MEGGER Grounding Resistance Tester MIT230 Shanghai

Since the 1950s, the 4 mA to 20 mA current loop has been the standard for industrial analog signals. The main advantage of this signal standard is that there is almost no attenuation when the signal is transmitted through long cables, making it more robust in industrial and spot environments that are prone to EMI. On the contrary, if voltage output is used, long cables (greater than 10 meters) will experience voltage drop due to cable resistance, resulting in sensor data loss and incorrect readings.

The reference design shown in Figure 1 consists of a single axis ADXL1002 MEMS accelerometer, whose analog voltage output is converted into a 4 mA to 20 mA signal standard by an AD5749 voltage to current converter. The input voltage (VIN) of AD5749 ranges from 0 V to 4.096 V, while the analog output voltage (VOUT) of ADXL1002 ranges from 0 V to VDD, so VDD must be set to 4.096 V. Therefore, LT6654AMPS6-4.096 is selected to provide a 4.096 V voltage, with a temperature stability of 10 ppm/° C in the temperature range of -55 ° C to 125 ° C. A 2-pole RC low-pass filter with a bandwidth of 36 kHz and a 3 dB bandwidth is placed between VOUT and VIN. This filter is used to limit broadband noise and attenuate the 200 kHz noise component from the internal clock of ADXL1002; According to the sampling rate and filtering characteristics of the DAQ circuit used in the application, this noise may cause in band aliasing.

AD5749 directly converts the voltage output signal of ADXL1002 into a current output of 4 mA to 20 mA, with minimal impact on the size of the printed circuit board (PCB), and provides a bandwidth of up to 50 kHz and good immunity.

Many 4 mA to 20 mA drivers on the market consist of current output digital to analog converters (DACs) that require SPI or I2C external controllers. Another advantage of the AD5749 4 mA to 20 mA driver is its independent working mode (hardware mode).

In hardware mode, the HW_SELECT pin is set to high level. The R0 to R3 and RSET pins are all connected to a low level to set the output range of AD5749 to 4 mA to 20 mA, which means there is no need for an external microcontroller to configure the output range of AD5749. To improve the stability of the output current over the entire temperature range, an external low drift resistor should be connected between the REXT1 and REXT2 pins.

The DAQ front-end circuit (not included) only requires one current to voltage (I-V) conversion amplifier. The mutual impedance (I-V resistance) must be set according to the input range of the DAQ front-end circuit.

Figure 2 shows an example of the current output (IOUT) of the circuit during manual shaking (black line). 0g水The flat corresponds to the middle range of IOUT, which is 12 mA for configurations ranging from 4 mA to 20 mA. The full-scale range (FSR) is also highlighted by a gray dashed line for reference.