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参数资料
| 型号： | LM1894MX/NOPB |
| 厂商： | National Semiconductor |
| 文件页数： | 17/19页 |
| 文件大小： | 0K |
| 描述： | IC NOISE REDUCT SYS DYNAM 14SOIC |
| 标准包装： | 2,500 |
| 类型： | 动态噪音抑制 |
| 应用： | 车载系统，播放器，电视 |
| 安装类型： | 表面贴装 |
| 封装/外壳： | 14-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 14-SOICN |
| 包装： | 带卷 (TR) |
| 配用： | LM1894MTBD-ND - BOARD EVALUATION LM1894 |
| 其它名称： | *LM1894MX *LM1894MX/NOPB LM1894MX |
SNAS551C – DECEMBER 1994 – REVISED APRIL 2013
External Component Guide
(Figure 1)
Component
Value
Purpose
C1
0.1
μF–100 μF
May be part of power supply, or may be added to suppress power supply
oscillation.
C2, C13
1
μF
Blocks DC, pin 2 and pin 13 are at DC potential of VS/2. C2, C13 form a low
frequency pole with 20k RIN.
C14
25
μF–100 μF
Improves power supply rejection.
C3, C12
0.0033
μF
Forms integrator with internal gm block and op amp. Sets bandwidth
conversion gain of 33 Hz/
μA of gm current.
C4, C11
1
μF
Output coupling capacitor. Output is at DC potential of VS/2.
C5
0.1
μF
Works with R1 and R2 to attenuate low frequency transients which could
disturb control path operation.
C6
0.001
μF
Works with input resistance of pin 6 to form part of control path frequency
weighting.
C8
0.1
μF
Combined with L8 and CL forms 19 kHz filter for FM pilot. This is only
required in FM applications(1).
L8, CL
4.7 mH, 0.015
μF
Forms 19 kHz filter for FM pilot. L8 is Toko coil CAN-1A185HM(1)(2).
C9
0.047
μF
Works with input resistance of pin 9 to form part of control path frequency
weighting.
C10
1
μF
Set attack and decay time of peak detector.
R1, R2
1 k
Ω
Sensitivity resistors set the noise threshold. Reducing attenuation causes
larger signals to be peak detected and larger bandwidth in main signal path.
Total value of R1 + R2 should equal 1 k
Ω.
R8
100
Ω
Forms RC roll-off with C8. This is only required in FM applications.
(1)
When FM applications are not required, pin 8 and pin 9 hook-up as follows:
(2)
Toko America Inc., 1250 Feehanville Drive, Mt. Prospect IL 60056
Circuit Operation
The LM1894 has two signal paths, a main signal path and a bandwidth control path. The main path is an audio
low pass filter comprised of a gm block with a variable current, and an op amp configured as an integrator. As
seen in Figure 11, DC feedback constrains the low frequency gain to AV = 1. Above the cutoff frequency of the
filter, the output decreases at
6 dB/oct due to the action of the 0.0033 μF capacitor.
The purpose of the control paths is to generate a bandwidth control signal which replicates the ear's sensitivity to
noise in the presence of a tone. A single control path is used for both channels to keep the stereo image from
wandering. This is done by adding the right and left channels together in the summing amplifier of Figure 11. The
R1, R2 resistor divider adjusts the incoming noise level to open slightly the bandwidth of the low pass filter.
Control path gain is about 60 dB and is set by the gain amplifier and peak detector gain. This large gain is
needed to ensure the low pass filter bandwidth can be opened by very low noise floors. The capacitors between
Copyright 1994–2013, Texas Instruments Incorporated
7
Product Folder Links: LM1894
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