ATA5745 datasheet(18/43 Pages) ATMEL

18 / 43 page

4596A–RKE–05/06

ATA5745/ATA5746 [Preliminary]

Sensitivity Reduction

The output voltage of the RSSI amplifier is internally compared to a threshold voltage V

Th_red.

Th_red is determined by the value of the external resistor RSense. RSense is connected between

the pins SENSE and VS3V_AVCC (see Figure 10-1 on page 28). The output of the comparator

is fed into the digital control logic. By this means, it is possible to operate the receiver at a lower

sensitivity.

If the level on input pin SENSE_CTRL is low, the receiver operates at full sensitivity.

If the level on input pin SENSE_CTRL is high, the receiver operates at a lower sensitivity. The

reduced sensitivity is defined by the value of R

Sense, the maximum sensitivity by the sig-

nal-to-noise ratio of the LNA input. The reduced sensitivity depends on the signal strength at the

output of the RSSI amplifier.

Since different RF input networks may exhibit slightly different values for the LNA gain, the sen-

sitivity values given in the electrical characteristics refer to a specific input matching. This

matching is illustrated in Figure 2-1 on page 6 and exhibits the best possible sensitivity.

If the sensitivity reduction feature is not used, pin SENSE can be left open, pin SENSE_CTRL

must be set to GND.

To operate with reduced sensitivity, pin SENSE_CTRL must be set to high before the RX signal

path will be enabled by setting pin RX to high (see Figure 4-1 on page 19). As long as the RSSI

level is lower than V

Th_red (defined by the external resistor RSense) no data stream is available on

pin DATA_OUT (low level on pin DATA_OUT). An internal RS flip-flop will be set to high the first

time the RSSI voltage crosses V

Th_red, and from then on the data stream will be available on pin

DATA_OUT. From then on the receiver also works with full sensitivity. This makes sure that a

telegram will not be interrupted if the RSSI level varies during the transmission. The RS flip-flop

can be set back, and thus the receiver switched back to reduced sensitivity, by generating a pos-

itive pulse on pin ASK_NFSK (see Figure 4-2 on page 19). In FSK mode, operating with reduced

sensitivity follows the same way.

ATA5745 Datasheet(PDF) 18 Page - ATMEL Corporation