Problem of True RMS Conversion of Sine Wave

S3S4S5S6

Problem of True RMS Conversion of Sine Wave [Copy link]

 

They all use RMS-DC chips such as AD637. If the waveform is just a single sine wave, can this chip be omitted? Full-wave rectification and filtering can be used to output DC. The main reason is that AD637 is too expensive, more than 400 RMB per chip.

In addition, in what fields are RMS-DC chips more commonly used? At present, I know that they seem to be used in multimeters (guessing)

maychang

"Is it possible to not use this chip and use full-wave rectification and filtering to output DC?"

Full-wave rectification or bridge rectification can only obtain the AC average value (strictly speaking, the absolute value), but cannot obtain the accurate effective value.

S3S4S5S6

maychang posted on 2022-3-27 16:08 "Is it possible not to use this chip and use full-wave rectification and filtering to output DC?" Full-wave rectification, or bridge rectification, only...

What is the principle of this type of chip? Can it be constructed with op amps, similar to instrumentation amplifiers, and implemented through three op amp circuits?

S3S4S5S6

maychang posted on 2022-3-27 16:08 "Is it possible not to use this chip and use full-wave rectification and filtering to output DC?" Full-wave rectification, or bridge rectification, only...

Is there any precision rectifier circuit that can be realized without a diode? The main reason is that the diode has a voltage drop, the minimum is 200mV

There will be problems when my signal is less than 200mV

maychang

S3S4S5S6 Published on 2022-3-27 17:08 What is the principle of this type of chip? Can it be constructed with op amps, similar to instrumentation amplifiers, and implemented through three op amp circuits?

The so-called true RMS chip has a nonlinear circuit inside and can measure the effective value of AC regardless of the waveform (non-sinusoidal waveforms can also measure the effective value).

Using ordinary op amps and triodes, we can construct logarithmic circuits and antilogarithmic circuits. We can obtain effective values from logarithmic circuits and antilogarithmic circuits, but it is quite troublesome and difficult to get accurate results by ourselves.

maychang

S3S4S5S6 Published on 2022-3-27 17:11 Is there any precision rectifier circuit that can be realized without diodes? The main reason is that the diode has a voltage drop, the minimum is 200mV. When my signal is less than 200mV...

A precision rectifier circuit can be constructed using an operational amplifier. However, the precision rectifier circuit measures an average value, and cannot obtain an effective value.

maychang

S3S4S5S6 Published on 2022-3-27 17:08 What is the principle of this type of chip? Can it be constructed with op amps, similar to instrumentation amplifiers, and implemented through three op amp circuits?

If the frequency of the signal to be measured is relatively low, for example, less than 100 Hz, it can be implemented using a microcontroller with AD conversion.

High-speed AD conversion is performed within one cycle of AC to obtain multiple instantaneous values of the AC signal. These instantaneous values are calculated using a single-chip microcomputer to obtain the AC effective value.

There are many microcontrollers with AD conversion, and the price is not expensive, but you need to write the software yourself.

maychang

S3S4S5S6 Published on 2022-3-27 17:11 Is there any precision rectifier circuit that can be realized without diodes? The main reason is that the diode has a voltage drop, the minimum is 200mV. When my signal is less than 200mV...

For ordinary diode rectification, let alone a signal amplitude less than 200mV, even if the amplitude is 1V or even 2V, the error caused by the forward voltage drop of the rectifier tube is already intolerable.

There are many such precision rectifier circuits and they are not complicated.

lugl4313820

maychang published on 2022-3-27 17:28 For ordinary diode rectification, let alone the signal amplitude is less than 200mV, even if the amplitude is 1V or even 2V, the error caused by the forward voltage drop of the rectifier tube is already intolerable. ...

How to choose an amplifier to measure the small change of NTC temperature voltage? Can you provide some help?

maychang

lugl4313820 Published on 2022-3-28 09:38 How to choose an amplifier to measure the small change of NTC temperature voltage? Can you provide some help?

"How to choose an amplifier to measure small changes in NTC temperature and voltage"

Choose an op amp with low offset voltage and low temperature drift to amplify and then measure. Op amps with low offset and low temperature drift are usually called precision op amps. Among them, the one with the smallest offset temperature drift is the chopper-stabilized zero op amp.

S3S4S5S6

maychang published on 2022-3-27 17:28 For ordinary diode rectification, let alone the signal amplitude is less than 200mV, even if the amplitude is 1V or even 2V, the error caused by the forward voltage drop of the rectifier tube is already intolerable. ...

"There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

S3S4S5S6

S3S4S5S6 Published on 2022-3-27 17:11 Is there any precision rectifier circuit that can be realized without diodes? The main reason is that the diode has a voltage drop, the minimum is 200mV. When my signal is less than 200mV...

The picture above shows the half-wave and full-wave rectifier circuits composed of ADI's AD823. When I built the actual circuit,

The waveform after output rectification is the yellow waveform in the figure, with a trapezoid in the middle, not the half sine wave shown in the figure.

However, the simulated waveform can match the one in the figure. I suspect that the power supply voltage of the op amp is not enough, resulting in clipping. After trying to increase the power supply voltage, it still cannot be solved. I don't know where the problem lies.

maychang

S3S4S5S6 posted on 2022-3-28 20:44 "There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

"There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

maychang

S3S4S5S6 posted on 2022-3-28 20:44 "There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

"There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

maychang

S3S4S5S6 posted on 2022-3-28 20:44 "There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

"There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

运放精密全波整流电路.pdf (575.3 KB, downloads: 6)

2022-3-28 20:57 上传

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S3S4S5S6

maychang posted on 2022-3-28 20:54 "There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

I mean without the diode.

S3S4S5S6

maychang posted on 2022-3-28 20:57 "There are many such precision rectifier circuits, and they are not complicated." Can you recommend one?

grateful!

maychang

S3S4S5S6 posted on 2022-3-28 20:57 I mean the one without diode

In the circuits listed above, the forward voltage drop of the diode has been largely overcome by the high gain of the op amp and can be ignored (the effect of the diode is below mV).

maychang

S3S4S5S6 Published on 2022-3-28 20:53 The above picture shows the half-wave and full-wave rectifier circuits composed of ADI's AD823. When I built it in real life, the waveform after output rectification was the yellow one in the picture...

This circuit uses a single power supply.

It is not clear from the picture whether it can perform precision rectification. Please post the datasheet of AD823 to see.

S3S4S5S6

maychang posted on 2022-3-28 21:06 This circuit uses a single power supply. Whether it can play a precise rectification role cannot be seen in the figure. Please post the datasheet of AD823 for reference.

This circuit is mentioned in the "Operational Amplifier Precision Full-Wave Rectification Circuit" you just uploaded.