True Rms Detector

National Semiconductor Corporation

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True rms Detector
True rms Detector
National Semiconductor
Linear Brief 25
June 1973
INTRODUCTION
The op amp precision Rectifier circuits have greatly eased
the problems of AC to DC conversion It is possible to mea-
sure millivolt AC signal with a DC meter with better than 1%
accuracy Inaccuracy due to Diode turn-on and nonlinearity
is eliminated and precise rectification of low level signals is
obtained
Once the signal is rectified it is normally filtered to obtain a
smooth DC output The output is proportional to the average
value of the AC input signal rather than the root mean
square With known input waveforms such as a sine trian-
gle or square this is adequate since there is a known pro-
portionality between rms and average values However
when the waveform is complex or unknown a direct readout
of the rms value is desirable
The circuit shown will provide a DC output equal to the rms
value of the input Accuracy is typically 2% for a 20 V
p-p
input signal from 50 Hz to 100 kHz although it’s usable to
about 500 kHz The lower frequency is limited by the size of
the Filter Capacitor Further since the input is DC coupled it
can provide the true rms equivalent of a DC and AC signal
Basically the circuit is a precision absolute value circuit con-
nected to a one-quadrant multiplier divider Amplifier A1 is
the absolute value amplifier and provides a positive input
current to amplifiers A2 and A4 independent of signal polari-
ty If the input signal is positive A1’s output is clamped at
b
0 6V D2 is reverse biased and no signal flows through
R5 and R6 Positive signal current flows through R1 and R2
into the summing junctions of A2 and A4 When the input is
negative an inverted signal appears at the output of A1
(output is taken from D2) This is summed through R5 and
R6 with the input signal from R1 and R2 Twice the current
flows through R5 and R6 and the net input to A2 and A4 is
positive
Note 1
Note 2
All Resistors are 1% unless otherwise specified
Note 3
All Diodes are 1N914
Note 4
Supply voltage
g
15V
TL H 8474 – 1
LB-25
C
1995 National Semiconductor Corporation
TL H 8474
RRD-B30M115 Printed in U S A