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heat the thermistor comes from the circuit being measured. If the circuit being measured can support the heating current, then it is possible to make a post-measurement calculation to correct the effect, as the impedance of the heating element is known. If the signal is small then a pre-amplifier is necessary, and the measuring capabilities of the instrument will be limited by this pre-amplifier. In radio frequency (
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limitation of the operational amplifier used to create the absolute value (especially at low input signal levels) tends to make the second method the poorest at high frequencies, while the FET method can work close to VHF. Specialist techniques are required to produce sufficiently accurate integrated
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The technique is not very precise but it will measure any waveform at any frequency (except for extremely low frequencies, where the thermistor's thermal capacitance is too small so that its temperature is fluctuating too much). A big drawback is that it is low-impedance: that is, the power used to
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include a function to give the RMS value of a waveform. The precision and the bandwidth of the conversion is entirely dependent on the analog to digital conversion. In most cases, true RMS measurements are made on repetitive waveforms, and under such conditions digital oscilloscopes (and a few
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Thermal converters have become rare, but are still used by radio hams and hobbyists, who may remove the thermal element of an old unreliable instrument and incorporate it into a modern design of their own construction. Additionally, at very high frequencies
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in a specific configuration which multiplies the input signal by itself (squares it), averages the result with a capacitor, and then calculates the square root of the value (via a multiplier/squarer circuit in the feedback loop of an
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sophisticated sampling multimeters) are able to achieve very high bandwidths as they sample at much higher sampling frequency than the signal frequency to obtain a stroboscopic effect.
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True RMS provides a more correct value that is proportional to the square root of the average of the square of the curve, and not to the average of the absolute value. For any given
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circuits for complex analog calculations, and very often meters equipped with such circuits offer true RMS conversion as an optional extra with a significant price increase.
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This principle was exploited in early thermal converters. The AC signal would be applied to a small heating element that was matched with a
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Tyler, Les; Kirkwood, Wayne (2008). "12.3.4 Dedicated Analog
Integrated Circuits for Audio Applications". In Glen Ballou (ed.).
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value that would be required to get the same heating effect. For example, if 120 V AC RMS is applied to a resistive
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188:) work, the low impedance is not necessarily a drawback since 50 ohm driving and terminating impedances are widely used.
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and applying a correction factor. The value of the correction factor applied is only correct if the input signal is
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If a waveform has been digitized, the correct RMS value may be calculated directly. Most digital and PC-based
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work. The circuitry before time averaging is particularly crucial for high-frequency performance. The
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may be used to directly create the square-law transfer function, before time-averaging.
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it would heat up by exactly the same amount as if 120 V DC were applied.
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National
Semiconductor - LB-25 True rms Detector (Linear Brief 25), June 1973
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A list of digital multimeters having true rms feature in them.
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as a means of deriving the square-law transfer function
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323:limitations which makes them unsuitable for most
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319:Unlike thermal converters they are subject to
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351:Handbook for Sound Engineers. Fourth Edition
232:of the input signal, which is fed into a
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77:Learn how and when to remove this message
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40:This article includes a list of general
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413:Springer Vieweg, 2013, 8. Aufl., S. 2.
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109:the signal is often converted into a
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450:Electronic amplifiers
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306:Blackmer RMS detector
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238:exponential amplifier
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157:The RMS value of an
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101:True RMS Multimeter
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59:introducing
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178:thermistor
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42:references
329:slew rate
321:bandwidth
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200:RF work.
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130:waveform
18:True RMS
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