A5191HRT AMIS HART™ Modem
4.4.2 Bias Current Resistor
Data Sheet
The A5191HRT requires a bias current resistor to be connected between OCBIAS and V
SS
. The bias current controls the operating parameters of the internal
operational amplifiers and comparators.
The value of the bias current resistor is determined by the reference voltage IAREF and the following formula:
The recommended bias current resistor is 500KW when IAREF is equal to 1.235 V
DC
.
In Figure 8 all external capacitor values have a tolerance of ±5 percent and the resistors have a tolerance of ±1 percent, except the 3MW which has a
tolerance of ±5 percent. External to the A5191HRT, the filter exhibits a three-pole, high-pass filter at 624Hz and a one-pole, low-pass filter at 2500Hz.
Internally, the A5191HRT has a high-pass pole at 35Hz and a low-pass pole at 90kHz. The low-pass pole can vary as much as ±30 percent. The input
impedance of the entire filter is greater than 150MW at frequencies below 50kHz.
Figure 8: Receive Filter Schematic
AMI Semiconductor - Rev. 2.0, Mar. 05
www.amis.com
6
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