High sensitivity I-V conversion circuit and design method thereof

Abstract

The present application belongs to the technical field of signal test and measurement, and particularly relates to a high-sensitivity I-V conversion circuit and a design method thereof, which can improve the current measurement sensitivity by two orders of magnitude under the condition that the load voltage is the same. The present application realizes a high-sensitivity I-V conversion function through a two-stage active deep negative feedback circuit structure, and can increase the resistance value of a 100-fold current sampling resistor under the premise of keeping the load voltage unchanged, so as to greatly improve the voltage output by the I-V conversion circuit, and at the same time solve many problems caused by the great influence of the traditional I-V conversion circuit on the impedance of the current loop to be measured.

Description

Technical Field

[0001] This invention belongs to the field of signal testing and measurement technology, specifically relating to a high-sensitivity IV conversion circuit and its design method. Background Technology

[0002] In high-precision digital multimeter measurement circuits, an IV conversion circuit is often used to convert the measured current signal into a voltage signal: A DC current signal is input, conditioned by the IV conversion circuit to output a DC voltage signal, which is then processed by an analog-to-digital converter and digital signal processing to obtain the current value; an AC current signal is input, conditioned by the IV conversion circuit to output an AC voltage signal, which is then processed by an RMS conversion circuit, an analog-to-digital converter, and digital signal processing to obtain the current value. The performance indicators of the standard sampling resistor in the IV conversion circuit, such as its resistance value, directly determine the sensitivity of the current detection circuit; a higher sensitivity indicates higher current measurement accuracy. However, traditional IV conversion circuits only contain one or more standard sampling resistors of different specifications, selected according to the current measurement range. Since V=I×R, when the output voltage V of the IV conversion circuit is measured and the resistance value R is known, the measured current I can be calculated. Traditional IV conversion circuits, such as... Figure 1 As shown, the current detection sensitivity of the current in the current conversion circuit is not high. Summary of the Invention

[0003] In view of this, the present invention provides a high-sensitivity IV conversion circuit and its design method, which can improve the current measurement sensitivity by two orders of magnitude, or 100 times, under the same load voltage conditions.

[0004] To achieve the objectives of this invention, the following technical solutions are provided.

[0005] A high-sensitivity IV conversion circuit includes a standard sampling resistor Rs, and further includes: a first buffer unit, a second buffer unit, and a feedback amplification unit; The standard sampling resistor Rs is connected between the first node A and the second node B; The second buffer unit has a high input impedance, and its input is connected to the first node A; The input terminal of the first buffer unit is connected to the output terminal of the second buffer unit, and the output terminal of the first buffer unit is connected to the second node B; The input of the feedback amplification unit is connected to the first node A, and its output is fed back to the feedback input of the second buffer unit. The feedback amplification unit is configured to amplify the voltage signal of the first node A and provide negative feedback, such that the equivalent input impedance of the second node B to ground is one-Nth of the resistance value of the standard sampling resistor Rs, where N is the absolute value of the amplification factor of the feedback amplification unit.

[0006] The second buffer unit includes a first operational amplifier U3, whose non-inverting input is connected to the first node A as the input of the second buffer unit.

[0007] The first buffer unit includes a second operational amplifier U1 and a third operational amplifier U2. The input terminal of the second operational amplifier U1 is connected to the output terminal of the first operational amplifier U3, and the output terminal of the second operational amplifier U1 is connected to the input terminal of the third operational amplifier U2. The output terminal of the third operational amplifier U2 is connected to the second node B as the output terminal of the first buffer unit.

[0008] The feedback amplification unit includes a fourth operational amplifier U4 and an external resistor network. The external resistor network is configured to make the fourth operational amplifier U4 an inverting amplifier with a gain N of 100.

[0009] The amplifier in the second buffer unit is an operational amplifier with a JFET or CMOS input stage.

[0010] The standard sampling resistor Rs is a precision foil resistor.

