cheap & easy way to measure mic polarization at the capsule

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dfuruta

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Joined
Apr 8, 2010
Messages
237
With a (very) crude differential voltmeter.  Schematic attached.  For the "NULL" ammeter, use a normal DMM in DC voltage mode.

Component values could use some tweaking.  Particularly, if you don't need to measure up to 120V, use a lower voltage.  For the 120V:120V, I used back to back 120V:12V xfmrs.

To use this, start with the pot all the way down and adjust it up slowly until you've got a null.  Unplug the DUT and measure the voltage across the pot with your DMM.

Built the circuit and confirmed that it gets as close as my clumsy fingers will go to a 9V battery through a 1G resistor, and to the polarization voltage on an MCA SP1, checked against a Keithley 619.

Would love to hear the problem with this sort of thing.  Seems easy enough (too easy?).
 

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mmm do you think the impedance of the multimeter is high enough to not load the capsule? a tipical impedance for volt meter is 10Mohms, it is very low compared to 1G.  provably the error will be always less when the "null" voltage is near zero but there should be an error always isn't? the point if it is low enough.

I can be very wrong if so please enlight me.

cheerz
Rafael
 
That's exactly how it should be done, it has a name, it's called opposition method or something like that if the translation is good.

The proper way to do it is with two multimeters, one as a zero detector and the other at the pot decider so you don't insert a load into the pot as a new error source. The zero detector could be used as voltage or current, it's always done for high impedance sources, but you have to take into account how much current will mean how much voltage to know the definition you need as a zero current detector, if you use a voltage detector the impedance of the zero detector will still be loading badly the DUT, so for 1GΩ DUT and 10MΩ meter, you will have 1% of the voltage, so the error will be 100 times higher than the null error, usually accounted as one or half less significant digit of the meter, but could be lower if a 50% duty cycle on the sign detector is used. Note that the error in the zero detector is not critical as lone as it's properly zeroed before starting, so you can archive errors lower than the instrument errors, the error on the other meter, measuring the voltage across the pot is the one that counts, let's say you try to measure 50V with a 1%+1LSD DMM at the pot, 200V range, 0.2V as 1LSD plus 0.5V as 1% of the reading, you have 0.7V error, so you probably want to detect your zero to be less than a few mV, so you are still on the order of magnitude of error, too much definition and you won't get the zero by tweaking your source. Also you want your source to have a low noise so it doesn't affect the reading, using two or three stages RC filtering or even a regulator to get rid of as much noise as you can is not a bad idea.

12afael said:
mmm do you think the impedance of the multimeter is high enough to not load the capsule? a tipical impedance for volt meter is 10Mohms, it is very low compared to 1G.  provably the error will be always less when the "null" voltage is near zero but there should be an error always isn't? the point if it is low enough.

I can be very wrong if so please enlight me.

cheerz
Rafael

The error will always be there, the thing is we can know how big it is and make it as small as we want (with certain limits of course but probably good enough for a few digits reading) The thing about a good measurement is always know how much error we are making, so we are not fooling ourselves, the error will still be there but as I mentioned, we will know how much it will be, even if we can't cancel it mathematically, we can know a range where our measurement is valid, the difference with this method is that we are measuring within about 2% with the example I quoted, rather than measuring 1/10th of the actual value (1000% error)

dfuruta, if you came out with this method by yourself you could be pretty proud, I've seen people who have seen this schematic scratching the head for quite some time before getting it, most indirect measuring methods are quite counterintuitive, and being able of using an instrument to get a measurement better than it's own error is quite good, since your zero detector could have 10% error and still be good enough to get 0.1% reading at the end of the day. Of course the other instrument must be better than 0.1% but it doesn't need to be high impedance at all, as long as you can feed it from your source, it may be getting few mA and being quite a load for the pot, but still being good enough, since you know, as your zero detector is saying, that the voltage at the pot is the same (within the margin) than at the DUT.

JS
 
Joaquins nailed it!

This is, of course, a very old circuit - I work in a surplus shop, and I've sold a number of potentiometers and differential voltmeters.  I was just surprised that I couldn't find any posts mentioning this as the cheapest & easiest way to measure polarization for mics.  Much cheaper than an electrometer!

I suggested using a DMM in voltage mode as the null detector since most people won't have a nanoammeter, and normal 20uA/50uA movements won't move enough to get a good null.  A DMM in voltage mode is close enough to a precise but inaccurate nanoammeter for our purposes (and since it's just a null detector, accuracy doesn't matter).  To be clear, I'm not suggesting one should use the null meter to actually measure voltage - in my schematic it's an ammeter, but with the DMM set to "voltage" for a finer reading.

For greater accuracy, the pot could be a lower value - the value in my drawing was just what I had at the moment.  More filtering on the power supply is certainly advisable!
 
The pot doesn't need to be lower value, it wouldn't improve anything but power dissipation and increased ripple, maybe lower thermal noise but shouldn't be a problem measuring few tens of volts. Since you are measuring it already loaded if using two DMM at the same time.

Other way would be using a fast DMM with a max/peak hold and a low leakage cap, connecting the cap to the circuit and waiting it enough for charging, then measuring with the multimeter and you will get a lower value than expected, for the leakage of the cap and the discharging by the time the DMM gets the measurement, it's way more crude and inaccurate but also needs almost anything, modern low definition DMM are quite fast, and having a good poly cap by hand, probably a nF or so, would give a much close reading than just attaching the DMM to the DUT, at least good when in the road or to get a first approach. Is not as smart as the opposition method nor precise, but could be useful for someone who doesn't have a low noise variable PS with high enough voltage or fine enough definition.

JS
 
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