TDA5212_06 INFINEON [Infineon Technologies AG], TDA5212_06 Datasheet - Page 23

• Manufacturer Part Number: TDA5212_06
• Description: ASK/FSK 915MHz Single Conversion Receiver
• Manufacturer: INFINEON [Infineon Technologies AG]

Wireless Components
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 4-1
The LNA automatic gain control circuitry consists of an operational transimpe-
dance amplifier that is used to compare the received signal strength signal
(RSSI) generated by the Limiter with an externally provided threshold voltage
U
value between approximately 0.8 and 2.8V to provide a switching point within
the receive signal dynamic range.
This voltage U
can be generated by attaching a voltage divider between the 3VOUT pin (i.e.
Pin 24) which provides a temperature stable 3V output generated from the inter-
nal bandgap voltage and the THRES pin. If the RSSI level generated by the
Limiter is higher than U
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 volt-
age.
thres
. As shown in the following figure the threshold voltage can have any
LNA Automatic Gain Control Circuitry
thres
Pins:
RSSI > U
RSSI < U
is applied to the THRES pin (Pin 23) The threshold voltage
thr e s h o ld
thr e s h o ld
4 - 2
thres
24
: I
: I
, the OTA generates a positive current I
loa d
loa d
20kΩ
=4.2µA
= -1.5µA
+3.1 V
C
4
R1
U
I
C
lo ad
U
U
U
U
c
c
c max
c min
:< 2.6V : Gain high
:> 2.6V : Gain low
23
= 1.67V
= V
OTA
C C
U
thr e s h o ld
- 0.7V
Gain control
R2
v oltage
Specification, December 2006
VCC
RSSI (0.8 - 2.8V)
LNA
Applications
TDA 5212
load
LNA_autom.wmf
. This