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HART Modem ASIC | HT1200M | Page
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The recommended bias current resistor is 500 ohm; when IAREF is equal to 1.235
V
DC
.
In Figure 8 all external capacitor values have a tolerance of 15 percent and the
resistors have a tolerance of 1 percent, except the 3 ohm; which has a tolerance of 15
percent. External to the HT1200M, the filter exhibits a three-pole, high-pass filter at
624 Hz and a one-pole, low-pass filter at 2500 Hz.
Internally, the HT1200M has a high-pass pole at 35 Hz and a low-pass pole at 90
kHz. The low-pass pole can vary as much as 130 percent. The input impedance of
the entire filter is greater than 150 ohm; at frequencies below 50 kHz.
Figure 8 Receive Schematic
OSCILLATOR
The HT1200M requires a 460.8 kHz clock signal on OXTL. This can be provided by
an external clock or external components may be connected to the HT1200M
internal oscillator.
Internal Oscillator Option
The oscillator cell will function with either a 460.8 kHz crystal or ceramic resonator.
A parallel resonant ceramic resonator can be connected between OXTL and IXTL.
Figure 9 illustrates the crystal option for clock generation using a 460.8 kHz (1
percent tolerance) parallel resonant crystal and two tuning capacitors. The actual
values of the capacitors may depend on the recommendations of the manufacturer of
the resonator. Typically, capacitors in the range of 100 pF to 470pF are used.