[0011] The present invention also provides a design method for the high-sensitivity IV conversion circuit described herein, comprising the following steps: Calculate and determine the resistance value of the standard sampling resistor Rs based on the target current range and the desired output voltage. An operational amplifier with high input impedance is selected as the core amplifier of the second buffer unit; The first buffer unit is constructed by selecting an operational amplifier with low offset voltage and low noise. The feedback amplification unit is configured with a precision resistor network to set its precise amplification factor N; A phase compensation network is introduced into the feedback loop of the second buffer unit and the feedback amplification unit to ensure the overall stability of the circuit. The assembled circuit is calibrated to eliminate systematic errors introduced by component tolerances and misalignments.

[0012] Beneficial effects 1. This invention achieves a high-sensitivity IV conversion function through a two-stage active deep negative feedback circuit structure. While keeping the load voltage constant, the resistance value of the current sampling resistor can be increased by 100 times, thereby greatly improving the output voltage of the IV conversion circuit. At the same time, it solves many problems caused by the large influence of the impedance of the current loop under test in traditional IV conversion circuits.

[0013] 2. This invention creatively solves the core contradiction in traditional IV conversion circuits—the inability to simultaneously achieve high sensitivity and low load voltage—through a two-stage active deep negative feedback circuit architecture. By introducing only a minimal load voltage (typically only one percent of that in traditional circuits), it achieves a 100-fold increase in current detection sensitivity. This allows the circuit to convert weak current signals into voltage signals with sufficiently large amplitude, easily measured by downstream circuitry, without interfering with the normal operation of the circuit under test, significantly improving measurement accuracy and signal-to-noise ratio.

[0014] 3. In the design of the circuit of the present invention, it is permissible to use a precision sampling resistor with a resistance value up to 100 times that of the traditional scheme, thereby obtaining an output voltage with a 100-fold increase in amplitude under the same measured current. At the same time, through the internal feedback mechanism, this high resistance value is "virtually" converted into an extremely low equivalent input impedance and presented to the circuit under test.

[0015] 4. This invention provides a systematic design method, from parameter calculation, core component selection, stability design to final calibration, to guide the realization of a practical circuit with high linearity and high stability. It can be widely used in multi-range high-precision digital multimeters, picoammeters and other precision weak current measuring instruments ranging from microamperes to milliamperes, significantly improving the performance of such instruments. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a traditional IV conversion circuit.

[0017] Figure 2 This is a schematic diagram of a two-stage active deep negative feedback IV conversion circuit according to an embodiment of the present invention. Detailed Implementation

[0018] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0019] This embodiment provides a high-sensitivity IV conversion circuit, which is a two-stage active deep negative feedback IV conversion circuit. Let Ix be the measured current, Rs be the standard sampling resistor, the voltage at point A be V2, the voltage at point B be V1, the gains of the four amplifiers U1 to U4 be A1 to A4 respectively, and Rin be the equivalent input impedance of U3. Circuit analysis shows that: 1 Since the equivalent input impedance Rin of U3 is very high, reaching up to TΩ, the above equation can be simplified to: 2 During current detection, the measured current Ix can be calculated by measuring the voltage V2 at point A, i.e.: 3 It can be concluded that when the value of Rs is fixed, the output voltage V2 is linearly proportional to Ix, and the linearity of the IV conversion circuit depends on the performance of Rs.

[0020] Since V1 and V2 are in the same current loop and have the same current, according to Formula 2, the equivalent impedance at point B is 0.01 times that at point A. Compared to the traditional IV conversion circuit, for the same load voltage, the resistance of the current sampling resistor can be increased by 100 times. In other words, when using the same current sampling resistor, the output voltage of the IV conversion circuit proposed in this invention is 100 times that of the traditional IV conversion circuit, thereby improving the current detection sensitivity of the measurement circuit by 100 times.

[0021] For example, in a digital multimeter with a 10µA current range, a 100kΩ precision foil resistor is used for current sampling. The output voltage of the IV conversion circuit of this invention is 1V, while the actual equivalent shunt resistance of the circuit under test is only 1kΩ, resulting in a load voltage of only 10mV, which greatly reduces the influence of the measurement circuit on the current circuit under test.

