Tutorial of measuring current with funnel amplifier

Accurate current measurement is not as easy as voltage measurement. This measurement becomes more difficult when the current you are trying to measure flows through a load connected to a relatively high power supply voltage. Current detection resistors are also called shunt resistors. Because of their high measurement accuracy, low temperature coefficient, and relatively low cost, they have become a technology for measuring current. Due to the low impedance of this type of resistor, it is usually necessary to boost the lower voltage across it. This task is usually done by a current sense amplifier connected in a low-side or high-side configuration.
However, when the load is driven by a relatively high voltage power supply (for example: industrial control applications), the sense resistor can be much larger without competing for too much drive voltage from the load. Compared with the voltage generated by the detection current flowing through a low-impedance shunt resistor (the value is usually measured in milliohms or microohms), these increased resistances will generate a much larger current detection voltage. In high-power industrial applications ranging from motor control to power conversion, these detection voltages can often be as high as several volts.
This kind of detection voltage often needs to be attenuated and level shifted before it can be applied to analog-to-digital converters (ADCs) that usually use unipolar 3 V or 5 V power supplies. The attenuation and level shift signal conditioning chain is sometimes called the funnel signal chain, because the detected voltage signal gradually narrows as it passes through the ADC's signal conditioning chain. The traditional way to reduce or narrow these detection voltages is to use passive attenuation, but a differential funnel amplifier can also be used. This method can improve measurement accuracy while reducing the number of components.
funnel amplifier can perform up to three signal conditioning tasks:
At the end of the analog front end (AFE) signal chain, the detected voltage is attenuated to a level acceptable to the ADC.
Perform level conversion (level shift) as needed, for example in the high-voltage side detection design.
can have the differential output required to drive a fully differential ADC.
If designers need to measure small signals on hundreds of volts and extremely high common-mode voltages, please refer to Art Pini’s article "Measure small signals on high voltages and avoid sensor ground loops."
High-pressure side and low-pressure side detection overview
As shown in Figure 1, the common current monitoring signal chain configuration includes shunt resistor, AFE, ADC and system controller. An operational amplifier or a dedicated current-sense amplifier converts the small differential voltage generated across the shunt resistor into the larger output voltage required by the ADC.




Figure 1: A simple current measurement method is to use a shunt resistor (on the left). The voltage generated by the resistor itself is proportional to the current flowing through it. The sense amplifier adjusts the signal to meet the input requirements of the ADC.
Low-voltage side current measurement places a shunt resistor between the active load and ground. Low-voltage side current measurement is easier to implement because the detection voltage across the shunt resistor is referenced to ground. However, the measurement configuration on the low-voltage side has an obvious disadvantage: the shunt resistor is located between the load and ground, which means that the load is not referenced to ground. In addition, the leakage current on the sneak path from the load to the ground cannot be detected.
High-voltage side current measurement inserts a shunt resistor between the power supply and the active load. Figure 2 shows the circuit used for low-side and high-side current measurement.


