Methods for Characterizing System Bandwidth in Complex Current Sensor Applications

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/ 2019-04-03 / Tech

Introduction

 

It is very challenging to drive high currents at high frequencies in a controlled manner. Even with the right equipment, due to the unique and complex system of the specific application, it is impossible to make the external current flow easily due to problems such as load conditions or switching circuit timing.

 

Methods for Characterizing System Bandwidth in Complex Current Sensor Applications


This article describes several methods for measuring and characterizing system bandwidth, with an emphasis on impulse response methods.

 

Allegro's ACS720 is a high precision and highly integrated current sensor IC with very low temperature sensor drift and dual overcurrent fault channels with a typical bandwidth of With 120 kHz and an external filter function, it is the perfect device to verify these test methods.

 

The functional block diagram of the ACS720 is shown in Figure 1. The highlighted part is the filter circuit. For more information, please refer to the Allegro website ACS720 device data sheet.


Methods for Characterizing System Bandwidth in Complex Current Sensor Applications

Figure 1: ACS720 block diagram The highlighted portion is an external filter circuit.

Bandwidth Measurement


The block diagram in Figure 2 shows the standard workbench settings for system bandwidth testing.


The network analyzer provides control signals to the wideband amplifier that can cover AC currents from 10 Hz to hundreds of kHz. The high-bandwidth precision current monitor can be used to monitor the input current, and the analog output of the ACS720 can be directly measured. The network analyzer can compare the sensor output (channel B) to the reference sensing probe (channel A).


Estimate system bandwidth using rise time


Frequency scan test for applications with minimal switching circuitry It is the most practical method. Consider the simplified inverter schematic in Figure 3, which shows the common sensing locations of the Allegro integrated current sensor (CS).


Figure 3: Simplified Three-phase inverter schematic.

Figure 3: Simplified Three-phase inverter schematic.


One solution for bandwidth verification is to switch the FET in a single-phase circuit at various frequencies while observing the output. however,Width adjustment of the switching frequency is not feasible, and accurate in-phase measurement of the reference current can be very challenging.


The second solution is to short the single-phase high and low sides in a single-phase circuit and use an external power supply to provide the sweep current.

However, other components such as large capacitors on the circuit or small loads on the FET can suppress the amplifier's ability to scan high-frequency currents. In this case, an alternative test method can be used: the rise time method. Rise time is defined as the time it takes for the system to rise from a steady state value to 90% in response to a fast input current step.


The inverter system of Figure 3 can be made to have a fast input step by simply shorting the high and low sides of the single phase. In this case, a short shoot-through event is allowed, and then the rise time of the sensor output can be measured, and then the measured rise time can be used to approximate the bandwidth of the system by the following equation:

Equation 1:

The low-pass filter SPICE model with a cutoff frequency of 120 kHz is used here to illustrate the method, as shown in Figure 4.


Figure 4: Cutoff frequency is 120 kHz The low pass filter SPICE model.

Figure 4: Cutoff frequency is 120 kHz The low pass filter SPICE model.


The cutoff frequency of the low pass filter is calculated using the following equation: approximately 120 kHz:

Equation 2:


A voltage step of 0 to 5V at VIN input , the rise time is 100ns,The rise time of VOUT was measured to be 2.92 μs. The simulation results are shown in Figure 5.


Figure 5: 120 kHz The impulse response of the low pass filter.

Figure 5: 120 kHz The impulse response of the low pass filter.


The bandwidth of the low-pass filter can be confirmed using the measured rise time and bandwidth approximation equation:

Equation 3:


It should be noted that the rise time of the input signal directly affects the response of the system. In this test, a relatively fast input rise time is recommended (

Approximate rise time approximation

The typical internal bandwidth of the ACS720 data sheet is 120 kHz. The device under test in Figure 6 A bandwidth of approximately 128 kHz is shown.


Figure 6: The Bode plot of the ACS720 shows the amplitude and phase relative to the sinusoidal input current.

Figure 6: The Bode plot of the ACS720 shows the amplitude and phase relative to the sinusoidal input current.

Use The current step generator measures the rise time of the ACS720, and the rise time is <>

Figure 7: ACS720 impulse response with 0 nF filter capacitor.

Figure 7: ACS720 impulse response with 0 nF filter capacitor.

Measure the average rise time of the ACS720 to 2.8μs, insertable bandwidth approximation formula:

Equation 4:


The rise time results in Figure 7 are closely related to the measured bandwidth in Figure 6. Although the correlation can be further improved by using a faster input step, in Figure 7 The 1μs pulse is sufficient for approximation.


Add a 4. on the ACS720 FILTER pin.The 7nF capacitor reduces the sensor's cutoff frequency to 18.8kHz. As shown in Figures 8 and 9, the frequency response and rise time are retested after the filter capacitor is added.


Figure 8: ACS720 and Bode plot of 4.7nF filter capacitor.

Figure 8: ACS720 and Bode plot of 4.7nF filter capacitor.


Figure 9: 4.7nF filtering The ACS720 impulse response of the capacitor.

Figure 9: 4.7nF filtering The ACS720 impulse response of the capacitor.

Using the measured rise time and approximate equations, you can confirm the new sensor bandwidth:

Eq. 5:

Conclusion< /strong>

The system is too complicated,The rise time method is an effective way to characterize system bandwidth when standard test procedures cannot be used or when there is no suitable test equipment.


Keywords: Sensor
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