AD5700-1BCPZ-R5 Просмотр технического описания (PDF) - Analog Devices

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AD5700-1BCPZ-R5

Low Power HART Modem

Analog Devices 

Data Sheet

AD5700/AD5700-1

APPLICATIONS INFORMATION

SUPPLY DECOUPLING

It is recommended to decouple the VCC and IOVCC supplies with

10 μF in parallel with 0.1 μF capacitors to ground. For many

applications, 1 μF in parallel with 0.1 μF ceramic capacitors to

ground should be sufficient. The REG_CAP voltage of 1.8 V is

used to supply the AD5700/AD5700-1 internal circuitry and is

derived from the VCC supply using a high efficiency clocking

LDO. Decouple this REG_CAP supply with a 1 μF ceramic

capacitor to ground. It is also required to decouple the REF pin

with a 1 μF ceramic capacitor to ground. Place decoupling

capacitors as close to the relevant pins as possible.

For loop-powered applications, it is recommended to connect a

resistance in series with the VCC supply to minimize the effect of

any noise, which may, depending on the system configuration, be

introduced onto the loop as a result of current draw variations

from the AD5700/AD5700-1. For typical applications, 470 Ω of

resistance has proven most effective. However, depending on the

application conditions, alternative values may also be acceptable

(see R1 in Figure 31).

TRANSIENT VOLTAGE PROTECTION

Many industrial control applications have requirements for

HART-enabled current input and output modules. Figure 28

shows an example of a HART-enabled current input module

that contains transient voltage protection circuitry, which is

very important in harsh industrial control environments.

The module is powered from a 24 V field supply, and the 250 Ω

load is within the low impedance module itself. This configuration

is in contrast to Figure 29, which demonstrates a secondary HART

device, in which the load is outside of the module. For transient

voltage protection, a 10 V unidirectional (for protection against

positive high voltage transients) transient voltage suppressor (TVS)

is placed at the connection point of the current input module.

The TVS component that is used in a given application circuit

must have power ratings that are appropriate to the individual

system. When choosing the TVS, low leakage current is also an

important specification for maintaining the accuracy of the analog

current input. In the event of a transient spike, the 22 Ω series

resistor acts as a current limiting resistor for the FSK output pin.

The FSK input pin is inherently protected by the 150 kΩ resistor,

which forms part of the recommended external filter circuitry

at the FSK input. The voltage divider, made up of both a 75 kΩ

resistor and a 22 kΩ resistor, is used to maintain a 0.75 V dc bias

at the field side of the FSK output switch.

VLOOP

24V

10V

400W

6.8nF

2.2µF

22Ω

3.3V

75kΩ

22kΩ

150kΩ

150pF

300pF 1.2MΩ

1.2MΩ

3.3V

10nF

1µF

VCC

HART_OUT

TXD

RXD

AD5700/

AD5700-1 RTS

REF

CD

ADC_IP

AGND

MICRO-

CONTROLLER

250Ω

20kΩ

10µF

ADC

Figure 28. Current Input Module, HART Circuit

39V

1500W

2.2µF

50V

4.7Ω

6.8nF 0.5W

50V

3.3V

75kΩ

10V

400W

22kΩ

20Ω

10nF

150kΩ

150pF

300pF 1.2MΩ 1µF

1.2MΩ

3.3V

VCC

HART_OUT

AD5700/

AD5700-1

REF

ADC_IP

AGND

TXD

RXD

RTS

CD

Figure 29. Secondary HART Device

Rev. G | Page 17 of 24

HOST

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