SP000016381 View Datasheet(PDF) - Infineon Technologies

Part Name

Description

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SP000016381

ASK/FSK Single Conversion Receiver

Infineon Technologies 

4

Applications

TDA 5200

ASK Single Conversion Receiver

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.

R4

R5

Pins:

Uthreshold

24

23

+3V

RSSI (0.8 - 2.8V)

OTA

VCC

Iload

RSSI > Uthreshold: Iload=4.2µA

RSSI < Uthreshold: Iload= -1.5µA

LNA

Gain control

voltage

4

UC

Uc:< 2.6V : Gain high

Uc:> 2.6V : Gain low

C

Ucmax= VCC - 0.7V

Ucmin = 1.67V

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 Uthres. As shown in the following figure the threshold voltage can have any value between approximately

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

This voltage Uthres is applied to the THRES pin (Pin 23). The threshold voltage can be generated by attaching a

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 Uthres,

the OTA generates a positive current Iload. This yields a voltage rise on the TAGC pin (Pin 4). Otherwise, the OTA

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.

Data Sheet

22

Revision 3.0, 2010-12-28

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