a5191hrt AMI Semiconductor, Inc., a5191hrt Datasheet - Page 5

• Manufacturer Part Number: a5191hrt
• Description: Amis Hart Modem
• Manufacturer: AMI Semiconductor, Inc.

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A A 5 5 1 1 9 9 1 1 H H R R T T A A M M I I S S H H A A R R T T ™ ™ M M o o d d e e m m
A A M M I I S S e e m m i i c c o o n n d d u u c c t t o o r r - - R R e e v v . . 2 2 . . 0 0 , , M M a a r r . . 0 0 5 5
The output of the demodulator is qualified with the carrier detect signal (OCD), therefore, only IRXA signals large enough to be detected (100mV
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 4 . . 2 2 T T r r a a n n s s m m i i t t - - S S i i g g n n a a l l S S h h a a p p e e r r
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
4 4 . . 3 3 C C a a r r r r i i e e r r D D e e t t e e c c t t C C i i r r c c u u i i t t r r y y
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 . . 4 4 A A n n a a l l o o g g R R e e c c e e i i v v e e r r C C i i r r c c u u i i t t r r y y
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
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
DC
, OTXA will have a voltage swing from approximately 0.25 to 0.75 V
Figure 6: OTXA Waveform (1200Hz)
p-p
.
5
DC
.
Figure 7: OTXA Waveform (2200Hz)
DC
reference (Analog Devices AD589) is suitable as
Data Sheet
p-p