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參數(shù)資料
| 型號： | LTC491IS |
| 廠商： | LINEAR TECHNOLOGY CORP |
| 元件分類： | 通用總線功能 |
| 英文描述： | Differential Driver and Receiver Pair |
| 中文描述： | LINE TRANSCEIVER, PDSO14 |
| 封裝： | PLASTIC, SOIC-14 |
| 文件頁數(shù)： | 9/12頁 |
| 文件大?。?/td> | 236K |
| 代理商： | LTC491IS |
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LTC491
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Cable Termination
The proper termination of the cable is very important.
If the cable is not terminated with it’s characteristic
impedance, distorted waveforms will result. In severe
cases, distorted (false) data and nulls will occur. A quick
look at the output of the driver will tell how well the cable
is terminated. It is best to look at a driver connected to the
end of the cable, since this eliminates the possibility of
getting reflections from two directions. Simply look at the
driver output while transmitting square wave data. If the
cable is terminated properly, the waveform will look like a
square wave (Figure 13).
Rt
DRIVER
DX
RECEIVER
RX
Rt = 120
Rt = 47
Rt = 470
LTC491 TA14
PROBE HERE
Figure 13. Termination Effects
If the cable is loaded excessively (47
), the signal initially
sees the surge impedance of the cable and jumps to an
initial amplitude. The signal travels down the cable and is
reflected back out of phase because of the mistermination.
When the reflected signal returns to the driver, the ampli-
tude will be lowered. The width of the pedestal is equal to
twice the electrical length of the cable (about 1.5ns/foot).
If the cable is lightly loaded (470
), the signal reflects in
phase and increases the amplitude at the driver output. An
input frequency of 30kHz is adequate for tests out to 4000
feet of cable.
AC Cable Termination
Cable termination resistors are necessary to prevent un-
wanted reflections, but they consume power. The typical
differential output voltage of the driver is 2V when the
cable is terminated with two 120
resistors, causing
33mA of DC current to flow in the cable when no data is
being sent. This DC current is about 60 times greater than
the supply current of the LTC491. One way to eliminate the
unwanted current is by AC coupling the termination resis-
tors as shown in Figure 14.
LTC491 TA15
120
RECEIVER
RX
C
C = LINE LENGTH (ft) x 16.3pF
Figure 14. AC Coupled Termination
The coupling capacitor must allow high-frequency energy
to flow to the termination, but block DC and low frequen-
cies. The dividing line between high and low frequency
depends on the length of the cable. The coupling capacitor
must pass frequencies above the point where the line
represents an electrical one-tenth wavelength. The value
of the coupling capacitor should therefore be set at 16.3pF
per foot of cable length for 120
cables. With the coupling
capacitors in place, power is consumed only on the signal
edges, and not when the driver output is idling at a 1 or 0
state. A 100nF capacitor is adequate for lines up to 4000
feet in length. Be aware that the power savings start to
decrease once the data rate surpasses 1/(120
×
C).
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| LTC491IS#PBF | 功能描述:IC TXRX RS485 LOW POWER 14-SOIC RoHS:是 類別:集成電路 (IC) >> 接口 - 驅動器，接收器，收發(fā)器 系列:- 產品培訓模塊:RS-232 & USB Transceiver 標準包裝:2,000 系列:- 類型:收發(fā)器 驅動器/接收器數(shù):1/1 規(guī)程:RS232 電源電壓:3 V ~ 5.5 V 安裝類型:表面貼裝 封裝/外殼:16-SSOP（0.209"，5.30mm 寬） 供應商設備封裝:16-SSOP 包裝:帶卷 (TR) 其它名稱:296-19849-2 |
| LTC491IS#PBF | 制造商:Linear Technology 功能描述:Driver/Receiver IC |
| LTC491IS#TR | 功能描述:IC TXRX RS485 LOW POWER 14-SOIC RoHS:否 類別:集成電路 (IC) >> 接口 - 驅動器，接收器，收發(fā)器 系列:- 標準包裝:27 系列:- 類型:收發(fā)器 驅動器/接收器數(shù):3/3 規(guī)程:RS232，RS485 電源電壓:4.75 V ~ 5.25 V 安裝類型:表面貼裝 封裝/外殼:28-SOIC（0.295"，7.50mm 寬） 供應商設備封裝:28-SOIC 包裝:管件 |
| LTC491IS#TRPBF | 功能描述:IC TXRX RS485 LOW POWER 14-SOIC RoHS:是 類別:集成電路 (IC) >> 接口 - 驅動器，接收器，收發(fā)器 系列:- 標準包裝:27 系列:- 類型:收發(fā)器 驅動器/接收器數(shù):3/3 規(guī)程:RS232，RS485 電源電壓:4.75 V ~ 5.25 V 安裝類型:表面貼裝 封裝/外殼:28-SOIC（0.295"，7.50mm 寬） 供應商設備封裝:28-SOIC 包裝:管件 |
| LTC491ISPBF | 制造商:Linear Technology 功能描述:RS-485/RS-422 Driver/Receiver SOIC14 |
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