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EEVBlog 121GW Discussion thread

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J-R:
The mV relative mode statements in the manual makes no sense to me.  If the zero calibration point is off far enough that you need to REL the meter before taking a measurement, then the final reading will be off as well.  If you REL out noise/interference, same issue.

I think the larger issue here is that you need a way to verify your readings.  Don't blindly trust one DMM.  Using the function generator and the ratio transformer, I can easily make any of my DMMs display garbage values where they really shouldn't.  But frequently one DMM pulls through where another falls flat.  The inability of the 34401A and the 287 to display very low mV AC values surprised me a bit.  But they also both destroy the 121GW and Brymens with DCV accuracy all day long...

Keyview:

--- Quote from: J-R on March 21, 2021, 06:44:53 am ---Don't blindly trust one DMM.

--- End quote ---
This is a key point. If I had (besides budget) more room, I would get a Keysight 34465 bench meter and a good handheld DMM, say Fluke 287/289.
(Btw, the Flukes are here in Europe twice the price of the US market prices.)

I am interested in low voltage AC (down to a few mV) for some audio connected measurements.
Ideally, I would find a handheld DMM with the accuracy of the 34465, but this does not exist.
I would also like to simply trust the product specifications to avoid having several quality devices that can measure (by specification) quite accurately down to 1 mV AC.
Not sure, if it is smart to fully trust the specifications. Maybe this is indeed blind trust.


--- Quote from: J-R on March 21, 2021, 06:44:53 am --- The inability of the 34401A and the 287 to display very low mV AC values surprised me a bit.

--- End quote ---
Actually, the 287 does not meet its specifications with your observation that 0.150 mV 1 kHz is displayed as 0.
The accuracy as stated by Fluke should be 0.4% + 45 digits (0.001 mV) in this case.

dcac:

--- Quote from: J-R on March 21, 2021, 06:44:53 am ---The mV relative mode statements in the manual makes no sense to me.  If the zero calibration point is off far enough that you need to REL the meter before taking a measurement, then the final reading will be off as well.  If you REL out noise/interference, same issue.

--- End quote ---

I believe the reason for using REL in the lowest mV range is to compensate for any temporary offset drift (usually) caused by temperature variations. In DC mV mode the signal passes through PTCs which can exhibit thermocouple behavior - then there’s i.e. also possible drift in the pre-amp and ADC.

In AC mV mode the signal has more or less the same path as DC but is then AC coupled right before the AD8436 RMS converter - so here it’s really only if the converter exhibits any drift you’d want to use the REL to cancel it out.

This description is for the 121GW but many other DMMs has much the same problem.

dcac:

--- Quote from: Keyview on March 21, 2021, 12:36:17 pm ---
--- Quote from: J-R on March 21, 2021, 06:44:53 am ---Don't blindly trust one DMM.

--- End quote ---
This is a key point. If I had (besides budget) more room, I would get a Keysight 34465 bench meter and a good handheld DMM, say Fluke 287/289.
(Btw, the Flukes are here in Europe twice the price of the US market prices.)

I am interested in low voltage AC (down to a few mV) for some audio connected measurements.
Ideally, I would find a handheld DMM with the accuracy of the 34465, but this does not exist.


--- End quote ---

Perhaps a dedicated AC mV meter would be a better option - I don’t really have any specific to suggest though.

When I needed to measure sub mV Audio signals I built an external AC amplifier based on the OP37 opamp. Placed it in a battery powered small shielded box I could attach directly to a handheld DMM. I could set the amplification to 10, 100 or 200X and here the frequency response was perfectly flat 20Hz - 20KHz and a bit beyond. I don’t know if any such device exists ready to buy though.

Keyview:

--- Quote from: dcac on March 21, 2021, 01:48:01 pm ---When I needed to measure sub mV Audio signals I built an external AC amplifier based on the OP37 opamp. [...] I don’t know if any such device exists ready to buy though.

--- End quote ---
I have never soldered before, so I would have to buy it - if it is for sale.
Your approach sounds somewhat similar to the eevblog uCurrent, but I am not sure if it measures uV AC.
Besides, the uCurrent is out of stock everywhere.

Please let me know if anyone has an idea where to get a (reasonably priced) AC amplifier to use with a DMM.

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