Figure 2: The low-voltage side current measurement circuit places the current detection resistor between the active load and ground, while the high-voltage side measurement circuit places the current detection resistor between the power supply and the load.
Compared with low-voltage side current measurement, high-voltage side current measurement has two key advantages:
The short circuit to the ground inside the load can be easily detected through the sneak path, because the short-circuit current generated will flow through the shunt resistor and form a detection voltage across its both ends.
The high-voltage side current measurement does not take the ground as a reference, so the differential ground voltage caused by the large current flowing through the system ground plane will not affect the measurement.
High-side current measurement also has an obvious disadvantage: the detection voltage is superimposed on a relatively large common-mode voltage.
Whether it is low-voltage side measurement or high-voltage side measurement, the detection voltage generated by the load operating under high voltage and large current can easily exceed the input voltage rating, and even exceed the ADC power rail used to convert the detection voltage into a digital value. Some kind of attenuation is required in this case. In addition, the detection voltage depends on the large voltage offset measured on the high-voltage side, usually as high as tens or even hundreds of volts. In these cases, level conversion is required to make the detection voltage within the rated input voltage range of the ADC.
The funnel amplifier integrates a high matching resistor that has been fine-tuned before leaving the factory, and the voltage gain and offset can be set. Compared with designs based on discrete, unmatched resistors, these internal resistors have better performance and higher accuracy, while reducing the number of components. The high-performance ADCs used in these current-sensing applications may have differential inputs, so some funnel amplifiers have differential output functions that can drive these differential ADCs correctly.
Introduce two funnel amplifiers
Analog Devices' LT1997 funnel amplifier (LT1997-2 and LT1997-3) and AD8475 fully differential funnel amplifier are examples of fully integrated precision resistors. All three devices can be used to perform similar signal conditioning tasks, but their functions are quite different.
Two of the LT1997 gain-selectable funnel amplifiers are attenuated (funnel) differential amplifiers, which can convert a larger differential signal into a lower voltage range compatible with the ADC input. Both LT1997 funnel amplifiers integrate a precision operational amplifier and a set of highly matched internal resistors on one chip. These two devices can perform voltage attenuation and level shift without additional external components. Figure 3 is an internal schematic diagram showing the components of the LT1997-2 amplifier in the DFN package, and Figure 4 is the internal schematic diagram of the LT1997-3 amplifier in the MSOP package.


Figure 3: The LT1997-2 amplifier contains multiple matched resistors, which can be combined to produce multiple high-precision fractional gains and attenuations.


Figure 4: The LT1997-3 amplifier contains multiple matched resistors, which can be combined to produce multiple high-precision fractional gains and attenuations.
Please note that although the architectures of these two devices are very similar and the part numbers are very similar, the resistance values are quite different. Please also note that the MSOP package splits the internal resistor connected to the REF pin in the DFN package into two larger resistors connected to pins REF1 and REF2.
When connected in parallel, the resistances in the two packages are the same. However, this feature of the MSOP package allows the two resistors to be connected to the power rail to establish a midpoint voltage reference at the positive input of the internal amplifier, and No additional components are required. This discrete resistor configuration exists in both LT1997-2 and LT1997-3 MSOP packages.
In order to generate various amplifier gains, you can connect the internal input resistor of LT1997. In order to achieve the task of funneling, input resistors can be connected to generate a variety of attenuation settings for forming a funnel amplifier. Table 1 lists the 38 fractional attenuation settings that can be achieved using the internal positive input resistor of the LT1997-2 amplifier, and Table 2 lists the 30 settings that can be achieved using the internal positive input resistor of the LT1997-3.


Table 1: The matched positive input resistors of the LT1997-2 amplifier can be used in combination to produce multiple fractional attenuation levels.


