Probe Interface Circuit for High-Impedance pH Signal Buffering
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing interface units for coupling measuring probes to measuring systems face issues with high temperature resistance, current leakage, and signal distortion, especially when used with ion-sensitive or solid-state pH sensors, leading to inaccurate measurements and potential damage during autoclaving processes.
Innovation Solution
An interface unit with a decoupling circuit using transistors with insulated gates and operational amplifiers to buffer the input signal, maintaining low current flow and reducing residual current effects, while allowing for a wide range of input voltages without saturation or distortion, and compensating for manufacturing tolerances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the input stage circuit is switched off or in an unpowered state during high temperature process, then the impedance of the input stage circuit falls to a low level, but the residual current rises between the sensing element and the input stage circuit, causing signal drifts and damages to the sensor element
Solution Approach 1:
The patent introduces an intermediary component (such as a high-impedance buffer circuit or isolation amplifier) between the sensing element and the input stage circuit. This intermediary maintains high impedance even when the main input stage is powered down, thereby blocking residual current flow while still allowing the sensing element to function. The intermediary acts as a mediator that protects the sensor from harmful effects of the input stage's low-impedance state.
2Reliability
If the measuring probe is exposed to high temperature for sterilization, then the sterilization effect is achieved, but the resistance of the sensing element decreases drastically, causing current flow issues
Solution Approach 1:
The patent employs parameter changes by dynamically adjusting the input impedance of the measuring circuit based on temperature conditions. At high temperatures during sterilization, the circuit switches to a high-impedance mode to compensate for the decreased resistance of the sensing element. This prevents excessive current flow while maintaining measurement capability. The system monitors temperature and adapts its electrical parameters accordingly.
3Measurement precision
If the impedance of the input stage circuit is at a high level during normal operations, then the voltage measurement is accurate, but when switched off the impedance falls to a low level causing current leakage
Solution Approach 1:
The patent implements a dynamic impedance control mechanism where the input stage circuit can switch between high-impedance and low-impedance states based on operational requirements. During active measurement, the circuit maintains high impedance for accurate voltage measurement. When powered down or in standby mode, it transitions to a high-impedance hold state rather than falling to low impedance, thereby preventing current leakage while maintaining measurement readiness.
4Reliability
If the electrostatic discharge protection diode is used in the Operational Amplifier, then protection is provided, but at high temperatures the isolating characteristics decrease and short circuits may occur
Solution Approach 1:
The patent removes or extracts the electrostatic discharge protection diode from the operational amplifier circuitry to eliminate the high-temperature failure mode. Instead, it implements alternative protection mechanisms that are temperature-resistant, such as external protection circuits or different architectural approaches that do not rely on ESD diodes with poor high-temperature characteristics. This extraction eliminates the source of the problem while maintaining necessary protection functions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The interface unit effectively reduces residual current influence on the sensor elements, maintains accurate measurements across a wide temperature range, and extends the life span of sensors by minimizing lithium consumption, especially for solid-state pH sensors.
Implementation Method 1
a first transistor (210) with an insulated gate terminal coupled to the input (110)
Implementation Method 2
it is usual to couple the output of the sensing element of the measuring probe to an input stage circuit provided with an Operational Amplifier
Data Source
Figure 1~3
Figure 2
Figure 4
AI summary
The present disclosure relates an interface unit comprising: • an input for receiving an input voltage from an electrochemical measuring probe; • a first transistor provided with a first insulated gate terminal coupled to the input, a first source terminal coupled to a first terminal of a first resistor, and a first drain terminal coupled to a first terminal of a second resistor; the second terminal of the first resistor being coupled to a point where a constant voltage is maintained to a constant value; the first transistor being arranged to generate in the second resistor a second resistor voltage, according to the input voltage ; • a first operational amplifier provided with a first non-inverting input coupled to a point of the circuit where a reference voltage is maintained to a constant value; with a first inverting input coupled to the first drain terminal of the first transistor; and with a first output coupled to a second terminal of the first resistor; • a second transistor provided with a second source terminal coupled to a second terminal of a third resistor, and a second drain terminal coupled to a first terminal of a fourth resistor; a first terminal of the third resistor being coupled to the second terminal of the first resistor; the second terminal of the fourth resistor being coupled to a point where the constant voltage is maintained ; • a second operational amplifier provided with a second non-inverting input coupled to the second drain terminal of the second transistor; and, with an a second inverting input coupled to the first inverting input of the first operational amplifier; and, with a second output coupled a second insulated gate terminal of the second transistor and to the output of the interface unit. The first operational amplifier is arranged to provide a variable tension to the first source terminal of the first transistor, according to the comparison between the reference voltage and the second resistor voltage, in order to control the operating point of the first transistor.