U2538B_05 Datasheet, PDF (4/9 Pages) ATMEL Corporation – IR Receiver for Data Communication

3. Functional Description
3.1 Input Stage (TIA)
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 separate 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 kΩ), 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 kΩ). If the input resistance is too
high, the useful signal will be lost due to the junction capacitance of the photodiode.
3.2 Controlled Gain Amplifier (CGA)
The controlled gain amplifier accounts for the greatest part of the voltage gain and can be con-
trolled via the voltage at CAGC (pin 2). Gain control is needed to support the interference
suppression of the detector. High-pass behavior results from the capacitive coupling of the indi-
vidual stages. The cut-off frequency is approximately 20 kHz.
3.3 Bandpass Filter (BPF)
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 3-1). The following formula can be used for calculating the resistor, R_f0:
R_fO(kΩ)
=
---⁸----⁸---⁵---⁵----- – 13
f₀(kHz)
where: 20 kHz < f₀ < 60 kHz
Figure 3-1. 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
4717B–IRRC–09/05