General Description
The DS8500 is a single-chip modem with Highway
Addressable Remote Transducer (HART) capabilities
and satisfies the HART physical layer requirements.
The device integrates the modulation and demodulation
of the 1200Hz/2200Hz FSK signal, has very low power
consumption, and needs only a few external compo-
nents due to the integrated digital signal processing.
The input signal is sampled by an analog-to-digital con-
verter (ADC), followed by a digital filter/demodulator.
This architecture ensures reliable signal detection in
noisy environments. The output digital-to-analog con-
verter (DAC) generates a sine wave and provides a
clean signal with phase-continuous switching between
1200Hz and 2200Hz. Low power is achieved by dis-
abling the receive circuits during transmit and vice
versa. The DS8500 is ideal for low-power process con-
trol transmitters.
Applications
4–20mA Loop-Powered Transmitters for
Temperature, Pressure, Flow, and Level
Measurement
HART Multiplexers
HART Modem Interface Connectivity
Features
♦Single-Chip, Half-Duplex, 1200bps FSK
Modulation and Demodulation
♦Digital Signal Processing Provides Reliable Input
Signal Detection in Noisy Conditions
♦Sinusoidal Output Signal with Lowest Harmonic
Distortion
♦Few External Components Enable a Space-Saving
Solution
♦Standard Component 3.6864MHz Crystal
♦Complies to HART Physical Layer Requirements
♦2.7V to 3.6V Operating Voltage
♦285µA (max) Current Consumption
♦Space-Saving, 5mm x 5mm x 0.8mm, 20-Pin TQFN
Package
DS8500
HART Modem
________________________________________________________________
Maxim Integrated Products
1
THIN QFN
(5mm
×
5mm)
TOP VIEW
19
20
18
17
7
6
8
DVDD
RST
OCD
9
DVDD
FSK_IN
FSK_OUT
AVDD
AGND
12
DGND
45
15 14 12 11
D_OUT
D_IN
DGND
XTAL2
XTAL1
RTS
DGND REF
3
13
DGND
16 10 XCEN
DGND
DS8500
+*EP
*EXPOSED PAD.
Pin Configuration
Ordering Information
Rev 1; 2/09
For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642,
or visit Maxim’s website at www.maxim-ic.com.
PART TEMP RANGE PIN-PACKAGE
DS8500-JND+ -40°C to +85°C 20 TQFN-EP*
+
Denotes a lead(Pb)-free/RoHS-compliant package.
*
EP = Exposed pad.
DS8500
HART Modem
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
RECOMMENDED DC OPERATING CONDITIONS
(VDVDD = VAVDD = 2.7V to 3.6V, TA= -40°C to +85°C.) (Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Note 1: Specifications to -40°C are guaranteed by design and are not production tested.
Note 2: Active currents are measured when the device is driven by an external clock XCEN = 1 condition.
Note 3: Guaranteed by design and not production tested.
Note 4: Accuracy is guaranteed based on the external crystal or clock provided.
Voltage Range on All Pins (including AVDD,
DVDD) Relative to Ground .................................-0.5V to +3.6V
Voltage Range on Any Pin Relative to
Ground Except AVDD, DVDD .............-0.5V to (VDVDD + 0.5V)
Operating Temperature Range ...........................-40°C to +85°C
Storage Temperature Range .............................-65°C to +150°C
Soldering Temperature...........................Refer to the IPC/JEDEC
J-STD-020 Specification.
PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS
Digital Supply Voltage VDVDD 2.7 3.6 V
Analog Supply Voltage VAVDD V
AVDD = VDVDD 2.7 3.6 V
Ground GND AGND = DGND 0 0 V
Digital Power-Fail Reset Voltage VRST Monitors VDVDD 2.59 2.64 2.69 V
Active Current IDD V
AVDD = VDVDD = 2.7V (Note 2) 285 μA
Input Low Voltage VIL DGND 0.30 x
VDVDD V
Input High Voltage VIL 0.75 x
VDVDD V
DVDD V
Output Low Voltage VOL I
OL = 4mA DGND 0.4 V
Output High Voltage VOH I
OH = -4mA 0.8 x
VDVDD V
I/O Pin Capacitance CIO Guaranteed by design (Note 3) 15 pF
RST Pullup Resistance RRST 19 45 k
Input Leakage Current XTAL,
RST IILRX -30 +30 μA
Input Leakage Current All Other
Pins IIL -2 +2 μA
Input Low Current for RST I
IL1 V
IN = 0.4V 170 μA
CLOCK SOURCE
External Clock Frequency fHFIN -1% 3.6864 +1% MHz
VOLTAGE REFERENCE
Internal Reference Voltage VREF 1.23 V
FSK INPUT
Input Voltage Range at FSK_IN 0 VREF V
FSK OUTPUT
Output Voltage at FSK_OUT VOUT AC-coupled max 30k load 400 500 600 mVP-P
For a mark -1% 1200 +1%
Frequency of FSK_OUT (Note 4) For a space -1% 2200 +1% Hz
DS8500
HART Modem
_______________________________________________________________________________________ 3
Pin Description
PIN NAME FUNCTION
1, 2 DVDD Digital Supply Voltage
3, 9, 16,
17, 18 DGND Digital Ground
4RST Active-Low Reset, Digital Input/Output. This pin includes an internal pullup resistor and is driven low
as an output when an internal reset condition occurs.
5 OCD
Carrier Detect, Digital Output. A logic-high indicates a valid carrier detection on FSK_IN.
OCD = 1 when FSK_IN amplitude is greater than 120mVP-P.
OCD = 0 when FSK_IN amplitude is less than 80mVP-P.
6 RTS
Request to Send, Digital Input. When set high, the device is put into the demodulator mode. A logic-low
puts the device into modulator mode.
7 XTAL1 Crystal Pin or Input for External Clock at 3.6864MHz
8 XTAL2 Crystal Pin or Output of the Crystal Amplifier
10 XCEN
External Clock Enable, Digital Input. When set high, this pin allows the user to drive an external clock
signal through XTAL1. When in this mode, XTAL2 should be left unconnected. An external crystal must
be connected between XTAL1 and XTAL2 when set low.
11 AVDD Analog Supply Voltage
12 FSK_OUT
FSK Out, Analog Output. Output of the modulator. Provides a phase-continuous, FSK-modulated output
signal (1200Hz and 2200Hz output frequencies) to the 4–20mA current loop interface circuit.
13 REF
Reference, Analog Output. The internal voltage reference is provided as output. This pin must be
connected to a 0.1μF capacitor.
14 FSK_IN
FSK In, Analog Input. Input for the FSK-modulated HART receive signal from the 4–20mA current loop
interface circuit.
15 AGND Analog Ground
19 D_OUT Digital Data Out, Digital Output. Output from the demodulator.
20 D_IN Digital Data In, Digital Input. Input to the modulator.
— EP Exposed Pad. Should be connected to ground (DGND, AGND).
Block Diagram
DS8500
CRYSTAL
OSCILLATOR
CLOCK
GENERATOR
XTAL1
RST AVDDAGNDDGNDDVDD
POWER
MONITOR
XTAL2
Rx
DEMODULATOR
DIGITAL
FILTER
SAMPLE/HOLD
ADC
OCD
D_OUT
Tx
MODULATOR
RTS
D_IN
XCEN
FSK_IN
REF
DAC FSK_OUT
VREF
1.23V
DS8500
Introduction to HART
HART is a backward-compatible enhancement to exist-
ing 4–20mA instrumentation networks that allows two-
way, half-duplex, digital communication with a
microcontroller-based field device. The digital signal is
encoded on top of the existing instrumentation signal.
Communication is accomplished through a series of
commands and responses dependent on the specific
protocol and network topology. The DS8500 does not
implement any portion of the communication protocol; it
only handles the modulation and demodulation of the
encoded information. Digital data is encoded using fre-
quency-shift keying (FSK), which is illustrated in Figure
1. A “1” is identified as a mark symbol and is represent-
ed with a center frequency of 1.2kHz. A “0” is identified
as a space symbol and is represented with a center fre-
quency of 2.2kHz. This allows a throughput of 1.2kbps,
with each symbol occupying an 833µs slot.
Functional Description
The DS8500 modem chip consists of a demodulator, car-
rier detect, digital filter, ADC for input signal conversion, a
modulator and DAC for output signal generation, and
receive and transmit state machine blocks to perform the
HART communication. The
Block Diagram
illustrates
the interface between various blocks of circuitry.
