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AN8008 US $19, 9999count, 1uV, 0.01uA, 0.01Ohm, 1pF resolution meter

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ocw:

--- Quote ---Or instead of going through all that, just feed it close to the maximum current of the range and measure the voltage across the terminals.
--- End quote ---

I don't understand why people like to refer to the burden voltage.  There are an infinite number of actual burden voltages which obviously depend on the actual current being measured rather than the maximum current of the range.  There are typically two or three shunt resistances.  That's why I have their values listed on the meter.  With those two or three figures you can calculate the actual burden voltage.  For the lower currents where one of the resistors is used, it probably has a relatively insignificant burden voltage.  However, at the high current limit of that one resistor the readings may have an accuracy way out of its rated specification.  Some lower price meters have protection diodes across those resistors which start to conduct even when measuring a current within its ratings.  I find that in my tests.

I already own two of the Keithley electrometers for low current measurement with insignificant shunt resistance/burden voltage.  I've made my own version of Dave's uCurrent using an ADA4530-1 (which I don't believe was available when Dave designed his circuit).  My circuit can measure femtoamps with an accuracy exceeding my ability to guarantee its accuracy.  While a commercial equivalent would need greater protection than my design, it should not add that much to the price of the mid to upper level multi-meters.

janekm:

--- Quote from: ocw on July 17, 2017, 01:30:40 am ---I've made my own version of Dave's uCurrent using an ADA4530-1 (which I don't believe was available when Dave designed his circuit).  My circuit can measure femtoamps with an accuracy exceeding my ability to guarantee its accuracy.  While a commercial equivalent would need greater protection than my design, it should not add that much to the price of the mid to upper level multi-meters.

--- End quote ---
Even if it had been available, that one chip seems to cost several times the uCurrent BOM ;)

ocw:

--- Quote ---Even if it had been available, that one chip seems to cost several times the uCurrent
--- End quote ---

There are currently 67 ADA4530-1's available for $23.49 each from Mouser as compared to $69.66 USD for a uCurrent.  That was Google's translation for AU$89.00

alm:

--- Quote from: ocw on July 17, 2017, 01:30:40 am ---I don't understand why people like to refer to the burden voltage.  There are an infinite number of actual burden voltages which obviously depend on the actual current being measured rather than the maximum current of the range.

--- End quote ---
Well yes, the burden voltage is a maximum per range. A worst case that you can take into account as you design your setup. It will likely be very close to 0 V at 0 A ;). You can quantify it for intermediate values if you are so inclined, but no need to confuse it with accuracy.


--- Quote from: ocw on July 17, 2017, 01:30:40 am ---There are typically two or three shunt resistances.

--- End quote ---
Which is why there may be a burden voltage spec per current range.


--- Quote from: ocw on July 17, 2017, 01:30:40 am ---However, at the high current limit of that one resistor the readings may have an accuracy way out of its rated specification.

--- End quote ---
Is that rated specification for a current source with such a low output impedance? Are you also measuring voltage with a 1 MOhm series resistor? Then your electrometer would definitely be much more accurate :P.


--- Quote from: ocw on July 17, 2017, 01:30:40 am ---Some lower price meters have protection diodes across those resistors which start to conduct even when measuring a current within its ratings.  I find that in my tests.

--- End quote ---
The effect at full scale currents should be included in the burden voltage specs.


--- Quote from: ocw on July 17, 2017, 01:30:40 am ---While a commercial equivalent would need greater protection than my design, it should not add that much to the price of the mid to upper level multi-meters.

--- End quote ---
The fuses will still represent a significant resistance, so I am not so sure how much you would gain by reducing the shunt resistance. This is also why the µCurrent does not have a fuse (and is not marked CAT III ;)).


--- Quote from: ocw on July 17, 2017, 01:49:37 am ---
--- Quote ---Even if it had been available, that one chip seems to cost several times the uCurrent
--- End quote ---

There are currently 67 ADA4530-1's available for $23.49 each from Mouser as compared to $69.66 USD for a uCurrent.  That was Google's translation for AU$89.00

--- End quote ---
You left off the word BOM in the quote. If the µCurrent sold for US$ 70, then I would expect US$ 23 to be close to the BOM costs.

Mark Hennessy:
I express burden voltage as volts-per-amp, or millivolts-per-milliamp, or whatever.

However, it can only be approximate because - as I've mentioned at least once already - the fuse is non-linear.

Take a typical 20mm 500mAT fuse - the sort found in cheap multimeters for the mA range - and measure the voltage across it for a range of currents. Last time I did this, the value of resistance at ~500mA was practically double what it was at ~100mA. I've no idea if "proper" HRC fuses are better or worse; at ~£15 a go, that's a lot to risk if something goes wrong. Someone else can run that experiment ;)

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