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SP000012902 View Datasheet(PDF) - Infineon Technologies

Part NameDescriptionManufacturer
SP000012902 ASK Single Conversion Receiver Infineon
Infineon Technologies Infineon
SP000012902 Datasheet PDF : 38 Pages
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4
Applications
TDA 5201
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:< 2.6V : Gain high
UC 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
21
Revision 1.6, 2010-12-21
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