Audio ICs Band-pass filter for spectrum analyzer BA3835S / BA3835F The BA3835S and BA3835F are 5-band, band-pass filter ICs that use microprocessor time division to produce serial output for spectrum analyzer displays. To minimize the number of external components required and enable compact and reliable designs, all of the capacitors for the filters are on the chip. Applications CD radio cassette players, mini-component stereo systems, car stereos Features 1) Five band-pass filter elements for spectrum analyzer displays. 2) High-speed readout (output response time : 5µsec. Typ.). 3) Discharge time constant circuit for each band is on the chip. 4) Differential input amplifier rejects common-mode noise. 5) Single 5V power supply. Absolute maximum ratings (Ta = 25C) Recommended operating conditions (Ta = 25C) 601 Audio ICs Block diagram 1 (BA3835S) 602 BA3835S / BA3835F Audio ICs BA3835S / BA3835F Block diagram 2 (BA3835F) 603 Audio ICs BA3835S / BA3835F Electrical characteristics (unless otherwise noted, Ta = 25C, VCC = 5V, RL = 10MΩ, VAIN = 30dBV and SEL = 1) 604 Audio ICs BA3835S / BA3835F Measurement circuit (BA3835S) 605 Audio ICs Output select logic table Timing chart 606 BA3835S / BA3835F Audio ICs BA3835S / BA3835F Application example (BA3835S) 607 Audio ICs BA3835S / BA3835F Operation notes (1) Frequency characteristics The frequency characteristics of this IC are determined by the resistor connected between the RREF terminal and GND. For the specification conditions, the value of this resistor is 100kΩ. If it is necessary to set the frequency characteristics accurately, use a variable resistor (note : all bands will shift together). (2) Load characteristics To convert the bias sense output signal to the GND sense signal, the IC performs a V / I conversion, and then an I / V conversion using a 10kΩ resistor (typ.) for the output. Therefore, if the load circuit connected to the A OUT pin has a MOS structure there is no problem (eg. microprocessor input port), but if the connected circuit has a low input impedance, it may cause the output level to drop. (3) External resistor for the control pin When using a common port for the output select control and FL drive, you must add a diode and resistor as shown in Fig. 4 to prevent the FL drive “L” voltage from destroying the IC. In this case, the “L” voltage applied to the internal comparator input terminal V1 is given by : V1 = R1)R R1)R2)R Vref To maintain a noise margin of at least 2.5V with respect to the comparator threshold level Vref, the representative values for Vref, R1 and R2 are 1.5V, 20kΩ, and 10kΩ respectively. This gives : 20kΩ)R 20kΩ)10kΩ)R 608 1.5V)0.25V<1.5V And from this, the following condition is obtained : R < 30kΩ In this case, the “L” level voltage V2 for the IC will be : V2 < 0.75V (4) Recommended operating ranges Provided that the IC is operated within the recommended operating conditions and the recommended temperature range, the basic circuit functions are guaranteed. Within these ranges, ratings for electrical characteristics for conditions other than those stipulated cannot be guaranteed, but the inherent function of the bandpass filter will be maintained. (5) Application circuit Provided the recommended circuit constants are used, the application circuit should function correctly. However, we recommend that you confirm the characteristics of the circuits in actual use and pay due attention to the cautionary notes given below. If you change the circuit constants, check both the static and transient characteristics of the circuit, and allow sufficient margin to accommodate variations between both ICs and external components. Note, also, that Rohm has not been able to conduct a sufficient study into patent rights. Audio ICs BA3835S / BA3835F (6) Output offset voltage The relationship between the output offset voltage and the output selection cycle (cycle time) for this IC is shown in Fig.5. The maximum output offset voltage of 150mV that is given in the electrical characteristics table is under the condition that Ts = 200ms. When Ts is greater than 50ms, the graph of the output offset voltage is a straight line at 150mV. When Ts is below 50ms, due to transient characteristics of the peak hold circuit, the graph is a line sloping downwards to the right. In other words, the shorter the cycle, the larger the output offset voltage. Furthermore, the output offset voltage may shift due to soldering or other temperature stresses from the surroundings. Therefore, when setting the spectral analyzer light level, take into consideration the points given above and make sure that it does not light by mistake during quiescent periods. Use the chart below as a guide for this, and, if necessary, leave even a larger margin. Electrical characteristics curves 609 Audio ICs External dimensions (Units: mm) 610 BA3835S / BA3835F