CN-0278 Circuit Note
Application Note AN-1065 describes a manner in which the
AD5420 I
OUT
DAC can be configured for HART communication
compliance. AN-1065 outlines how the AD5700 HART modem
output can be attenuated and ac coupled into the AD5420 via the
CAP2 pin. The same is true of the AD5422. However, if the
application involves a particularly harsh environment, an
alternative circuit configuration can be used which offers better
power supply rejection characteristics. This alternative circuit
requires the use of the external R
SET
resistor and involves coupling
the HART signal into the R
SET
pin of the AD5420 or AD5422.
The CN-0270 describes this solution for the AD5420, typical of
line-powered transmitter applications. The current circuit note
is relevant to the AD5422, which, unlike the AD5420, offers
both a voltage and a current output pin, and so is particularly
useful in PLC/distributed control system (DCS) applications.
The AD5422 is available in both 40-lead LFCSP and 24-lead
TSSOP packages and the relevance of this, to the circuit
characteristics, is examined in the Circuit Description section.
This circuit adheres to the HART physical layer specifications as
defined by the HART Communication Foundation, for example,
the output noise during silence and the analog rate of change
specifications.
For many years, 4 mA to 20 mA communication has been used
in process control instrumentation. This communication method is
reliable and robust, and offers high immunity to environmental
interference over long communication distances. A limitation,
however, is that only 1-way communication of one process
variable at a time is possible.
The development of the highway addressable remote transducer
(HART) standard provided highly capable 2-way digital
communication, simultaneously with the 4 mA to 20 mA analog
signaling used by traditional instrumentation equipment. This
allows for features such as remote calibration, fault interrogation,
and transmission of additional process variables. Put simply,
HART is a digital two-way communication in which a 1 mA
peak-to-peak, frequency-shift-keyed (FSK) signal is modulated
on top of the 4 mA to 20 mA analog current signal.
CIRCUIT DESCRIPTION
Figure 1 shows the manner in which the AD5422 can be combined
with the AD5700 HART modem and a UART interface to
construct a HART-capable 4 mA to 20 mA current output, typical
of PLC and DCS systems. The buffer connected to the +V
SENSE
pin is not necessary if the application does not require the I
OUT
and V
OUT
pins to be shorted. The HART_OUT signal from the
AD5700 is attenuated and ac-coupled into the R
SET
pin of the
AD5422. If the external R
SET
resistor is not being used, an
alternative method of connecting the AD5422 and the AD5700
via the CAP2 pin can be found in
Application Note AN-1065, as
previously described. This method is only relevant to the 40-lead
LFCSP package option of the AD5422 because the lower pin-
count 24-lead TSSOP package does not contain a CAP2 pin.
While the method described in the current circuit note requires
the use of the external R
SET
resistor, in return, it provides better
power supply rejection performance than the alternative
application note solution. The use of either solution results in
the AD5700 HART modem output modulating the 4 mA to 20 mA
analog current (as shown in Figure 2) without affecting the dc level
of the current. The diode protection circuitry (D1 to D4) is
discussed in more detail in the Transient Voltage Protection
section.
STOP
START
8-BIT DATA + PARITY
TXD
HART_OUT
"1" = MARK
1.2kHz
"0" = SPACE
2.2kHz
10803-002
Figure 2. AD5700/AD5700-1 Sample Modulator Waveform
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