参数资料
型号: LTC1968IMS8#TRPBF
厂商: Linear Technology
文件页数: 7/28页
文件大小: 296K
描述: IC CONVERTER RMS-DC PREC 8MSOP
标准包装: 2,500
电流 - 电源: 2.3mA
电源电压: 5.0V
安装类型: 表面贴装
封装/外壳: 8-TSSOP,8-MSOP(0.118",3.00mm 宽)
供应商设备封装: 8-MSOP
包装: 带卷 (TR)
7
LTC1968
1968f
GND (Pin 1): Ground. The power return pin.
IN1 (Pin 2): Differential Input. DC coupled (polarity is
irrelevant).
IN2 (Pin 3): Differential Input. DC coupled (polarity is
irrelevant).
V
OUT
 (Pin 5): Output Voltage. Pin 5 is high impedance. The
RMS averaging is accomplished with a single shunt ca-
pacitor from Pin 5 to OUT RTN. The transfer function is
given by:
V
OUTRTN  Average IN  IN
OUT


(
)
=
(
)
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
?/DIV>
2  1
2
U
U
PI FU CTIO S
OUT RTN (Pin 6): Output Return. The output voltage is
created relative to this pin. The V
OUT
 and OUT RTN pins
are not balanced and this pin should be tied to a low
impedance, both AC and DC. Although Pin 6 is often tied
to GND, it can also be tied to any arbitrary voltage:
GND < OUT RTN < (V
+
  Max Output)
V
+
 (Pin 7): Positive Voltage Supply. 4.5V to 5.5V.
ENABLE (Pin 8): An Active-Low Enable Input. LTC1968 is
debiased if open circuited or driven to V
+
. For normal
operation, pull to GND.
APPLICATIO S I FOR ATIO
U
U
U
RMS-TO-DC CONVERSION
Definition of RMS
RMS amplitude is the consistent, fair and standard way to
measure and compare dynamic signals of all shapes and
sizes. Simply stated, the RMS amplitude is the heating
potential of a dynamic waveform. A 1V
RMS
 AC waveform
will generate the same heat in a resistive load as will 1V DC.
Mathematically, RMS is the Root of the Mean of the
Square:
V    V
RMS
=
2
+

R
1V DC
R
1968 F01
SAME
HEAT
1V AC
RMS
R
1V (AC + DC) RMS
Figure 1
Alternatives to RMS
Other ways to quantify dynamic waveforms include peak
detection and average rectification. In both cases, an
average (DC) value results, but the value is only accurate
at the one chosen waveform type for which it is calibrated,
typically sine waves. The errors with average rectification
are shown in Table 1. Peak detection is worse in all cases
and is rarely used.
Table 1. Errors with Average Rectification vs True RMS
AVERAGE
RECTIFIED
WAVEFORM
V
RMS
(V)    ERROR*
Square Wave
1.000   1.000   11%
Sine Wave
1.000   0.900   *Calibrate for 0% Error
Triangle Wave    1.000   0.866   3.8%
SCR at 1/2 Power,  1.000   0.637   29.3%
?= 90?/DIV>
SCR at 1/4 Power,  1.000   0.536   40.4%
?= 114?/DIV>
The last two entries of Table 1 are chopped sine waves as
is commonly created with thyristors such as SCRs and
Triacs. Figure 2a shows a typical circuit and Figure 2b
shows the resulting load voltage, switch voltage and load
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