A5191HRTNGEVB
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Figure 5. Power and RESETB Waveform During Startup, Showing 2.63 ms Startup Delay
C1, C2 and C6 are 100 nF ceramic decoupling capacitors
located directly adjacent to each power pin. For analog
power pins, an additional large−value ceramic capacitor
may be needed in addition to the 100 nF decoupling
capacitor when the application is intended for high−noise
environments.
For loop powered devices, additional decoupling with a
large value capacitor is advised to prevent digital noise from
being transmitted on the current loop.
The ferrite beads FB1, FB2 and FB3 in series with power
supply lines help to reduce EMI.
Reference voltages and comparator bias
A5191HRT needs an external analog reference voltage
for the receiver or demodulator (RX) comparator and carrier
detect (CD).
The AREF reference voltage sets the trip point of the
demodulation operational amplifier of the 5191HRT. The
AREF reference voltage is also used in setting the DC
operating point of the received signal after it has passed
through the band−pass receive filter. The ideal value for the
AREF reference voltage depends on the voltage supply, and
is chosen roughly half−way the operating range of the
operational amplifiers. This ensures the range of the
operational amplifier is maximized. For operation at 3 V, a
1.24 V reference voltage is recommended. For operation at
5 V, a 2.5 V reference voltage is recommended.
For A5191HRTNGEVB, the TLV431 shunt regulator is
used with an internal reference of 1.24 V. This reference is
compared against the output voltage, and the shunt transistor
base is adjusted until it sinks enough current to drop the
output to 1.24 V.
A simple low pass filter formed by R12 and C11 is added
to increase reference stability. A slight voltage drop is
observed over this filter caused by loading of the reference
voltage. However, the voltage drop
and the influence on the
operation of the IC is minimal. Measurements show a
voltage drop of 22 mV over R12, indicating a current of 22
m
A. Of this current ca. 5
m
A is consumed by the CDREF
resistor division. The rest (ca. 17
m
A) is used internally by
the IC through the AREF pin. Current consumption through
the CDREF pin is negligible.
The CDREF reference voltage sets the threshold for the
carrier detect comparator. As the received signal is biased at
AREF, the difference between CDREF and AREF will
determine the minimum amplitude needed for the carrier
detect comparator to flip. A (AREF−CDREF) of 80 mV
corresponds to signal of approximately 100 mV
peak−to−peak at the input of the receive filter. The CDREF
reference voltage on the A5191HRTNGEVB is generated
by a resistor division of the AREF reference. This will create
an extra load on the low pass filter of AREF. However, the
drop on the resistor of the low pass can be considered
negligible.
An external resistor is required to set the bias current. The
voltage over the bias resistor is regulated to AREF, so that
the resistor determines a bias current. This bias current
