Products > Test Equipment
AN8008 US $19, 9999count, 1uV, 0.01uA, 0.01Ohm, 1pF resolution meter
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floobydust:

--- Quote from: electronic_eel on July 29, 2017, 09:31:55 pm ---
--- Quote from: floobydust on July 22, 2017, 08:14:49 pm ---Temperature - I enabled in EEPROM on the mV function, works °C/°F and factory CJC CAL is already set at 25°C fixed.

--- End quote ---
Cold Junction Temperature fixed to 25°C?

Does that mean there is no thermistor or other means in the ic to measure the temperature inside the multimeter to have a somewhat proper cold junction temp? Or is it just that you calibrate the proper cjc at 25°C?

--- End quote ---

The DMM IC has an internal temperature sensor (diode?), but needs to be calibrated.
The EEPROM comes with default values and I'm not sure how accurate they are:
0BH, 0CH: 00FAH: Ambient temperature (25.0°C)
0DH~0FH: 03BE00H: Ambient temperature Default ADC value

A one-point CAL is supported (offset?), you basically tell the meter what the ambient temperature is (25C).
Kalvin:

--- Quote from: evava on July 29, 2017, 06:14:18 pm ---AN8008 problem -  bias current on mV range is too big:

When meter in mV range, connected 10.1MOhm resistor causes 0.101mV reading - non of my other meters does that to such an extent - it means input bias current is about 10pA!
Smaller resistors cause proportionally smaller value on display and vice versa.
Can be used for measuring resistors up to 9999MOhms on 99.99mV range, thats nice ;D 

Can someone check, whether it is just my meter this bad/nice?

--- End quote ---

I tried with my two AN8008 meters with 10 Mohm and 1 Mohm resistors in mVDC range. The meters were too sensitive to how I held the probes in order to make any definitive conclusions.

However, when I first short-circuit the probes (display shows 0uV) and then let the probes hang in the air freely, the display shows increasing negative offset voltage. With 10 Mohm resistor I can get fluctuating readings around -125 uV ... -150 uV and with 1 Mohm around -10 uV ... -18 uV. I repeated the tests with my other AN8008 and obtained about similar results: Open leads increasing negative offset voltage, 10 Mohm resistor giving around -130 uV, 1 Mohm resistors giving around -10uV ... -15 uV.

So, I can repeat your findings. Whether or not this is an issue - it depends on what you are measuring. If you are measuring a circuit with an impedance of 100 kohm or less, this will not be an issue. However, if you are measuring very high impedance circuits (with impedance > 100 kohm) and want to get accurate uV measurements this may be an issue.
amspire:

--- Quote from: floobydust on July 29, 2017, 10:00:22 pm ---
--- Quote from: evava on July 29, 2017, 06:14:18 pm ---AN8008 problem -  bias current on mV range is too big:

When meter in mV range, connected 10.1MOhm resistor causes 0.101mV reading - non of my other meters does that to such an extent - it means input bias current is about 10pA!
Smaller resistors cause proportionally smaller value on display and vice versa.
Can be used for measuring resistors up to 9999MOhms on 99.99mV range, thats nice ;D 

Can someone check, whether it is just my meter this bad/nice?

--- End quote ---

I also measure 0.100mV across a 10MEG resistor, for 10pA or 0.01nA leakage current. Second try it hones in on -0.135mV. Using no leads and Faraday shield.
Johnson-Nyquist noise of a 10MEG resistor is significant here. Try add a disc cap in parallel ;)

--- End quote ---
There can also be the issue with a big resistor that the AC pickup is saturating the DC amplifier causing an apparent voltage. A better way to test for offset current is probably to use a capacitor such as a 10nF capacitor and measure the time constant. At 10pA, you should get -1mV/second voltage.

I tested my Richmeter 101 (Aneng AN8001) and got -18mV over 60 seconds with a 360pF capacitor. That corresponds to 360pF x -0.018/60 = -0.1pA offset current on the mV range.  I put the capacitor directly in the multimeter socket to eliminate any additional effect of the leads and to minimize AC pickup.

Edit: looks like there may be a capacitance inside the meter and 360pF is not enough. Definitely need a much bigger capacitor. Repeated the test with a good quality 2.68uF capacitor. The meter shows 0000 until the count gets to 0005 - I think that quirk has been mentioned before. After 60 seconds, the voltage was -0.18mV. That is a current of 2.68uF x 0.00018 / 60 = 8pA.
amspire:
Repeated my test with an AN8001 type meter over 5 minutes and the result was -5.7pA. Looks like the mV range input impedence is pretty high - well over 1G ohm. Probably hard to measure with the offset current present.
floobydust:
You have to be careful measuring uV at high-Z (10MEG)
Many effects you would not normally notice come to surface, such as contact potential and thermal EMF from dissimilar metals, AC hum and RFI.
You need a shielded, grounded work area. You need symmetric metallurgy on the leads and plugs etc. otherwise you get a net thermocouple.
The Metrology boyz surely have more guidance  :-/O

Connect your scope (10:1 10MEG) to the multimeter with both on lowest mV scale.
Notice you can see on the scope the DMM analog input circuit pulses, I about 1mVpp at 3.3kHz
The DMM IC has CAZ chopper before the input buffer op-amps, so I guess this is making some noise that shows up at 10MEG ohm. Can't see it at 1MEG though.
Adding 820-1,000pF eliminates most of it; with 10nF it is all gone. The readings shift maybe 10uV adding the cap.

So some of the displayed uV offset is resistor noise, DMM IC leakage current and CAZ noise, with high source resistance.
It is not entirely picoamps of leakage.
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