U2538B Datasheet, PDF (4/9 Pages) ATMEL Corporation – IR RECEIVER FOR DATA COMMUNICATION

Functional Description
Input Stage (TIA)
Controlled Gain
Amplifier (CGA)
Bandpass Filter (BPF)
The input stage provides the necessary bias voltage for the photo diode and ensures
decoupling of the useful signal. This involves processing the DC and AC portions in sep-
arate parts of the circuit: the bias voltage (BIAS) and the transimpedance amplifier
circuit (TIA). The bias voltage circuit operates like a load resistor with respect to the
photo diode, the value of which is low for DC and low-frequency signals (3 to 100 kW),
but as high as possible for the operating frequency (100 kHz to 1 MHz) depending on
the input current). The ac portion of the input signal feeds an inverted amplifier with a
sufficiently low input resistance (Z_i < 10 kW). If the input resistance is too high, the useful
signal will be lost due to the junction capacitance of the photodiode.
The controlled gain amplifier accounts for the greatest part of the voltage gain and can
be controlled via the voltage at CAGC (Pin 2). Gain control is needed to support the
interference suppression of the detector. High-pass behaviour results from the capaci-
tive coupling of the individual stages. The cut-off frequency is approximately 20 kHz.
The bandpass filter basically consists of integrated components. An external resistor
determines the mid-frequency. The filter quality is about 7 and is practically independent
of the selected mid-frequency (see Figure 3). The following formula can be used for cal-
culating the resistor, R_f0:
R_fO(kW )
=
---⁸----⁸---⁵---⁵----- – 13
f₀(kHz)
where: 20 kHz < f₀ < 60 kHz
Figure 3. Characteristic of the Bandpass Filter
120
100
80
60
40
20
0
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
f/f₀
4 U2538B
4717A–IRRC–05/03