Wide-dynamic range electrometer with a fast response

• US 8,415,942 B2
• Filed: 12/16/2011
• Issued: 04/09/2013
• Est. Priority Date: 07/16/2009
• Status: Active Grant

• Current Assignee: MKS Instruments Incorporated
• Original Assignee: MKS Instruments Incorporated
• Inventors: Fletcher, William Roger
• Primary Examiner(s): BENITEZ ROSARIO, JOSHUA
• Application Number:
  US13/328,765
• Publication Number:
  US 20120086434A1
• Time in Patent Office: 480 Days
• Field of Search: 324/73.1, 324/123.R, 324/123.C, 324/124, 324457-457
• US Class Current: 324/123.R
• CPC Class Codes: G01R 15/09 Autoranging circuits
• CPC Class Codes: G01R 15/165 measuring electrostatic potential, e.g. with electrostatic voltmeters or electrometers, when the design of the sensor is essential electrometers with passively moving electrodes G01R5/28; measuring electrostatic fields G01R29/12; measuring charge G01R29/24; measuring in circuits with high internal resistance G01R19/0023
• CPC Class Codes: G01R 19/0023 Measuring currents or voltages from sources with high internal resistance by means of measuring circuits with high input impedance, e.g. OP-amplifiers electrostatic instruments G01R5/28; measuring electrostatic potential G01R15/165; measuring electrostatic fields G01R29/12; amplifiers per se H03F
• CPC Class Codes: G01R 19/0061 Measuring currents of particle-beams, currents from electron multipliers, photocurrents, ion currents; Measuring in plasmas
• CPC Class Codes: H03F 2200/261 Amplifier which being suitable for instrumentation applications
• CPC Class Codes: H03F 2203/45138 Two or more differential amplifiers in IC-block form are combined, e.g. measuring amplifiers
• CPC Class Codes: H03F 2203/45518 the FBC comprising one or more diodes and being coupled between the LC and the IC
• CPC Class Codes: H03F 2203/45522 the FBC comprising one or more potentiometers
• CPC Class Codes: H03F 2203/45528 the FBC comprising one or more passive resistors and being coupled between the LC and the IC
• CPC Class Codes: H03F 2203/45534 the FBC comprising multiple switches and being coupled between the LC and the IC
• CPC Class Codes: H03F 2203/45536 the FBC comprising a switch and being coupled between the LC and the IC
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit

Abstract

A method and apparatus for measuring current includes sensing a first voltage at the output of an amplifier and computing a current based on the first voltage and the resistance of a first resistive element, which is electrically coupled between an inverting input of the amplifier and the output of the amplifier, if the first voltage is below a predetermined level. The method also includes sensing a second voltage at the output of a buffer and computing a current based on the first and second voltages and the resistances of the first resistive element and a second resistive element, which is electrically coupled between the inverting input of the amplifier and an input of the buffer and is also electrically coupled to the output of the amplifier through a at least one diode, if the voltage output from the amplifier is above the predetermined level.

Claims

1. An electrometer, comprising:

an amplifier having an inverting input, a non-inverting input, and an output;

first and second resistive elements each having a first end and a second end, the first ends of the first and second resistive elements electrically coupled to the inverting input of the amplifier and the second end of the first resistive element electrically coupled to the output of the amplifier;

a buffer having a high-impedance input and an output, the second end of the second resistive element electrically coupled to the input of the buffer, wherein a change to an input voltage of the buffer results in a substantially equal change to an output voltage of the buffer; and

at least one diode electrically coupling the output of the amplifier to the second end of the second resistive element.

2. The electrometer of claim 1 wherein the at least one diode comprises a first diode and a low-leakage and low-capacitance second diode having a first end and a second end, the first end of the second diode electrically coupled to the input of the buffer, the second end of the second diode electrically coupled to the first diode.

3. The electrometer of claim 2 further comprising a third resistive element electrically coupling the second end of the second diode to ground.

4. The electrometer of claim 2 wherein the first diode is a small signal silicon diode.

5. The electrometer of claim 2 further comprising a temperature controller configured to stabilize the temperature of at least the second diode, the amplifier, and the buffer.

6. The electrometer of claim 1 wherein the at least one diode comprises pairs of diodes, each pair being electrically coupled in parallel and arranged with reverse polarity.

7. A method for measuring current, comprising:

sensing a first voltage at the output of an amplifier;

computing a current based on the first voltage and the resistance of a first resistive element electrically coupled between an inverting input of the amplifier and the output of the amplifier if the first voltage is below a predetermined level; and

sensing a second voltage at the output of a buffer, the buffer configured such that a change to an input voltage of the buffer results in a substantially equal change to an output voltage of the buffer;

computing a current based on the first and second voltages and the resistances of the first resistive element and a second resistive element, the second resistive element being electrically coupled between the inverting input of the amplifier and an input of the buffer, if the voltage output from the amplifier is above the predetermined level.