A5191HRT AMIS HART™ Modem
Data Sheet
The output of the demodulator is qualified with the carrier detect signal (OCD), therefore, only IRXA signals large enough to be detected (100mV
p-p
typically) by the carrier detect circuit produce received serial data at ORXD.
Maximum demodulator jitter is 12 percent of one bit given input frequencies within HART specifications, a clock frequency of 460.8kHz (±1.0 percent)
and zero input (IRXA) asymmetry.
4.2 Transmit-Signal Shaper
The transmit-signal shaper generates a HART compliant FSK modulated signal at OTXA. Figure 6 and Figure 7 show the transmit-signal forms of the
A5191HRT.
For IAREF = 1.235 V
DC
, OTXA will have a voltage swing from approximately 0.25 to 0.75 V
DC
.
Figure 6: OTXA Waveform (1200Hz)
Figure 7: OTXA Waveform (2200Hz)
4.3 Carrier Detect Circuitry
The carrier detect comparator shown in Figure 8 generates logic low output if the IRXAC voltage is below ICDREF. The comparator output is fed into a
carrier detect block (see Figure 4). The carrier detect block drives the carrier detect output pin OCD high if INRTS is high and four consecutive pulses out
of the comparator have arrived. OCD stays high as long as INRTS is high and the next comparator pulse is received in less than 2.5ms. Once OCD goes
inactive, it takes four consecutive pulses out of the comparator to assert OCD again. Four consecutive pulses amount to 3.33ms when the received signal
is 1200Hz and to 1.82ms when the received signal is 2200HZ.
4.4 Analog Receiver Circuitry
4.4.1 Voltage References
The A5191HRT requires two voltage references, IAREF and ICDREF.
IAREF sets the dc operating point of the internal operational amplifiers and comparators. A 1.235 V
DC
reference (Analog Devices AD589) is suitable as
IAREF.
The level at which OCD (carrier detect) becomes active is determined by the dc voltage difference (ICDREF - IAREF). Selecting a voltage difference of 0.08
V
DC
will set the carrier detect to a nominal 100 mV
p-p
.
AMI Semiconductor - Rev. 2.0, Mar. 05
www.amis.com
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