Integrated electric field sensor

Abstract

An electric field sensor includes one or more sensing electrodes connected to an integrated amplifier that bootstraps all parasitic capacitances at the sensor input to provide for a very high input impedance without the need for neutralization or other adjustments and calibration. The integrated amplifier for the electric field sensor further includes low-noise ESD / biasing structures to stabilize the DC-potential of the sensor with a minimum amount of added noise, leakage and parasitic capacitance.

Description

RELATED APPLICATIONS[0001]This application claims the benefit of U.S. Provisional Patent Application No. 61 / 350,449, filed Jun. 1, 2010, which is hereby incorporated by reference in its entirety.FIELD OF THE INVENTION[0002]The present invention relates to high input impedance sensors and circuits for measurements of electric fields.BACKGROUND OF THE INVENTION[0003]Non-contact, electric field measurements have been a challenge due to the need for constructing low-noise amplifiers with extremely high input impedance (>100 fF∥10TΩ) and low noise (>0.1 fA / Hz1 / 2). Prior art solid-state electric field sensors, such as those described in U.S. Pat. No. 6,686,800, US 2011 / 0043225, and U.S. Pat. No. 7,439,746, used for both free-space and biological applications, have relied on commercially available ‘discrete’ operational amplifiers or instrumentation amplifiers. One example is the TI INA116, from Texas Instruments, Dallas, Tex., which has an input impedance typically on the order of 2...

AI Technical Summary

Benefits of technology

[0012]Integration of the input biasing on-chip according to the present invention make it possible to achieve higher impedance, lower noise, lower capacitance and additional bootstrapping. Alternatively, an integrated approach makes it possible to build a sensor that can operate without the need for conductive biasing and operate entirely as a floating gate. In such a mode, the sensor input is initialized by using a non-volatile charge write method (e.g. hot carrier injection, electron tunneling) to set the input transistor's gate voltage. Since there is no conductive path at the sensor input, input leakage, drift, and current noise can be eliminated.

Problems solved by technology

Non-contact, electric field measurements have been a challenge due to the need for constructing low-noise amplifiers with extremely high input impedance (>100 fF∥10TΩ) and low noise (>0.1 fA / Hz1 / 2).

Any circuit element having a conductance or capacitance and connected to the sensor input necessarily degrades the input impedance.

However, there are always circuit elements connected as part of the circuit's normal operation (e.g., an amplifying transistor, biasing resistor, and shield) and other parasitic byproducts (e.g., neighboring electrical connections to the input).

Although active shielding has been effective in the prior art for minimizing the input capacitance on the packaging and circuit board level, its efficacy is reduced for reducing the internal capacitance of a discrete amplifier.

Additionally, attempts at implementing a high impedance amplifier using discrete components (e.g., transistors, resistors, capacitors) with bootstrapping have become difficult, if not impossible, due to the lack of suitable discrete FET parts with appropriate specifications (e.g., low gate leakage for JFETs, and low leakage ESD for MOSFETs).

Overcoming the internal input capacitance within the discrete amplifier has required the use of a positive feedback network that comprises of a second amplifier, with gain greater than unity, driving a neutralization capacitor, a technique known to a person skilled in the art Implementation is difficult due to the need for manual calibration and tuning.

In addition, the use of neutralization is additionally imprecise due to the non-linear input capacitance (e.g., P-N junction capacitance of protection diodes) of a typical discrete amplifier, which may vary across operating conditions making the entire process inherently imprecise and difficult to manufacture.

This method is effective but has an additional set of limitations, including that: 1) it requires a careful selection of components including the specific discrete amplifier part since this mode necessarily operates the part outside of its recommended usage; 2) a large voltage minimum supply range is necessary (>5-10V) since the supply must accommodate both the primary amplifier (3V) plus an additional overhead required to operate the power supply bootstrap circuit (˜3-5V); and 3) stable operation is difficult to achieve due to the multiple feedback paths involved in bootstrapping of all the ports in a discrete amplifier.

Additionally, DC biasing the sensor input has been difficult due to the need for high resistance (>100 GΩ), low-noise (1 / 2), low-leakage (<20 fA), elements.

The use of discrete components is subject to the following disadvantages: 1) low noise biasing resistors (1 / 2 current noise) are not commercially available and can be only implemented at great cost; 2) other input bias techniques, such as diodes, can provide lower noise but add additional leakage and capacitance to the input; and 3) discrete components add more parasitic capacitance and leakage than integrated versions of the same, adding noise to the sensor.

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