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U2532B Datasheet(PDF) 3 Page - TEMIC Semiconductors |
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U2532B Datasheet(HTML) 3 Page - TEMIC Semiconductors |
3 / 10 page ![]() 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. |