equation: /,_/, is the threshold current; κ is the divider from the gate to the channel; Ur is the thermal voltage, V_w is the bulk bias; V70 is the threshold voltage; is the gate voltage; V, is the source voltage; and V,/ is the drain voltage. [S.C. Liu, "analog VLSI. Circuits and Principles," Bradford Books (2002); Degnan, B.P, et a/. , "Passgate resistance estimation based on the compact E V model and effective mobility," IEEE International Symposium on Circuits and Systems, 2009, at pp. 2765-2768].

[0048] The mathematical form of In¹ ( 1 + e ^{J 2)}) interpolates between regions, and in saturated operation this equation reduces to

In this equation: W is the drawn width; L is the drawn length; // is mobility; C_nx is the oxide capacitance, and the remaining terms have the same meaning as in the earlier equation.

[0049] When these conditions are met to keep the device in subthreshold operation, the output will result in a log-compressed voltage representation of the current for a significantly increased dynamic range. By utilizing a logarithmic-compression conversion technique, mechanical relays used in the prior art have been eliminated (which eliminates potentially harmful voltage spikes associated with range switching). Moreover, this technique is capable of delivering measurements at a rate of 15,000 samples per second (i.e. , between about 5,000 and about 15,000 samples per second), which is significantly greater than the 1 ,000 samples per second measurement speed of the Keithley 6485 Picoammeter.

[0050] FIGURE 4 shows the use of an operational amplifier 402 (such as a Converter AD8304 manufactured by Analog Devices, Norwood, MA) employed in a differential feedback loop 401 in order to implement a bidirectional continuously ranging ammeter (such as a picoammeter). The current 403 to be measured is probed at point A (408) and is connected to the negative input 404 of the operational amplifier 402. The operation amplification can be achieved by either a bipolar junction transistor (BJT) or a field-effect transistor (FET) with current from diffusion- based transport to provide a logarithmic voltage-current relationship.

[0051 ] As shown in FIGURE 4, the output of the operational amplifier at point B is connected to the base of both a PNP bipolar junction transistor 405 and an NPN bipolar junction transistor 406 to complete the differential feedback and allow the instrument to measure positive or negative current flow. (Alternatively, a positively doped field-effect transistor and a negative doped field-effect transistor can be utilized in place of the PNP bipolar junction transistor 105 and the NPN bipolar junction transistor 106, respectively).

[0052] To achieve a logarithmic output, either (i) the operation amplifier is a logarithmic operational amplifier and the transistors exhibit a linear behavior or (ii) the transistors exhibit a logarithmic behavior and the operation amplifier is a linear operational amplifier. For instance, the NPN bipolar junction transistor 106 in the Converter AD8304 can be modified such that it exhibits a logarithmic behavior. Converter AD8304 encompasses a linear operational amplifier with BJT transistor for use with receiver diode junctions for fiber optic applications. Converter AD8304 can be modified for use as an ammeter for four concurrent design decisions: limiting the current range that is seen into current input to minimize high-order effects in the depletion regions, supplying an adequate source voltage to maintain depletion width and thereby performance, modifying the output range, and intercept of the operational amplifier for coherent results.

[0053] Point B is also connected to an analog-to-digital converter 107 (such as an ADS8320 manufactured by Texas Instruments, Inc., Dallas, Texas) to provide a digital reading of the magnitude of the measured current.

[0054] The use of the operational amplifier 102 coupled with transistors 105 and 106 (one set of which exhibiting logarithmic behavior) simplifies the design by allowing the ammeter to be implemented as continuously ranging using a single operational amplifier rather than auto- ranging using an array of linear operational amplifiers. Logarithmic amplifiers and transistors exhibiting logarithmic behavior with varying characteristics are available to allow for current measurements of various ranges.

[0055] While not shown in FIGURE 4, the analog-to-digital converter 107 can be operatively connected to a microprocessor, an input, and output, and power circuitry.

[0056] FIGURE 5 shows the use of an operational amplifier 402 employed in a negative feedback loop 501 in order to implement a unidirectional ammeter. The circuit shown in FIGURE 5 is a simplified version of the circuit (shown in FIGURE 5), and is only capable of measuring current flow in a single direction. The current 502 to be measured is probed at point A (503) and is connected to the negative input 404 to the operational amplifier 402. The output of the logarithmic operational amplifier at point B (504) is connected to the base of an NPN bipolar junction transistor 405 to complete the negative feedback and allow the instrument to measure current flow. (Alternatively, a negative doped field-effect transistor can be utilized in place of the NPN bipolar junction transistor 406). Point B (504) is also connected to an analog- to-digital converter 407 to provide a digital reading of the magnitude of the measured current.

