4.1.4 External LNA Support

The sensitivity of the receiver can be increased by using an external low-noise amplifier. The ATA8510/15 offers the output pin 29/RX_ACTIVE, which can be used to bias the external LNA. The pin signalizes the active time of the receive path. The pin can supply 5 mA current for a 4.5-5.5V application, and 1.5 mA for a 1.9-3.6V application, while delivering a minimum of 0.9 x V_VS as supply voltage for the LNA (for more information, see parameter no. 15.60 in Electrical Characteristics).

The achievable sensitivity improvement can be calculated using a cascaded noise figure calculation according to Friis's formula. Be aware that Friis’s formula uses the noise factors instead of noise figures and a linear power gain. The noise figure of the ATA8510/15 is 7.5 dB in Low-Band and 8.5 dB in High-Band.

For example, an external LNA with a 3 dB noise figure and 10 dB gain improves the system noise figure in Low-Band from typical 7.5 dB to 3.9 dB. Therefore, the sensitivity values of parameters no. 4.90 to no. 5.60 in RF Receiving Characteristics are improved by typically 3.6 dB.