[0022] For example, in a digital multimeter with a 100µA current range, a 10kΩ precision foil resistor is used for current sampling. The output voltage of the IV conversion circuit of this invention is 1V, while the actual equivalent shunt resistance of the circuit under test is only 100Ω, and the resulting load voltage is only 10mV, which greatly reduces the influence of the measurement circuit on the current circuit under test.

[0023] For example, in a digital multimeter with a 1mA current range, a 1kΩ precision foil resistor is selected for the current sampling resistor. The output voltage of the IV conversion circuit of this invention is 1V, while the actual equivalent shunt resistance of the circuit under test is only 10Ω, and the resulting load voltage is only 10mV, which greatly reduces the influence of the measurement circuit on the current circuit under test.

[0024] For example, in a digital multimeter with a 10mA current range, a 100Ω precision foil resistor is used for current sampling. The output voltage of the IV conversion circuit of this invention is 1V, while the actual equivalent shunt resistance of the circuit under test is only 1Ω, resulting in a load voltage of only 10mV, which greatly reduces the influence of the measurement circuit on the current circuit under test.

[0025] This invention provides a high-sensitivity IV-to-Voltage conversion circuit based on a two-stage active deep negative feedback architecture and its systematic design method. The core of this solution lies in the fact that, through ingenious feedback network design, a high-multiplier and high-linearity voltage conversion of weak current signals is achieved under conditions of extremely low insertion impedance, i.e., minimal impact on the measured current loop, thereby increasing the current detection sensitivity to 100 times that of traditional solutions.

[0026] The specific circuit implementation is shown in the attached figure. Figure 2 As shown, a composite feedback loop consisting of four operational amplifiers U1-U4 is constructed. The measured current Ix flows through the precision sampling resistor Rs, generating an initial voltage V2 at point A. U3, acting as a non-inverting amplifier, drives the current buffer stage composed of U1 and U2, generating voltage V1 at point B. Crucially, U4 is configured as a -100-gain inverting amplifier, amplifying the voltage V2 at point A and feeding it back to the inverting input of U3, forming a globally deep negative feedback loop. This structure forces the voltage V1 at point B to automatically adjust to 1 / 100 of V2, i.e., V1 = V2 / 100. Since the voltage across Rs is (V1 - V2) ≈ -0.99V2, according to Ohm's law, this is equivalent to presenting a minimal input impedance of only Rs / 100 between point B and ground. Therefore, for the circuit under test, its load voltage is only Ix * (Rs / 100), significantly reducing its impact on the circuit under test.

[0027] The mechanism behind its enhanced sensitivity lies in the fact that the output voltage V2A used for measurement is effectively "amplified" compared to the voltage directly taken from the sampling resistor in a conventional circuit, under the same measured current Ix and the same sampling resistor Rs. Analysis shows that V2 ≈ Ix * Rs. This means that to generate the same output voltage V2 in a conventional circuit, a standard resistor with a resistance value 100 times higher must be used, which inevitably leads to an unbearable huge load voltage. This invention cleverly avoids this contradiction, allowing designers to choose a precision sampling resistor Rs with a resistance value 100 times higher, thereby increasing the IV conversion gain V2 / Ix by two orders of magnitude and greatly enhancing the ability to detect minute current signals.

[0028] The circuit design follows a systematic approach: First, the full-scale input of the subsequent ADC is typically matched based on the target current range and desired output voltage range. For example, the theoretical value of the sampling resistor Rs is determined by 1V: Rs = V2_full_scale / Ix_full_scale. Second, for core amplifier selection, U3 needs to have extremely high input impedance, such as JFET or CMOS input type, to ignore the shunt effect. U4 needs sufficient bandwidth and output drive capability to stably achieve a gain of -100. U1 and U2 should preferably be low offset voltage, low noise precision operational amplifiers. Third, the feedback resistor network needs to use high-precision, low-temperature-drift resistors to ensure the accuracy and stability of the 100-fold gain in this example. The stability design of the entire circuit is crucial; a phase compensation network should be appropriately introduced into the feedback loops of U3 and U4 to prevent oscillations caused by excessive loop gain. Finally, precise calibration is essential to eliminate systematic errors introduced by operational amplifier offset, resistor tolerances, etc., ensuring the absolute linearity and accuracy of the IV conversion.

[0029] For example, in the design of a 10µA full-scale ammeter, a traditional approach requires a 100kΩ resistor to achieve a 1V output voltage, which introduces a 1V load voltage into the circuit under test, typically unacceptable. Using the solution of this invention, a 100kΩ precision foil resistor can be safely used as Rs to obtain a 1V full-scale output voltage at point A, while the load voltage introduced to the circuit under test is only 10mV. The equivalent impedance at point B is 1kΩ, achieving a hundredfold increase in sensitivity without sacrificing the operating state of the circuit under test. This design method can be extrapolated to multiple ranges from microamps to milliamps, providing a high-performance IV conversion solution for high-precision digital multimeters, picoammeters, and other precision measuring instruments.

[0030] This invention includes, but is not limited to, the above embodiments. Any equivalent substitutions or partial improvements made under the spirit and principles of this invention shall be considered within the scope of protection of this invention.

Claims

1. A high-sensitivity IV conversion circuit, comprising a standard sampling resistor Rs, characterized in that, Also includes: First buffer unit, second buffer unit and feedback amplification unit; The standard sampling resistor Rs is connected between the first node A and the second node B; The second buffer unit has a high input impedance, and its input is connected to the first node A; The input terminal of the first buffer unit is connected to the output terminal of the second buffer unit, and the output terminal of the first buffer unit is connected to the second node B; The input of the feedback amplification unit is connected to the first node A, and its output is fed back to the feedback input of the second buffer unit. The feedback amplification unit is configured to amplify the voltage signal of the first node A and provide negative feedback, such that the equivalent input impedance of the second node B to ground is one-Nth of the resistance value of the standard sampling resistor Rs, where N is the absolute value of the amplification factor of the feedback amplification unit.

2. The high-sensitivity IV conversion circuit according to claim 1, characterized in that, The second buffer unit includes a first operational amplifier U3, whose non-inverting input is connected to the first node A as the input of the second buffer unit.

3. The high-sensitivity IV conversion circuit according to claim 2, characterized in that, The first buffer unit includes a second operational amplifier U1 and a third operational amplifier U2. The input terminal of the second operational amplifier U1 is connected to the output terminal of the first operational amplifier U3, and the output terminal of the second operational amplifier U1 is connected to the input terminal of the third operational amplifier U2. The output terminal of the third operational amplifier U2 is connected to the second node B as the output terminal of the first buffer unit.

4. The high-sensitivity IV conversion circuit according to any one of claims 1 to 3, characterized in that, The feedback amplification unit includes a fourth operational amplifier U4 and an external resistor network. The external resistor network is configured to make the fourth operational amplifier U4 an inverting amplifier with a gain N of 100.

5. The high-sensitivity IV conversion circuit according to claim 1, characterized in that, The amplifier in the second buffer unit is an operational amplifier with a JFET or CMOS input stage.

6. The high-sensitivity IV conversion circuit according to claim 1, characterized in that, The standard sampling resistor Rs is a precision foil resistor.

7. A design method for a high-sensitivity IV-to-Voltage conversion circuit as described in any one of claims 1 to 6, characterized in that, Includes the following steps: Calculate and determine the resistance value of the standard sampling resistor Rs based on the target current range and the desired output voltage. An operational amplifier with high input impedance is selected as the core amplifier of the second buffer unit; The first buffer unit is constructed by selecting an operational amplifier with low offset voltage and low noise. The feedback amplification unit is configured with a precision resistor network to set its precise amplification factor N; A phase compensation network is introduced into the feedback loop of the second buffer unit and the feedback amplification unit to ensure the overall stability of the circuit. The assembled circuit is calibrated to eliminate systematic errors introduced by component tolerances and misalignments.

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