Converting a DC milivoltmeter to AC

[red]

Hy I have this DC milivotmeter and I need to measure AC for determining the T/S parameters for drivers.
You may ask why not buy a multimeter?The answer is a lot cheaper to make a simple circuit than to buy a new multimeter right?
So I found a circuit that may solve my problem.This one in the pic.
In a simulation on electronics workbench the eror is about 10%.A bit much.So do you know a circuit better than this one?

[Frank Berry]

If you increase the value of the 100k feedback resistor on the #2 opamp (on the right), the gain and output voltage will come up.
Be sure to use high speed diodes.

[red]

Well I have to be sure that erors are still prety small but I don't know.Someone showed me another schematic that might work.He told me that this is used in multimeters.Maybe so.Anyway it has some erors that are corected by the R2/R1 amplification and this being almost 1.125 this wold corect the erors.As you can see in the pic the multimeter shows exactly what the source is.The analogue Voltmeter shows a value bigger than the multimeter.The advantage is that at bigger voltages the eror drops and is actualy showing the real value.

[red]

After playing around a bit I seem to have foud the answer (see this pic).
I only changed the R2 and made the opamp real).In a few days when I'll have the time I'll make it because seems the best choice(fewer precision resistors and only one opamp).I'll use the TL072 opamp or the TDA2330A (haven't decided yet.)Maybe you have a better ideea...

[mikee55]

Hi Red, I need to do this too. I just bought a new meter that measures low voltages,but silly me, it reads DC not AC. How are you getting on? Do I build your circuit? Please can you tell me what to do? And what do I need? I've got to tackle the volume used up by driver yet! Are you reffering to Rod Elliotts site for measuring T/S parameters.

PLEASE PLEASE talk to me. Pretty Please!!!

[AndrewT]

Hi Red,
your first post looked like a precision full wave rectifier.
You seem to have moved away from that. Go back.

The grounded non inverting inputs should have resistors to reduce the output offset of each opamp.
The resistor value should be equal to the parallel combination of resistors on the inverting input(s).

Could the offsets have been confusing the interpretation of the output readings?

Some very low offset opamps do not respond well to this standard technique. eg. op228

[AndrewT]

Hi,
to measure T/S you need accurate frequency measurement and a system that works at very low frequencies.
This is needed to allow you to see or read the slowly changing signal.
Try measuring 10Hz with a 2.5 samples/second multimeter to see what I mean (analogue might be better here).

An Fs above 30Hz just about avoids the problem

[janneman]

What you are building is a rectifier that makes half waves from sine waves. You need to put a capacitor at the output, and then measure with a DC meter. I saw some post measuring with an AC meter, which is wrong.
The precision rectifier has very specific ratios of the resistors which can be found on the net. Tweaking the resistors for one specific case means you can only measure this one specific case.

Find the precision rectifier on the net, check the resistor ratio, put the output in a cap so that the ripple is less than a few percent at your lowest freq (10Hz?), and then put on a DC meter and tweak THAT meter to give a reading correcsponding to the RMS value of the AC input sine wave. And no, it is not accurate for other waveforms than sines.

Jan Didden

[lineup]

Figure 4 shows the complete meter amplifier,
and uses a 2N5459 J-FET for the input.
---
... linear up to at least 80kHz

Audio Millivoltmeter - Rod Elliott - ESP (Original Design)

ESP Projects: Audio Test Equipment

Regards
and happy testing!
lineup

[red]

Hy guys,
I posted that schematic long ago. I tested it but it didn't work as expected. The problems seemed to be at first wiring the opamp, then the dc offset, then the diodes...I still haven't solved the problem.