Accurate measurement of weak current signal is one of the key links of various scientific analysis equipment, environmental monitoring and process control, especially when the weak current signal reaches pA or even fA level. How to measure weak signals? This has always been a topic that major instrument manufacturers talk about. How to design a product that detects weak current? This is also a huge challenge for designers.

Accurate measurement of weak current signal is one of the key links of various scientific analysis equipment, environmental monitoring and process control, especially when the weak current signal reaches pA or even fA level. How to measure weak signals? This has always been a topic that major instrument manufacturers talk about. How to design a product that detects weak current? This is also a huge challenge for designers.

For a long time, the AD549 chip solution based on ADI’s JFET amplifier has played a major role in the front-end of weak current sampling. It has been rampant in the instrumentation industry for more than 30 years and has endured. However, due to the obvious shortcomings of the JFET process chip, it lacks the low offset, low voltage noise, low temperature drift and high common mode characteristics of the Bipolar amplifier, and it is difficult to accurately compensate the characteristics at different temperatures. With the advancement of science and technology, ADI released the ADA4530-1 in 2015 based on a new CMOS process and through DigiTrim® technological innovation, which not only has extremely low bias current and ultra-high input impedance, but also has lower offset Voltage, better temperature drift characteristics, higher loop bandwidth and ESD protection function, etc. Figure 1 below is a comparison chart of ADA4530 and AD549. It is not difficult to find that even at high temperatures, ADA4530 still has good bias current characteristics.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 1 ADA4530-1 and AD549 performance comparison table

But the difference is that the CMOS process is often introduced in SMT packaging, which actually increases the difficulty of the designer in application. Because AD549 is a DIP package, users only need to vacate the input pins to get a good insulation effect, but ADA4530 cannot, so there are very high design requirements for how to choose the PCB board and PCB wiring.

(Note: The extremely low input bias current and extremely high input resistance requirements make the insulation resistance of the materials used to construct the circuit often the largest source of error. Any insulator with limited resistance and contact with high-impedance conductors will produce errors. Current, such as PCB laminates, cables and connector insulation)

In response to the above application problems, the Shanghai technical team of Excelpoint Shijian, a technology distributor that has cooperated with ADI for more than 30 years, has begun to conduct targeted research and technology accumulation on the ADA4530 chip since its pre-release. Based on how to fully utilize the performance of the ADA4530-1 through a good design, Excelpoint Shijian has continuously updated the design and research and development on the picoammeter current measurement evaluation kit. Since the launch of the ultra-high-precision picoammeter module EPSH-PAM2.0 in September 2020 Has been unanimously praised by users. Recently, the enhanced version EPSH-PAM2.0EP of the ultra-high-precision picoammeter module has been released, which provides end users with two subdivision options and accelerates the design of customer systems. (The following text will be described in PAM2.0EP).

As an advanced version of the picoammeter module, PAM2.0EP has stronger performance characteristics:

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module

In circuit design, ADA4530-1 performs IV conversion after passing 10GR transimpedance, and the second stage uses low-noise rail-to-rail zero-drift amplifier ADA4522-1 to condition the voltage signal output by ADA4530. It is worth mentioning that, in order to facilitate customers to use the front end of the module separately, Shijian added a gain controllable circuit to the ADA4522-1 (the gain controllable through the low Ron multiplexer ADG1608 and fixed resistance is matched to achieve gain controllable) and left Test point TP.

The analog-to-digital converter at the back end also uses ADI’s classic 24bit Δ-∑ ADC-AD7124. The AD7124 is more like a small acquisition system. The high-precision ADC core and the built-in PAG can flexibly adjust the signal gain and acquisition. At the same time, it also guarantees low gain error and high ENOB value. In addition, the three-state operating mode of the AD7124 is very suitable for product applications that require different power consumption fields. Its data acquisition rate can reach 19.2KSPS.

On the power supply side, Shijian uses two LDOs with low noise and high power rejection ratio, LTC3260 and LT1762. The former is a product with a super-functional Charge pump and a built-in positive and negative output LDO, which can be well guaranteed. The power supply requirements of the op amp, the latter provides a stable 3.3V supply to the digital part.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 2 PAM2.0EP system block diagram

The MCU part uses Microchip ARM Cortex-M0, which communicates with the PC through the built-in USB port of the microcontroller. The visual GUI interface is remarkable. The user can install the Labview GUI software to configure the various functions of the module in real time through the software interface on the PC, and let it give signal waveforms, histograms and statistical analysis. At the same time, the software also Support Excel data export, very suitable for users who need to do long-term test records.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 3 Appearance and software interface

The PAM2.0EP module has excellent linearity data, not only the test data is excellent at room temperature, but also the linearity performance under the full temperature range.

