Cannot measure sine wave (Vrms). So what is the other way to measure it?

[chellappan1975]

Hi All,

I need to design a simple debug tool for measuring the failures of audio boards.

Those audio boards are in Vrms voltage. I knew that I can't measure Vrms because it's a sine wave.

Please share your ideas.

 

[FvM]

I neither understand why you "can't measure Vrms" nor what the problem has to do with sine waves. Vrms is defined for any voltage waveform.

Testing an audio system usually involves connecting a test generator to the input and tracing the signal through the circuit up to the output. What's your particular problem with this procedure?

 

[chellappan1975]

I understood Vrms is AC voltage.

If I want to measure AC using ADC? How?

Anybody have ideas ?

 

[godfreyl]

You could just check the output with an oscilloscope or multimeter.

If you want to use an ADC, pass the signal to the ADC input through a capacitor, and bias the ADC input with a resistive divider between Vcc and ground. It's probably a good idea to connect a resistor in series with the ADC input too, to avoid overloading it.

 

[KonstanU]

As another way of biasing you can use circuit of Sum OpAmp.
This circuit is usual method to shift input signal.



If it is necessary I can show how to calculate value of resistors and Vref.

 

[FvM]

There's no specification, but I would expect an ADC that is not able to follow the AC instantaneous value. In this case, you would want to use a rectifier to measure the AC level.

 

[Syncopator]

Those audio boards are in Vrms voltage.

A sine wave test signal can be measured at various points in an amplifier with an ordinary digital or analogue multimeter set to a suitable a.c. voltage range. They are calibrated to display the r.m.s. value.

However, there will often be a d.c. component of voltage at the desired point of measurement. In such a case the d.c. must be isolated from the meter by using a capacitor in series with one of the meter's test probes (it doesn't matter which one). The value of the capacitor should be should be such that it has a low reactance, compared with the meter's impedance, at the frequency of the test signal.

I knew that I can't measure Vrms because it's a sine wave.

Whoever told you that was talking rubbish.

If I want to measure AC using ADC.

Why? You could do it, but that would be making an easy job much more complicated than it needs to be.
Use a multimeter, as I said above.

 

[yogece]

can you tell us what kind of problem more often you face with audio boards.why do you want to measure the Vrms of output from the amplifier.for example,if your amplifier gives the voltage gain of 10 you can generate a test signal(sinewave) of 1KHZ/1Vpp signal then by using CRO if you compare the input voltage and output voltage by using CRO i hope this would help you.if you are not satisfied with this reply just elabrate your problem

 

[FvM]

that would be making an easy job much more complicated than it needs to be.
Use a multimeter, as I said above.

Making an easy job much more complicated often spells academic project. :smile:

 

[chellappan1975]

Hi All,

Now I will explain deeper based on the question above that I shoot to u...

Actually, audio boards are tested in functional test and the reading are shown in Vrms. For people whom want to debug the failure boards, they need a simple tool to debug the boards. I'm planning to do something. Let me share with you.

Software (Computer) <-- Microcontroller <-- ADC <-- Measure voltage <-- Testpoint.

In the part of measure voltage, definitely the voltage reading is in Vrms unit, so how do I will convert the Vrms to DC?


Is my suggestion below correct...?

Use Rectifier with bypass capacitor to a voltage divider...

does it make sense to anyone? Please feedback to me...

 

[BradtheRad]

I suspect that you want to lift an AC signal into the DC range, 0 to 5V.

Schematic:



- - - Updated - - -

I see my schematic needs safeguards, in case the AC amplitude is great enough to extend above 5V or below 0.

It also is wise to install resistors, to limit current in case potentiometers are dialed to an extreme.

 

[alexan_e]

You can check an audio board using a pc oscilloscope like https://www.sillanumsoft.org/prod01.htm
It also includes many measuring tools (you have to calibrate the level first) and signal generator which means you can feed a signal and check the output.

 

[chellappan1975]

Hi,

How does a rectifier should be look like? For my case....

 

[BradtheRad]

Is my suggestion below correct...?

Use Rectifier with bypass capacitor to a voltage divider...

How does a rectifier should be look like? For my case....

Perhaps you are referring to a Villard circuit?

This consists of a diode and capacitor in series. It will lift an AC waveform into the positive region.

It is liable to distort your incoming signal. It will act as a filter by reducing high frequencies.

The following stage needs to be high impedance. If it is too low ohms, then you have a half-wave power supply with ripple voltage.

Here's a schematic to show how a Villard cell might be arranged to do what you ask: