NSC LH0091

LH0091 True RMS to DC Converter
General Description
Features
The LH0091, rms to dc converter generates a dc output
equal to the rms value of any input per the transfer function:
Y
Y
Y
EOUT(DC) e
0#
1 T
EIN2(t) dt
T 0
Y
Y
Y
The device provides rms conversion to an accuracy of 0.1%
of reading using the external trim procedure. It is possible to
trim for maximum accuracy (0.5 mV g 0.05% typ) for decade
ranges i.e., 10 mV x 100 mV, 0.7V x 7V, etc.
Y
Y
Low cost
True rms conversion
0.5% of reading accuracy untrimmed
0.05% of reading accuracy with external trim
Minimum component count
Input voltage to g 15V peak for VS e g 15V
Uncommitted amplifier for filtering, gain, or high crest
factor configuration
Military or commercial temperature range.
Block and Connection Diagrams
Dual-In-Line Package
Dual-In-Line Package
Order Number LH0091CD
See Package D16D
TL/H/5694 – 1
Simplified Schematic
TL/H/5694 – 2
Note: Dotted lines denote external connections.
C1995 National Semiconductor Corporation
TL/H/5694
RRD-B30M115/Printed in U. S. A.
LH0091 True RMS to DC Converter
September 1993
Absolute Maximum Ratings
Operating Temperature Range
LH0091C
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales
Office/Distributors for availability and specifications.
Supply Voltage
Storage Temperature Range
LH0091C
g 22V
Input Voltage
Output Short Circuit Duration
TMIN
b 25§ C
g 15V peak
TMAX
85§ C
b 25§ C to a 85§ C
Lead Temp. (Soldering, 10 seconds)
260§ C
Continuous
Electrical Characteristics VS e g 15V, TA e 25§ C unless otherwise noted
Transfer Function e EO(DC) e
0T# E
1 T
0
2
IN (t) dt
Parameter
Conditions
Min
Typ
Max
Units
ACCURACY (See Definition of Terms)
Total Unadjusted Error
50 mVrmssVINs7Vrms (Figure 1)
20, g 0.5
40, g 1.0
mV, %
Total Adjusted Error
50 mVrmssVINs7Vrms (Figure 3)
0.5, g 0.05
1, g 0.2
mV, %
Total Unadjusted Error vs Temperature
b 25§ C s TA s a 70§ C
Total Unadjusted Error vs Supply Voltage
0.25, g 0.2%
mV, %/§ C
1
mV/V
AC PERFORMANCE
Frequency for Specified Adjusted Error
Input e 7Vrms, Sinewave (Figure 3)
Input e 0.7Vrms, Sinewave (Figure 3)
Input e 0.1Vrms, Sinewave (Figure 3)
30
70
40
20
kHz
KHz
kHz
Frequency for 1% Additional Error
Input e 7Vrms, Sinewave (Figure 3)
Input e 0.7Vrms, Sinewave (Figure 3)
Input e 0.1Vrms, Sinewave (Figure 3)
100
200
75
50
kHz
kHz
kHz
Bandwidth (3 dB)
Input e 7Vrms, Sinewave (Figure 3)
Input e 0.7Vrms, Sinewave (Figure 3)
Input e 0.1Vrms, Sinewave (Figure 3)
2
1.5
0.8
MHz
MHz
MHz
Crest Factor
Rated Adjusted Accuracy Using the High
Crest Factor Circuit (Figure 5)
5
10
INPUT CHARACTERISTICS
Input Voltage Range
For Rated Performance
Input Impedance
g 0.05
4.5
g 11
Vpeak
5
kX
Output Short Circuit Current
22
mA
Output Impedance
1
X
OUTPUT CHARACTERISTICS
Rated Output Voltage
RLt2.5 kX
10
V
POWER SUPPLY REQUIREMENTS
Operating Range
Quiescent Current
g5
VS e g 15V
14
2
g 20
V
18
mA
Op Amp Electrical Characteristics VS e g 15V, TA e 25§ C unless otherwise noted
Parameter
Conditions
Typ
Max
Units
1.0
10
mV
4.0
200
nA
Input Bias Current
30
500
Input Resistance
2.5
MX
15
160
V/mv
g 10
g 13
V
RSs10 kX
90
dB
RSs10 kX
96
dB
25
mA
Slew Rate (Unity Gain)
0.5
V/ms
Small Signal Bandwidth
1.0
MHz
VOS
Input Offset Voltage
IOS
Input Offset Current
IB
RIN
AOL
Large Signal Voltage Gain
VOUT e g 10V, RLt2 kX
VO
Output Voltage Swing
R e 10 kX
VI
Input Voltage Range
CMRR
Common-Mode Rejection Ratio
PSRR
Supply Voltage Rejection Ratio
ISC
Output Short-Circuit Current
Sr
BW
Min
RSs10 kX
nA
g 10
V
Typical Performance Characteristics
Error vs Frequency
Error vs Frequency
Error vs Crest Factor
TL/H/5694 – 3
Typical Applications (All applications require power supply by-pass capacitors.)
TL/H/5694 – 4
CEXT t 1mF; frequency t 1 kHz
FIGURE 1. LH0091 Basic Connection (No Trim)
3
Typical Applications (Continued)
RT e 240k
CEXT t 1mF, f t 1 kHz
Note. The easy trim procedure is used for ac coupled input signals. It involves two trims and can achieve accuracies of 2 mV offset g 0.1% reading.
Procedure:
1. Apply 100 mV rms (sine wave) to input, adjust R3 until the output reads
100 mVDC.
2. Apply 5 Vrms (sine wave) to input, adjust R4 until the output reads 5 VDC.
3. Repeat steps 1 and 2 until the desired initial accuracy is achieved.
FIGURE 2. LH0091 ‘‘Easy Trim’’ (For ac Inputs Only)
R1 e dc symmetry balance
R2 e Input offset
Note. This procedure will give accuracies of 0.5 mV offset g 0.05% reading
for inputs from 0.05V peak to 10V peak.
R3 e Output offset
Procedure:
1. Apply 50 mVDC to the input. Read and record the output.
R4 e Gain adjust
2. Apply b 50 mVDC to the input. Use R2 to adjust for an output of the same
magnitude as in step 1.
3. Apply 50 mV to the input. Use R3 to adjust the output for 50 mV.
4. Apply b 50 mV to input. Use R2 to adjust the output for 50 mV.
5. Apply g 10V alternately to the input. Adjust R1 until the output readings
for both polarities are equal (not necessary that they be exactly 10V).
6. Apply 10V to the input. Use R4 to adjust for 10V at the output.
7. Repeat this procedure to obtain the desired accuracy.
FIGURE 3. LH0091 Standard dc Trim Procedure
Note. The additional op amp in the LH0091 may be used as a low pass filter
as shown in Figure 4.
R1 e R2 e 16k
C1 e C2 e 1mF
fO j 10 Hz
TL/H/5694 – 6
FIGURE 4. Output Filter Connection Using the Internal Op Amp
4
Typical Applications (Continued)
Note. When converting signals with a crest factor t 2, the LH0091 should
be connected as shown. Note that this circuit utilizes a 20k resistor to drop
the input current by a factor of five. The frequency response will correspond
to a voltage which is 1/5 eIN.
Note that the extra op amp in the LH0091 may be used to build a gain of 5
amplifier to restore the output voltage.
TL/H/5694 – 7
Note. Response time of the dc output voltage is dominated
by the RC time constant consisting of the total resistance
between pins 9 and 10 and the external capacitor, CEX.
FIGURE 5. High Crest Factor Circuit
Definition of Terms
Frequency for Specified Error: The error at low frequency
is governed by the size of the external averaging capacitor.
At high frequencies, error is dependent on the frequency
response of the internal circuitry. The frequency for specified error is the maximum input frequency for which the output will be within the specified error band (i.e., frequency for
1% error means the input frequency must be less than 200
kHz to maintain an output with an error of less than 1% of
the initial reading.
True rms to dc Converter: A device which converts any
signal (ac, dc, ac a dc) to the dc equivalent of the rms value.
Error: is the amount by which the actual output differs from
the theoretical value. Error is defined as a sum of a fixed
term and a percent of reading term. The fixed term remains
constant, regardless of input while the percent of reading
term varies with the input.
Total Unadjusted Error: The total error of the device without any external adjustments.
Crest Factor: is the peak value of a waveform divided by
the rms value of the same waveform. For high crest factor
signals, the performance of the LH0091 can be improved by
using the high crest factor connection.
Bandwidth: The frequency at which the output dc voltage
drops to 0.707 of the dc value at low frequency.
5
LH0091 True RMS to DC Converter
Physical Dimensions inches (millimeters)
Dual-In-Line Package
Order Number LH0091CD
NS Package D16D
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