Data Sheet

AD5700/AD5700-1

Rev. A | Page 13 of 20

THEORY OF OPERATION

Highway Addressable Remote Transducer (HART) Communica-

tion is the global standard for sending and receiving digital

information across analog wires between smart field devices

and control systems. This is a digital two-way communication

system, in which a 1 mA p-p frequency shift keyed (FSK) signal

is modulated on top of a 4 mA to 20 mA analog current signal.

The AD5700/AD5700-1 are designed and specified to operate

as a single-chip, low power, HART FSK half-duplex modem,

complying with the HART physical layer requirements

(Revision 8.1).

A single-chip solution, the AD5700/AD5700-1 not only inte-

grate the modulation and demodulation functions, but also

contain an internal reference, an integrated receive band-pass

filter (which has the flexibility of being bypassed if required),

and an internally buffered HART output, giving a high output

drive capability and removing the need for external buffering.

The AD5700-1 option also contains a precision internal RC

oscillator. The block diagram in Figure 1 shows a graphical

illustration of how these circuit blocks are connected together.

As a result of such extensive integration options, minimal

external components are required. The AD5700/AD5700-1

are suitable for use in both HART field instrument and master

configurations.

The AD5700/AD5700-1 either transmit or receive 1.2 kHz and

2.2 kHz carrier signals. A 1.2 kHz signal represents a digital 1,

or mark, whereas a 2.2 kHz signal represents a 0, or space.

There are three main clocking configurations supported by

these parts, two of which are available on the AD5700 option,

whereas all three are available on the AD5700-1 device:



External crystal



CMOS clock input



Internal RC oscillator (AD5700-1 only)

The device is controlled via a standard UART interface. The

relevant signals are RTS, CD, TXD, and RXD (see Table 6 for

more detail on individual pin descriptions).

FSK MODULATOR

The modulator converts a bit stream of UART-encoded HART

data at the TXD input to a sequence of 1200 Hz and 2200 Hz

tones (see Figure 17). This sinusoidal signal is internally buff-

ered and output on the HART_OUT pin. The modulator is

enabled by bringing the RTS signal low.

Figure 17. AD5700/AD5700-1 Modulator Waveform

The modulator block contains a DDS engine that produces a

1.2 kHz or 2.2 kHz sine wave in digital form and then performs

a digital-to-analog conversion. This DDS engine inherently

generates continuous phase signals, thus avoiding any output

discontinuity when switching between frequencies. For more

information on DDS fundamentals, see MT-085, Fundamentals

of Direct Digital Synthesizers (DDS). Figure 18 demonstrates a

simple implementation of this FSK encoding.

Figure 18. DDS-Based FSK Encoder

STOP

START

8-BIT DATA + PARITY

TXD

HART_OUT

"1" = MARK

1.2kHz

"0" = SPACE

2.2kHz

1.2kHz

WORD

2.2kHz

WORD

DDS

DAC

FSK

0

DATA

1

CLOCK