The quality inspection of the module is measured by Keithley 6220 source meter. Each product will be tested multiple times at different temperature points. If the data of 99 points is qualified, it is determined to be a product available for shipment. The 99-point test report will be issued. The measured points include:

1. Normal temperature 25⁰C, 11 data test points and 1 noise floor data point in the range of 0~100pA in steps of 10pA

2. Normal temperature 25⁰C, 21 data test points in the range of 0~20pA with a step of 1pA

3. When the temperature is 40⁰C, 55⁰C, 70⁰C, 85⁰C, 100⁰C, and 115⁰C, each temperature point is in the range of 0~100pA, with 11 data test points of 10pA, a total of 66 data

At room temperature 25°C

The IV curve of 0-100pA (see Figure 4) has a linearity better than one part per million (0.999999x).

The IV curve of 0-20pA (see Figure 5) has a linearity better than one in ten thousand (0.9999x).

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 4 IV curve test chart of 0-100pA (temperature 25°C)

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 5 0-20pA IV linear curve test (temperature 25°C)

From the test results in Figure 6, at each current input point, the current peak-to-peak deviation ΔIp-p caused by temperature versus current is about 1pA. The user can perform further linearization processing according to the temperature curve, combined with the field application, to provide accuracy.

From 40⁰C to 115⁰C, 7 points of temperature 25⁰C, 40⁰C, 55⁰C, 70⁰C, 85⁰C, 100⁰C, 115⁰C are selected at intervals of 15⁰C, and 11 points are tested from 0 to 100pA in steps of 10pA at each temperature point.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 6 Test chart of IV curve of 0-100pA from 40⁰C to 115⁰C

Ge Yixin, senior vice president of Shijian’s technical support department, introduced to us: “The upgraded version of PAM2.0EP is developed for weak current detection applications in a wide temperature range. Compared with the previous version, it has improved the full temperature range. We have done a lot of experiments and tests to ensure the performance and overall reliability under the conditions. The product maintains a fairly good noise level and linearity even at high temperatures. We will provide multi-point test data for each product, hoping to help users Save product development cycle.”

Accurate measurement of weak current signal is one of the key links of various scientific analysis equipment, environmental monitoring and process control, especially when the weak current signal reaches pA or even fA level. How to measure weak signals? This has always been a topic that major instrument manufacturers talk about. How to design a product that detects weak current? This is also a huge challenge for designers.

Accurate measurement of weak current signal is one of the key links of various scientific analysis equipment, environmental monitoring and process control, especially when the weak current signal reaches pA or even fA level. How to measure weak signals? This has always been a topic that major instrument manufacturers talk about. How to design a product that detects weak current? This is also a huge challenge for designers.

For a long time, the AD549 chip solution based on ADI’s JFET amplifier has played a major role in the front-end of weak current sampling. It has been rampant in the instrumentation industry for more than 30 years and has endured. However, due to the obvious shortcomings of the JFET process chip, it lacks the low offset, low voltage noise, low temperature drift and high common mode characteristics of the Bipolar amplifier, and it is difficult to accurately compensate the characteristics at different temperatures. With the advancement of science and technology, ADI released the ADA4530-1 in 2015 based on a new CMOS process and through DigiTrim® technological innovation, which not only has extremely low bias current and ultra-high input impedance, but also has lower offset Voltage, better temperature drift characteristics, higher loop bandwidth and ESD protection function, etc. Figure 1 below is a comparison chart of ADA4530 and AD549. It is not difficult to find that even at high temperatures, ADA4530 still has good bias current characteristics.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 1 ADA4530-1 and AD549 performance comparison table

But the difference is that the CMOS process is often introduced in SMT packaging, which actually increases the difficulty of the designer in application. Because AD549 is a DIP package, users only need to vacate the input pins to get a good insulation effect, but ADA4530 cannot, so there are very high design requirements for how to choose the PCB board and PCB wiring.

(Note: The extremely low input bias current and extremely high input resistance requirements make the insulation resistance of the materials used to construct the circuit often the largest source of error. Any insulator with limited resistance and contact with high-impedance conductors will produce errors. Current, such as PCB laminates, cables and connector insulation)

In response to the above application problems, the Shanghai technical team of Excelpoint Shijian, a technology distributor that has cooperated with ADI for more than 30 years, has begun to conduct targeted research and technology accumulation on the ADA4530 chip since its pre-release. Based on how to fully utilize the performance of the ADA4530-1 through a good design, Excelpoint Shijian has continuously updated the design and research and development on the picoammeter current measurement evaluation kit. Since the launch of the ultra-high-precision picoammeter module EPSH-PAM2.0 in September 2020 Has been unanimously praised by users. Recently, the enhanced version EPSH-PAM2.0EP of the ultra-high-precision picoammeter module has been released, which provides end users with two subdivision options and accelerates the design of customer systems. (The following text will be described in PAM2.0EP).

