Circuit Note

CN-0267

Rev. B | 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

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

For ease of in-circuit measurement, the DEMO-AD5700D2Z

has test points (T5, T6) on each side of the 10 Ω resistor in the

the voltage drop across the resistor to be measured, and the

current to be calculated without interrupting the supply current

or disturbing the circuit.

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

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

REST OF CIRCUIT

POWER SUPPLY

AD5421

VOLTAGE

REGULATOR