The input HART signal’s noise interference is attenuat-
ed by a one-pole highpass filter that is external to the
chip; the attenuated signal is digitized by the ADC and
filtered by the receive state machine. The transmit state
machine modulates the input to the HART-compliant
signal with the help of the modulator and the DAC.
Modulator
The modulator performs the FSK modulation of the digi-
tal data at the D_IN input. The FSK-modulated sinu-
soidal signal is present at the FSK_OUT output as
illustrated in Figure 1. The modulator is enabled by RTS
being a logic-low. The modulation is done between
1200Hz (mark) or 2200Hz (space) depending on the
logic level of the input signal. The modulator preserves a
continuous phase when switching between frequencies
to minimize the bandwidth of the transmitted signal.
Figure 2 illustrates an example waveform of the DS8500
in modulate mode. The data to be modulated is pre-
sented in a UART format (start, 8 data bits, parity, stop
bit) at D_IN. FSK_OUT shows the modulated output.
Demodulator
The demodulator accepts an FSK signal at the FSK_IN
input and reproduces the original modulating signal at
the D_OUT output. The HART signal should be present-
ed as an 11-bit UART character with a start, data, pari-
ty, and stop bits for proper operation of the
demodulator block. The nominal bit rate of the D_OUT
signal is 1200 bits per second. A simple RC filter is suf-
ficient for anti-aliasing. Figure 3 illustrates an example
waveform of the DS8500 in demodulate mode.
Applications Information
Figure 4 shows the typical application circuit. As the
DS8500 integrates a digital filter, only a simple passive
RC filter is required in front of the ADC. R3 and C3
implement a lowpass filter with a 10kHz cutoff frequen-
cy; C2 and R2/R1 implement a highpass filter with a
480Hz cutoff frequency. The resistor-divider formed by
R1 and R2 provides an input bias voltage of VREF/2 to
the ADC input (R1 = R2).
The output DAC provides a sine-wave signal, and C4
provides the AC-coupled signal output from the
DS8500. The typical value of C4 can be anything
greater than 20nF based on the application.
HART Modem
4 _______________________________________________________________________________________
1.2kHz MARK
"1"
2.2kHz SPACE
"0"
V
T
Figure 1. HART FSK Signal
DS8500
HART Modem
_______________________________________________________________________________________ 5
D_IN
FSK_OUT
START PARITY
STOP
8-BIT DATA
1200bps/833μs
Figure 2. Actual DS8500 Modulator Waveform
D_OUT
FSK_IN
START
STOP
8-BIT DATA
1200bps/833μs
ONE UART CHARACTER (START, 8 DATA BITS, PARITY, STOP)
PARITY
Figure 3. Actual DS8500 Demodulator Waveform
DS8500
HART Modem
6 _______________________________________________________________________________________
DS8500
CRYSTAL
OSCILLATOR
CLOCK
GENERATOR
XTAL1
RST
AVDD
POWER SUPPLY
2.7V TO 3.6V
AGNDDGNDDVDD
POWER
MONITOR
XTAL2
Rx
DEMODULATOR
DIGITAL
FILTER
SAMPLE/HOLD
ADC
OCD
D_OUT
Tx
MODULATOR
RTS
D_IN
XCEN
MICROCONTROLLER
FSK_IN
REF
DAC FSK_OUT
VREF
1.23V
3.6864MHz
3.6864MHz
CRYSTAL
R1
R2
C1
C3
C2
C4
R3
HART IN
4–20mA
DAC OUTPUT
HART AND
4–20mA
OUT
Figure 4. Typical Application Circuit
PACKAGE TYPE PACKAGE CODE DOCUMENT NO.
20 TQFN T2055+3 21-0140
Package Information
For the latest package outline information and land patterns, go to www.maxim-ic.com/packages.
DS8500
HART Modem
Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are
implied. Maxim reserves the right to change the circuitry and specifications without notice at any time.
Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________
7
© 2009 Maxim Integrated Products Maxim is a registered trademark of Maxim Integrated Products, Inc.
Revision History
REVISION
NUMBER
REVISION
DATE DESCRIPTION PAGES
CHANGED
0 10/08 Initial release. —
In the Electrical Characteristics table, changed the Frequency of FSK_OUT
parameter units from kHz to Hz. 2
1 2/09
Added the EP description to the Pin Description table. 3
