7
LTC1967
1967f
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. This is high impedance. The
RMS averaging is accomplished with a single shunt ca-
pacitor from this node to OUT RTN. The transfer function
is given by:
V
OUT RTN
Average IN
IN
OUT
–
–
(
)
=
)
2
1
2
PIU
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 it is often tied to
GND, it can 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. LTC1967 is
debiased if open circuited or driven to V
+
. For normal
operation, pull to GND.
APPLICATIOU
RMS-TO-DC CONVERSION
W
U
U
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
1967 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
(V)
1.000
0.900
0.866
0.637
WAVEFORM
Square Wave
Sine Wave
Triangle Wave
SCR at 1/2 Power,
Θ
= 90
°
SCR at 1/4 Power,
Θ
= 114
°
V
RMS
1.000
1.000
1.000
1.000
ERROR*
11%
*Calibrate for 0% Error
–3.8%
–29.3%
1.000
0.536
–40.4%
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