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ADUCM360 Arkusz danych(PDF) 5 Page - Analog Devices |
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ADUCM360 Arkusz danych(HTML) 5 Page - Analog Devices |
5 / 8 page Circuit Note CN-0267 Rev. A | Page 5 of 8 Output Noise During Silence Test When a HART device is not transmitting (silence), do not couple noise onto the network. Excessive noise may interfere with reception of HART signals by the device itself or other devices on the network. The voltage noise measured across a 500 Ω load in the loop must contain no more than 2.2 mV rms of combined broadband and correlated noise in the HART extended frequency band. In addition, the noise should not exceed 138 mV rms outside the HART extended frequency band. This noise was measured by a true rms meter across the 500 Ω load. This noise was measured directly for the out-of-band noise and measured through the HCF_TOOL-31 filter for the in-band noise. An oscilloscope was also used to examine the noise waveform. The noise was measured at the worse condition, which was 4 mA output current. The captured noise waveform is shown in Figure 4, and the results are summarized in Table 1. Figure 4. Output Noise During Silence Waveform Table 1. Output Noise During Silence Output Noise Measured (mV) Required (mV) Outside Extended Frequency Range 4.13 <138 Inside Extended Frequency Range 1.03 <2.2 Analog Rate of Change Test This specification ensures that when a device regulates the analog output current, the maximum rate of change of analog current does not interfere with HART communications. Step changes in current disrupt HART signaling. The worst-case change in the analog output current must not produce a disturbance higher than 15 mV peak, measured across a 500 Ω load in the HART extended frequency band. The AD5421 DAC and output driver are relatively fast. Therefore, to meet the required system specification, the output current change is controlled by combining hardware slew-rate limiting implemented at the AD5421 and a digital filter implemented in the ADuCM360 software. The hardware slew-rate limit is set by the capacitance connected to the AD5421 CIN pin. When a large step change is required in the analog output current value, the ADuCM360 software splits the output current change sent to the AD5421 DAC into a number of smaller subsequent steps. This test was performed using an oscilloscope coupled to the 500 Ω load through the HCF_TOOL-31 filter. The result is shown in Figure 5. Waveform CH1 shows the periodic steps between 4 mA and 20 mA, sensed directly across the 500 Ω load. Waveform CH2 is the signal captured on the HCF_TOOL-31 filter output, amplified 10×, within the 150 mV peak limits. Figure 5. Analog Rate of Change Waveform Circuit Power Consumption Two methods were used to evaluate the circuit power consumption performance. In the first method, the current from the AD5421 integrated voltage regulator output was measured. Considering the minimum analog output current of 4 mA and HART output ac modulation of 0.5 mA peak, the maximum current consumed by the circuit in normal mode operation must be less than 3.5 mA. The AD5421 requires a 0.3 mA maximum for its own operation, which leaves approximately 3.2 mA maximum current for the AD5421 REGOUT output. For ease of in-circuit measurement, the DEMO-AD5700D2Z has test points (T5, T6) on each side of the 10 Ω resistor in the REGOUT output filter, as shown in Figure 6. This setup allows the voltage drop across the resistor to be measured, and the current to be calculated without interrupting the supply current or disturbing the circuit. Figure 6. Measuring the AD5421 REGOUT Current Using Test Points CH1 20.0mV BW M 100ms CH1 –8.00mV <10Hz 1 CH1 p-p 44.8mV CH1 CYC RMS 4.64mV? CH1 NONE CH1 NONE CH2 OFF NONE MEASURE CH1 5.00V CH2 50mV BW CH1 4mA TO 20mA ACROSS 500Ω CH2 OUTPUT OF FILTER × 10 LIMIT = ±150mV M 25.0ms CH1 6.80V CH1 p-p 8.60V CH1 FREQ 8.123Hz? CH2 p-p 254mV CH2 MAX 134mV CH2 MIN –120mV MEASURE 1 2 VOLTMETER + T6 T5 10Ω 10µF 10µF REGOUT REST OF CIRCUIT POWER SUPPLY AD5421 VOLTAGE REGULATOR |
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