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| Audio ICs Ground isolation amplifier BA3121 / BA3121F / BA3121N The BA3121,BA3121F and BA3121N are ground isolation amplifiers developed for use in car audio applications. These ICs efficiently eliminate problems caused by wiring resistance, and remove noise generated by the electrical devices used in automobiles. The capacitance values of the external capacitors required for the ICs are small to allow compact and reliable set design. FApplications Car audio systems FFeatures 1) Large capacitors not required 2) High common-mode rejection ratio (57dB typ. at f = 1kHz). 3) Low noise (VNO = 3.5Vrms Typ.). FAbsolute maximum ratings (Ta = 25_C) 4) Low distortion (THD = 0.002% Typ.). 5) Two channels. FRecommended operating conditions (Ta = 25_C) 717 Audio ICs FBlock diagrams BA3121 / BA3121F / BA3121N 718 Audio ICs BA3121 / BA3121F / BA3121N FElectrical characteristics (unless otherwise noted, Ta = 25_C, VCC = 12V, f = 1kHz, Rg = 1.8k) 719 Audio ICs FMeasurement circuits BA3121 / BA3121F / BA3121N 720 Audio ICs FCircuit operation BA3121 / BA3121F / BA3121N Car-audio systems are earthed to the car body, and for this reason, electrical noise generated by the car electrics can enter the power amplifier input via the chassis, and become audible. The BA3121 makes use of the common-mode rejection characteristics of an operational amplifier to eliminate this noise. Without the BA3121 noise enters the power amplifier input directly, when used, the CMMR of operational amplifiers 1-A and 2-A eliminates the noise. Principles of noise elimination: To obtain the output voltage (eO) Vi = R4 (R3)R4) R2 e1 ) R1 S e2 R1)R2 R1 S Vi 1 eO = * 2 From 1 and 2 eO = * eO = * R1)R2 R2 e1 ) R1 R1 R2 R1 S (e1*e2) ) S R4 (R3)R4) S e2 R1R4*R2R3 S e2 R1 (R3)R4) Ideally, if R1R4 = R2R3, and e1 = e2, the noise voltage will become zero. However, due to mismatching between the resistors, difference in the noise voltages (e1 and e2), and tolerances in the operational amplifier, a noise voltage does result. With the BA3121, the elimination level of the noise is expressed as: CMMR = 20log (eO / ei) (ei = e1 = e2) Therefore, CMRR y 41dB can be guaranteed. FOperation notes (1) Maintain a ratio of 2: 1 for the values of the capacitors connected to pin 2 (Vm1) and pin 6 (Vm2) to keep the ripple rejection ratio stable. If this ratio is maintained, the ripple rejection ratio will not vary significantly even if the capacitance values are halved. (2) If the value of the capacitor connected to pin 2 (Vm1) in the example is doubled, the bass-region CMMR will be +6dB, and if it is halved, it will be *6dB (see Fig. 16). 721 Audio ICs FApplication example BA3121 / BA3121F / BA3121N 722 Audio ICs BA3121 / BA3121F / BA3121N FElectrical characteristics curves 723 Audio ICs BA3121 / BA3121F / BA3121N 724 Audio ICs FExternal dimensions (Units: mm) BA3121 / BA3121F / BA3121N 725 Audio ICs MEMO 726 Audio ICs MEMO 727 |
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