[0057] In an embodiment of the present invention, a linear CS3003 operational amplifier (manufactured by Cirrus Logic, Inc., Austin, Texas) and a ZU T71 TA BJT transistor (Diodes, Inc., Dallas, Texas) that is designed to source current can be used. The current output of such device has a logarithmic dependence on the voltage seen at the emitter.

[0058] In another embodiment of the present invention, an n-type transistor can be used in the same manner with some modifications. The nFET transistor has many operational modes. The subthreshold regime allows for an exponential current response to changes in gate voltage in the same way as the NPN with the added advantage of current isolation due to the gate capacitor of the nFET. The above-threshold region offers a quadratic current-voltage relationship. The nFET can also be used to force an exponential behavior as current is being sourced, not sunk from the device allowing low-current readings to attoampheres ( 10^'18 amperes) at the cost of dynamic range.

[0059] FIGURE 6 shows the use of an operational amplifier 402 employed in a positive feedback loop 601 in order to implement an alternative unidirectional ammeter. This embodiment is similar to that of FIGURE 5, with PNP bipolar junction transistor 406 being substituted for NPN bipolar junction transistor 405. (Alternatively, a positive doped field-effect transistor can be utilized in place of the PNP bipolar junction transistor 405).

[0060] In the ammeter of the current invention, the feedback look (i.e., the differential feedback loop 401 , the negative feedback loop 501 , or the positive feedback loop 601) and the analog-to- digital converter 407 are combine with other elements, such as, for example, a microprocessor, inputs, outputs (such as an LCD display), and power circuitry to provide the required voltages.

[0061 ] In an embodiment of the present invention, a linear CS3003 operational amplifier (Cirrus Logic) and a ZU T71 8TA BJT transistor (Diodes, Inc., Dallas, Texas) that is designed to source current have been used. The current output of such device has a logarithmic dependence on the voltage seen at the emitter.

[0062] In another embodiment of the present invention, an p-type transistor can be used in the same manner with some modifications. The pFET transistor has many operational modes. The subthreshold regime allows for an exponential current response to changes in gate voltage in the same way as the PNP with the added advantage of current isolation due to the gate capacitor of the pFET. The above-threshold region offers a quadratic current-voltage relationship. The pFET can also be used to force an exponential behavior as current is being sunk, not sourced from the device allowing low-current readings to attoampheres ( 10 ¹⁸ amperes) at the cost of dynamic range.

[0063] The ammeter of the present invention can work over a wide range of currents. Generally, such range of currents is limited by the type of logarithmic amplifier selected. Typically, most logarithmic amps are geared towards lower current applications and, when that is the circumstance, applying currents over a few amps will destroy the ammeter.

[0064] To adjust the range, the sensitivity of the output can be set based on parameters of the selected logarithmic amplifier. In effect, the output sensitivity is adjusted to the desired range to be measured. For instance, for a Converter AD8304, the slope and intercept of the amplifier can be adjusted per the guidelines in the Slope and Intercept Adjustment Section (pages 10- 12) of the Preliminary Technical Data for Logarithmic Converter AD8304, which is incorporated herein by reference. [0065] In embodiments of the present invention, it has been found that there can be a temperature dependence arising from the physics of Boltzman's distributions. The present invention will function without temperature compensation; however, in embodiments of the present invention, behavior could be improved by correlating the voltage value from the feedback loop in ammeters to a current value.

[0066] FIGURE 7 illustrates a comparison between the type of transient voltage that can be expected by the Keithley 6485 Picoammeter and an embodiment of the present invention when up-ranging with a full input signal (200μΑ on 200μΑ, up range to 2mA range). Curve 701 illustrates the type of transient voltage that can be expected by the Keithley 6485 Picoammeter and corresponds to the information of FIGURE 1 (which was reproduced from Keithley 6485 Application Guide, at 1-14. Curve 702 illustrates the type of transient voltage that was achieved by an embodiment of the present invention.

[0067] The present invention streamlines the testing and measuring of very-low electrical current in electronics, materials, and more. Compared to other prior art ammeters, the present invention is faster, simpler, more economically. It is also small enough to fit in the user's pocket.