As an advanced version of the picoammeter module, PAM2.0EP has stronger performance characteristics:

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module

In circuit design, ADA4530-1 performs IV conversion after passing 10GR transimpedance, and the second stage uses low-noise rail-to-rail zero-drift amplifier ADA4522-1 to condition the voltage signal output by ADA4530. It is worth mentioning that, in order to facilitate customers to use the front end of the module separately, Shijian added a gain controllable circuit to the ADA4522-1 (the gain controllable through the low Ron multiplexer ADG1608 and fixed resistance is matched to achieve gain controllable) and left Test point TP.

The analog-to-digital converter at the back end also uses ADI’s classic 24bit Δ-∑ ADC-AD7124. The AD7124 is more like a small acquisition system. The high-precision ADC core and the built-in PAG can flexibly adjust the signal gain and acquisition. At the same time, it also guarantees low gain error and high ENOB value. In addition, the three-state operating mode of the AD7124 is very suitable for product applications that require different power consumption fields. Its data acquisition rate can reach 19.2KSPS.

On the power supply side, Shijian uses two LDOs with low noise and high power rejection ratio, LTC3260 and LT1762. The former is a product with a super-functional Charge pump and a built-in positive and negative output LDO, which can be well guaranteed. The power supply requirements of the op amp, the latter provides a stable 3.3V supply to the digital part.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 2 PAM2.0EP system block diagram

The MCU part uses Microchip ARM Cortex-M0, which communicates with the PC through the built-in USB port of the microcontroller. The visual GUI interface is remarkable. The user can install the Labview GUI software to configure the various functions of the module in real time through the software interface on the PC, and let it give signal waveforms, histograms and statistical analysis. At the same time, the software also Support Excel data export, very suitable for users who need to do long-term test records.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 3 Appearance and software interface

The PAM2.0EP module has excellent linearity data, not only the test data is excellent at room temperature, but also the linearity performance under the full temperature range.

The quality inspection of the module is measured by Keithley 6220 source meter. Each product will be tested multiple times at different temperature points. If the data of 99 points is qualified, it is determined to be a product available for shipment. The 99-point test report will be issued. The measured points include:

1. Normal temperature 25⁰C, 11 data test points and 1 noise floor data point in the range of 0~100pA in steps of 10pA

2. Normal temperature 25⁰C, 21 data test points in the range of 0~20pA with a step of 1pA

3. When the temperature is 40⁰C, 55⁰C, 70⁰C, 85⁰C, 100⁰C, and 115⁰C, each temperature point is in the range of 0~100pA, with 11 data test points of 10pA, a total of 66 data

At room temperature 25°C

The IV curve of 0-100pA (see Figure 4) has a linearity better than one part per million (0.999999x).

The IV curve of 0-20pA (see Figure 5) has a linearity better than one in ten thousand (0.9999x).

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 4 IV curve test chart of 0-100pA (temperature 25°C)

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 5 0-20pA IV linear curve test (temperature 25°C)

From the test results in Figure 6, at each current input point, the current peak-to-peak deviation ΔIp-p caused by temperature versus current is about 1pA. The user can perform further linearization processing according to the temperature curve, combined with the field application, to provide accuracy.

From 40⁰C to 115⁰C, 7 points of temperature 25⁰C, 40⁰C, 55⁰C, 70⁰C, 85⁰C, 100⁰C, 115⁰C are selected at intervals of 15⁰C, and 11 points are tested from 0 to 100pA in steps of 10pA at each temperature point.

Excellence is just a passing line-Shijian launched an enhanced version of the ultra-wide temperature range, high-precision picoammeter module
Figure 6 Test chart of IV curve of 0-100pA from 40⁰C to 115⁰C

Ge Yixin, senior vice president of Shijian’s technical support department, introduced to us: “The upgraded version of PAM2.0EP is developed for weak current detection applications in a wide temperature range. Compared with the previous version, it has improved the full temperature range. We have done a lot of experiments and tests to ensure the performance and overall reliability under the conditions. The product maintains a fairly good noise level and linearity even at high temperatures. We will provide multi-point test data for each product, hoping to help users Save product development cycle.”

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