U2532B datasheet(3/10 Pages) TEMIC | IR Transmitter and Receiver

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U2532B

Preliminary Information

TELEFUNKEN Semiconductors

Rev. A1, 27-Sep-96

3 (10)

Functionality of the Various Circuit Blocks

Transimpedance Amplifier (TIA)

The input stage provides the necessary bias voltage for the

photodiode and ensures decoupling of the useful signal.

This involves processing the dc and ac portions in sepa-

rate parts of the circuit, BIAS (Bias voltage) and TIA. The

BIAS circuit separates the dc part (sunlight, incandescent

light) from the input signal. The ac portion of the input

current is fed to an inverting amplifier with a sufficiently

low input impedance (TIA). The TIA prevents the signal

slopes to be negatively affected by the junction capaci-

tance of the photodiode.

Controlled Gain Amplifier (CGA)

The CGA consists of two differential amplifiers. The total

gain is 17 in the high sense mode (Sensitivity Control In-

put “High”) and can be reduced to 8 if the Sensitivity

Control Input is set to “Low”. In the low sense mode a

BER of 1E-9 is guaranteed. In the high sense mode

eventually generated output pulses must be suppressed by

software handling. The lower and upper cut-off frequen-

cies of the amplifier are 25 kHz and 900 kHz

respectively. Additionally the overall gain can be

attenuated by 30 dB in 2 dB steps. The attenuation is digi-

tally controlled by the AGC (Automatic Gain Control)

circuit.

Comparator (COMP)

The comparator compares the output signal of the CGA

to an internal threshold voltage. The output of that

comparator is directly connected to an collector output

stage. An internal pull up resistor of 20 k

W is provided.

Automatic Gain Control (AGC)

The AGC adjusts the sensitivity of the receiver according

to the strength of the incoming signals. When the input

signal increases, the amplification of the CGA is reduced

to a value where a BER < 1E-9 is still guaranteed, but sig-

nals from disturbers can be effectively suppressed. With

this feature a proper data transmission can be maintained

also in the presence of energy saving lamps and ceiling

lamps that are common in offices. The dynamic range of

the AGC is max. 30 dB. This provides the ability to sup-

press also strong disturbers. As the AGC acts when the

input signal increases, the transmission distance is re-

duced if disturbances have to be suppressed. The AGC is

digitally controlled and therefore not dependent on any

time constant. The amplification of the CGA is set at ev-

ery input pulse or input burst for ASK-modulation and is

maintained until the next input signal is detected. The sig-

nal strength determines whether to reduce, increase or

maintain the gain. If no input signal is detected in 18 ms,

the AGC considers the data transmission to be finished

and increases the gain by a 2 dB step. Within every 18 ms

the gain is increased by an additional 2 dB step, until the

maximum gain of the CGA is reached. If a “High” signal

is applied at the reset (Pin 4), the AGC is set to maximum

sensitivity. Incoming signals don‘t influence the AGC.

Under this biasing condition it is possible to get maximum

transmission distance also in the presence of a known

strong disturber, if of course the signals of the disturber

can be succesfully suppressed by software handling in the

microprocessor. During “Power-On” the AGC is set to

maximum sensitivity. The gain of the AGC is maintained

while the transceiver is transmitting.

Transmitter

IRED Driver (DRV)

The IRED driver DRV is also monolithically integrated

on the transceiver chip providing a high impedance input

to drive a fast IR emitter diode. The “active high” input

signal drives the output stage. This stage mainly consists

of an input amplifier and an open collector NPN transistor

that is saturation controlled. The output transistor is capa-

ble of driving a lood current up to 1 A.

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