TDA 5201

ASK Single Conversion Receiver

Applications

Data Sheet

21

Revision 1.6, 2010-12-21

4

Applications

4.1

Choice of LNA Threshold Voltage and Time Constant

In the following figure the internal circuitry of the LNA automatic gain control is shown.

Figure 5

LNA Automatic Gain Control Circuitry

The LNA automatic gain control circuitry consists of an operational transimpedance amplifier that is used to

compare the received signal strength signal (RSSI) generated by the Limiter with an externally provided threshold

voltage

U

typically 0.8 V and 2.8 V to provide a switching point within the receive signal dynamic range.

This voltage

U

voltage divider between the 3VOUT pin (Pin 24) which provides a temperature stable 3 V output generated from

the internal bandgap voltage and the THRES pin. If the RSSI level generated by the Limiter is higher than

U

the OTA generates a positive current

I

generates a negative current. These currents do not have the same values in order to achieve a fast-attack and

slow-release action of the AGC and are used to charge an external capacitor which finally generates the LNA gain

control voltage.

Pins:

24

23

4

LNA

R4

R5

RSSI (0.8 - 2.8V)

Gain control

voltage

OTA

+3V

C