Table 2: The matched positive input resistors of the LT1997-3 amplifier can be used in combination to produce multiple decimal attenuation levels.
Table 1 and Table 2 show the various attenuation possibilities that can be achieved using only the built-in resistors of the LT1997-2 and LT1997-3 funnel amplifiers, but this is not their full function. In addition, you can use other internal resistors to program the amplifier gain, and then multiply the gain by the attenuation to get the amplifier's output. Of course, if the attenuation/gain combination achieved by the internal resistors is not suitable for the overall design requirements, external precision resistors can also be added to the circuit. However, the use of external partial vertical resistors lacks the advantage of strict factory matching that internal resistors have.
LT1997-2 and LT1997-3 funnel amplifiers can operate in a wide common-mode input voltage range (which can be 76 V higher than the negative power rail of the device). By using the device's internal input resistor in the voltage divider configuration, the analog INA input of the LT1997-3 can be safely driven up to ±160 V, and the INA input of the LT1997-2 can be driven up to ±255 V.
The strict matching of internal resistors enables the two devices to achieve a very high common-mode rejection ratio. This extreme ability to adapt to signals with higher common-mode voltages relies on what Analog Devices calls "Over-The-Top" operating capabilities. When the device is in Over-The-Top mode, it can withstand extreme common-mode voltages by weakening other specifications including linearity, input bias current, input offset current, differential input impedance, noise, and bandwidth. This function seems to need to weaken many parameters, but the advantage is that it can handle input voltages that are lethal to other operational amplifiers.
Both LT1997-2 and LT1997-3 amplifiers have all the specifications listed in the data sheet, and can operate under 5 V single-ended power supply and ±15 V power supply. In addition, these two devices can also be operated at 3.3 V-50 V. Operate within a wide range of supply voltage. It should be noted that the LT1997 amplifier has a single-ended output.
Fully Differential Funnel Amplifier
Analog Devices' AD8475 fully differential funnel amplifier can provide precision attenuation of 0.4 or 0.8, common-mode level shift, single-ended signal to differential signal conversion, and input overvoltage protection (Figure 5). The device contains a complete set of AFE components, including matched laser trimming input resistors and a precision differential amplifier. This amplifier can be used to connect industrial-grade signals to the differential input of a low-voltage, high-performance 16- or 18-bit single-supply SAR (successive approximation) ADC. The AD8475 amplifier can use a single power supply to process ±10 V signals, and when running on a single 5 V power supply, it can also provide overvoltage protection with a relative input voltage of up to ±15 V.


Figure 5: Analog Devices' AD8475 fully differential funnel amplifier uses matched internal laser trimming resistors to provide pin programmable gains of 0.8 and 0.4.
AD8475 has two standard gain options: 0.4 and 0.8. Use the input pin corresponding to the target gain to set the gain of the device.
The high-current differential output stage of the AD8475 funnel amplifier enables the amplifier to drive the switched capacitor front-end circuits of many ADCs with very small errors. In addition, the high-speed output of the slew-enhanced AD8475 enables it to stabilize to 18-bit accuracy and achieve a sampling rate as fast as 4 trillion times per second, so that high-speed current (and thus power) can be measured. The differential output of this amplifier can easily drive the inputs of SAR, ΣΔ, and pipelined ADCs.


The AD8475 amplifier shown in Figure 6 drives the differential input to the Analog Devices 18-bit, low-power AD7982 ADC that samples 1 trillion times per second.
Figure 6: The differential output of the AD8575 funnel amplifier can directly drive the differential input of ADCs like Analog Devices AD7982.
The differential input ADC is powered by a single power supply. The three sinusoidal waveforms describe a schematic diagram of all three signal processing tasks that the funnel amplifier can perform: attenuation, level shift, and differential drive. Note that the phase difference between the two sine waves at the top and bottom of the graph is 180°. These two waveforms demonstrate the differential drive capability of the AD8475 amplifier.
Analog Devices' ADR435 ultra-low noise XFET? The voltage reference is the 5 V reference voltage generated by the circuit.
The circuit in Figure 6 can adapt to the bipolar ±10 V AC input signal swing from the current sense resistor. This circuit can attenuate and level shift the input signal, and finally use a 4 V peak-to-peak signal swing centered on a 2.5 V DC offset to drive the input of the ADC to match the input requirements of the AD7982 ADC. A voltage divider composed of two 10 kΩ (kΩ) resistors (shown in the lower right corner of the figure) can generate a 2.5 V offset reference voltage for the VOCM input pin of the AD8475, which is used to set the output voltage offset of the amplifier shift. Design engineers can use this function to access the offset voltage required by the ADC used in the design.
  to sum up
Many industrial applications use relatively high voltages to drive loads. In this case, the analog front end of the high-side current measurement circuit must be able to accept an input signal voltage that is usually greater than its supply voltage. However, processing such input voltage requires signal attenuation and level shifting. The funnel amplifier is specifically designed for this type of signal conditioning task. It integrates factory-matched precision laser trimming resistors.
In addition, the funnel amplifier with differential output function can easily drive high-speed ADCs. These ADCs have switched capacitor front-end circuits and have very special drive requirements.