Rohm BA3826S Band-pass filter for spectrum analyzer Datasheet

Audio ICs
Band-pass filter for spectrum analyzer
BA3826S
The BA3826S is a 7-band (2), band-pass filter that uses microprocessor time division to output in serial to left and right
stereo channels. 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) Seven band-pass filter elements for spectrum analyzer displays.
2) Microprocessor time division used for serial output
on L and R channels.
3) All filter capacitors are built-in to minimize external attached requirements.
Absolute maximum ratings (Ta = 25C)
Recommended operating conditions (Ta = 25C)
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4) Low current dissipation (8mA Typ.).
5) Ideal for mini-component stereos and other stereo
equipment.
Audio ICs
BA3826S
Block diagram
Electrical characteristics (unless otherwise noted, Ta = 25C, VCC = 5V, Vee = *5V, RL = 10kΩ, and Rφ = 120kΩ)
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Audio ICs
Measurement circuit
Circuit operation
(1) Input switch circuit
The BA3826S alternately switches between the left and
right stereo channel signals using time division to serialize and process the signals.
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BA3826S
Audio ICs
(2) Band-pass filter circuit
This circuit extracts the required frequency range from
the input signal and amplifies it.
The center frequency is determined by the value of the
current flowing through the resistor connected to pin 14.
Q is set to 3.5 (Typ.).
Due to the characteristics of the band-pass filter circuit,
the circuit has a transition rise and fall characteristic. Due
to the transition rise characteristic, five input cycles are
required until peak output is reached.
When RESET is input, a built-in circuit cancels the fall
transition characteristic (input the reset signal for 0.5ms).
(3) DET circuit
The DET circuit detects the signal selected by the band
pass filter and performs peak hold.
The RESET Driver discharges the charge stored in the
capacitor. After the RESET input is released, the RESET
Driver discharges continuously for 14ms.
BA3826S
(4)
Timing
1) RESET timing
The RESET input is used as the RESET signal for the
band-pass filter circuit and the DET circuit. The setting
time is 0.5ms min.
When performing a RESET input, disable both the Lin
and Rin inputs. The synchronous shift in this case must
be "0.1ms max.
2) Output select timing
The audio input signal is not accepted when the RESET
input and L (R) inputs are low. There are no particular restrictions on the select input for the various frequencies,
but there are delay times until the peak voltage is settles
(see (a) and (b) below). Accordingly, set the read timing
for the DET signal after the longer of the delay times in
(a) and (b).
(a) For the band-pass filter, a sampling time of at least
5 cycles is required due to the transition rise characteristic.
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Audio ICs
(b) For DET, after release of RESET, there is a discharge time of 14mS (Max.)
The read enable period for Vo starts 0.5ms after the output switch logic settles.
(5) Application example with reduced cycle time
The transition characteristics for the band-pass filter are
shown in the graph below. The time for at least 5 fo cycles
is required until the DET output peaks. Therefore, the
lower the frequency, the longer the time required and fo1
requires the longest time. Refer to the values given below
for applications that require a shorter cycle time.
When fo1 = 60Hz, the time requires for output u 60% is
tg u 45ms.
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BA3826S
(6) Output switch circuit
This circuit uses time division switching to output the
peak hold signal from each DET.
Switching occurs when the control signal reaches 1.4V,
and when A, B, and C are all high, output is disabled
(GND level). Refer to the table below regarding the output switching logic.
The recommended output load resistance is 10kΩ.
Audio ICs
BA3826S
Input / output switching logic
Input switching logic
Output switching logic
Application example
Operation notes
(1) Application circuits
Provided the recommended circuit constants are used,
the application circuits 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.
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Audio ICs
(2) Frequency characteristics
The frequency characteristics for this IC are set by an external resistor. The value of this resistor for the specification conditions is 120kΩ. Use a potentiometer if you wish
to set the frequency characteristics accurately.
(3) Operating voltage range
Operation of the application example circuit is guaranteed provided that the VCC and Vee voltages are within
the specified power supply voltage range, and the ambient temperature is within the specified range. With regard
to the characteristic values, the output voltage level will
differ from the rated values for the specification conditions if the power supply voltage is reduced, but the inherent function of the band-pass filters is preserved.
Electrical characteristics curves
External dimensions (Units: mm)
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BA3826S
(4) Load resistance
The various circuit characteristics in this specification
sheet were measured using an output load resistance of
10kΩ. If the load resistance is too low, the I / O gain and
control range vary slightly. Be aware of this when connecting the next stage.
(5) Coupling capacitors
Set the polarity of the input electrolytic coupling capacitors after establishing the electrical potential relationship
with the connected input.
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