CN-0267 Datasheet, PDF (5/8 Pages) Analog Devices – Complete 4 mA to 20 mA Loop Powered Field Instrument with HART Interface

Circuit Note
CN-0267
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.
MEASURE
CH1
p-p
44.8mV
CH1
CYC RMS
4.64mV?
CH1
1
NONE
CH1
NONE
CH2
OFF
NONE
CH1 20.0mV ^BW
M 100ms
CH1 –8.00mV
<10Hz
Figure 4. Output Noise During Silence Waveform
Table 1. Output Noise During Silence
Output Noise
Measured
(mV)
Outside Extended Frequency Range 4.13
Inside Extended Frequency Range 1.03
Required
(mV)
<138
<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.
CH1
4mA TO 20mA
ACROSS 500Ω
1
CH2
OUTPUT OF
FILTER × 10
2
LIMIT =
±150mV
CH1 5.00V CH2 50mV ^BW
MEASURE
CH1
p-p
8.60V
CH1
FREQ
8.123Hz?
CH2
p-p
254mV
CH2
MAX
134mV
M 25.0ms CH1 6.80V
CH2
MIN
–120mV
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.
VOLTMETER
+
REST OF CIRCUIT
POWER SUPPLY
T6
10µF
T5
10Ω
10µF
AD5421
REG_{OUT VOLTAGE}
REGULATOR
Figure 6. Measuring the AD5421 REGOUT Current Using Test Points
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