[0068] The present invention allows for a continuous ranging feature that is superior to the conventional mechanical relay-based technology used by other ammeters. This feature allows for accurate, real-time current measurement over many decades of current, while eliminating (or significantly reducing) induced noise and transient voltages and their related issues. (In embodiments of the present invention, single measurement of 20 pA to 2mA can be performed at a rate of 1 ,000 samples per second).

[0069] Various types of uses for the ammeter of the present invention include:

[0070] Fixed current reference. The logarithmic operational amplifier can be used in combination with a voltage source to fix a current through a device. Since V=IR, if R is changing, V must be changed to keep I fixed. For example, such a fixed cu ent reference would be useful for semiconductor testing.

[0071] Detecting hidden signals. The fixed current reference would also useful for pulling out possible signals from noise. A broad spectrum signal could be run through a band of band-pass filters to do "base band" processing. When it is passed into a vector matrix multiplier, this will provide a vector of currents. The "fixed current" reference can then be used to determine the frequencies that might have hidden signals. For instance, this technique could be used to track frequency hopping radios.

[0072] Chaotic oscillator calibration. A logarithmic operational amplifier can be modified to calibrate a chaotic oscillator. Chaotic oscillators can be used to synchronize analog-base - transmissions and encrypt/decrypt them. This is a solution for solving the drone problem that the US Army is having with people stealing their feeds.

[0073] Heart-beat monitor. The ammeter of the present invention can be combined with a directional antenna to identify heart beats of mammals. For instance, such information can be used to locate or target the mammals.

[0074] Component, sensor, device I-V and material characterization. The ammeter of the present invention can be used to determine non-linear resistance across a material, such as silicon or plastics by setting a high voltage and reading the resulting current seen across the device under test.

[0075] Leakage current testing. The ammeter of the present invention can be used to characterize transistor leakage across junctions and to then predict the operating range of the transistors outside of the explicit measureable ammeter range.

[0076] Photodiode and dark current measurements. The ammeter of the present invention can be used to determine the voltage to current relationship of doped junctions, such as diodes. A specific application is the light-induced current from photodiodes, which are the basis of the digital camera. The dark current is the inherent junction leakage of diode in a zero-light condition.

[0077] Ion beam alignment. The ammeter of the present invention can be used to determine the path of an ion beam by collecting current on a sensor to determine the actual ion path.

[0078] Spectroscopy and fluorometry. The ammeter of the present invention can be used to read the current seen across a junction induced by the reflection of light, or other electromagnetic radiation across an object and then read as current flux across a sensor or sensor circuit.

[0079] Insulation resistance measurement. The ammeter of the present invention can be used to measure the current across an insulator by applying a high voltage conductor at one point of the material at a known distance from another conductor and measure the current flow between the two conductors.

[0080] Analog and mixed circuit analysis. The ammeter of the present invention can be used to set and read current biases in an analog system. Furthermore, the ammeter can be used to determine circuit behavior for a given input signal in the analog domain. The ammeter can also be used to determine if unauthorized circuits have been added to digital systems by an increase in current when the digital system is powered down. The ammeter can be used to characterize the functional performance of a digital system and determine other circuits have been switched on intentionally or unintentionally. The ammeter can be used to determine if a trusted digital system has been compromised electronically through increased current draw and one-time current events.

[0081 ] Nanomaterial testing. The ammeter of the present invention can be used to determine variable resistance across materials or devices, including conductance changes to due to electron spin properties or due to dopant changes. [0082] While embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described and the examples provided herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Accordingly, other embodiments are within the scope of the following claims. The scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.

[0083] The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated herein by reference in their entirety, to the extent that they provide exemplary, procedural, or other details supplementary to those set forth herein.

Claims

WHAT IS CLAI MED IS:

1 . An ammeter comprising:

(a) an amplifier;

(b) a nonlinear resistance circuit operatively connected to the amplifier, wherein the amplifier and nonlinear resistance circuit are operable for measuring current using a logarithmic-compression conversion technique.

2. The ammeter of Claim 1 , wherein the nonlinear resistance circuitry comprises:

(i) a feedback loop and

(ii) a first transistor operatively connected to the feedback loop.

3. The ammeter of Claim 2, wherein the amplifier is a linear amplifier and the first transistor has logarithmic behavior.

4. The ammeter of Claim 2, wherein the amplifier is a logarithmic amplifier and the first transistor has linear behavior.

5. The ammeter of Claims 1 -3 or 4, wherein the ammeter is a continuously ranging instrument for measuring small currents.

6. The ammeter of Claims 2-4 or 5, wherein

(a) the feedback loop is a differential feedback loop;

(b) the first transistor is operatively connected to the amplifier in the differential feedback loop, wherein (i) the amplifier and the first transistor are selected from the group consisting of

(A) the amplifier is a logarithmic amplifier and the first transistor has linear behavior; and

(B) the amplifier is a linear amplifier and the first transistor has logarithmic behavior, and

(ii) the amplifier and the first transistor are operable for performing the logarithmic-compression conversion technique.

The ammeter of Claims 2-5 or 6, further comprising a second transistor, wherein

(a) the nonlinear resistance circuit comprises the second transistor;

(b) the second transistor is opeialively connected to the feedback loop and the first transistor; and

(c) the amplifier, the first transistor, and second transistor are operable for performing the logarithmic-compression conversion technique.

The ammeter of Claim 7, wherein

(a) the first transistor is selected from the group consisting of a PNP bipolar junction transistor and a positively doped field-effect transistor; and

(b) the second transistor selected from the group consisting of a NPN bipolar junction transistor and a negatively doped field-effect transistor.

The ammeter of Claim 8, wherein

(a) the amplifier, the first transistor, and the second transistor are selected from the group consisting of (i) the amplifier is a logarithmic amplifier and both of the first transistor and the second transistor have linear behavior, and

(ii) the amplifier is a linear amplifier and both of the first transistor and the second transistor have logarithmic behavior;

(b) the amplifier, the first transistor, and the second transistor are operable for performing a logarithmic-compression conversion technique; and

(c) the ammeter is operable for use as a bidirectional continuously ranging ammeter.

10. The ammeter of Claim 9, wherein

(i) the first transistor is a PNP bipolar junction transistor, and

(ii) the second transistor is a NPN bipolar junction transistor.

1 1. The ammeter of Claims 2-5 or 6, wherein:

(a) the first transistor is selected from the group consisting of a bipolar junction transistor or a field-effect transistor; and

(b) the ammeter is operable for use as a unidirectional continuously ranging ammeter.

12. The ammeter of Claim 1 1 , wherein the first transistor is a NPN bipolar junction transistor.

13. The ammeter of Claim 1 1 , wherein the first transistor is a PNP bipolar junction transistor.

14. The ammeter of Claims 1 - 12 or 13, wherein the ammeter is a picoammeter.

1 5. The ammeter of Claims 1 - 12 or 13, wherein the ammeter is operable to measure a current in picoamps.

16. The ammeter of Claims 1 - 12 or 13, wherein the ammeter is operable to measure a current in milliamps.

17. The ammeter of Claims 1 - 15 or 16, wherein the ammeter is operable to measure at a rate between about 5,000 and about 15,000 samples per second.

18. The ammeter of Claims 1 -16 or 17, wherein the ammeter is not operable to perform auto- ranging.

19. The ammeter of Claims 1 - 16 or 17, wherein the ammeter does not comprise relays operable to switch between current ranges.

20. The ammeter of Claims 1 -18 or 19, wherein the amplifier and nonlinear resistance circuit are operable for performing a temperature compensation technique.

21 . The ammeter of Claims 1 - 19 or 20 further comprising:

(a) an analog-to-digital converter operatively connected to the amplifier and the nonlinear resistance circuit;

(b) a microprocessor operatively connected to the analog-to-digital converter;

(c) an input operatively connected to the microprocessor;

(d) an output operatively connected to the microprocessor; and

(e) power circuitry.

22. A method comprising the steps of:

(a) selecting an ammeter having a amplifier and a nonlinear resistance circuit operable for performing a logarithmic-compression conversion technique when measuring current; and

(b) using the ammeter to measure current.

23. The method of Claim 22, wherein said step of selection comprises selecting an ammeter of Claims 1 -20 or 21 .

24. The method of Claims 22 or 23, further comprising using the ammeter in a use selected from the group consisting of a fixed current reference; detecting hidden signals; chaotic oscillator calibration; heart beat monitoring; component, sensor, device I-V and material characterization; leakage current testing; photodiode and dark current measurements; ion beam alignment; spectroscopy and fluorometry; insulation resistance measurement; analog and mixed circuit analysis; nanomaterial testing; and combinations thereof.

25. The method of Claims 22-23 or 24, further comprising using the ammeter to measure between about 5,000 and about 15,000 samples per second.