General > General Technical Chat
How do you calibrate your gear.
<< < (4/4)
chinoy:

--- Quote from: rdl on November 04, 2023, 09:11:11 pm ---You might not want to spend the money but there are cheap voltage references available on eBay and probably on AliExpress.

--- End quote ---
Thank you that is the best lead I got so far.
I will sit down and list the values I am getting From each instrument. The next time I build one of these circuits. For now I have just used the newest Fluke I have.
JohanH:
When measuring small signals, particularly if they are high impedance circuits, the instrument you use will load the circuit, which affects readings. How much depends on the input impedance of the instrument, but also of your circuit. An oscilloscope has 1 MΩ to 10 MΩ of input impedance. That's like loading your circuit with a resistor of the same value. A modern cheap voltmeter typically also have around 1 MΩ of input impedance, whereas more professional ones have impedance measured in gigaohm (GΩ). Check the specifications!

As an example, I've built my own voltage references and I get different readings with the 34401A meter depending on if measured with its default 10 MΩ impedance mode and when changing to 1 GΩ (it sucks that the gigaohm mode can't be turned permanently on with this device). It's AC input is 1 MΩ in parallel with 100 pF.
EPAIII:
If you are going to compare the readings on two meters, they should both be connected to the test Voltage at the same time.




--- Quote from: bdunham7 on November 04, 2023, 10:53:23 pm ---To sort of answer the OP's question with an example, I set up the following small bench experiment:

I set up an AWG (SDG2042X) to put out 450mVRMS @ 5kHz plus the 15th harmonic @ 900mVp-p and combined that with 50mV of noise with a 20MHz BW.  Oddly the channel combine feature didn't work as I expected, but that's a different issue--it displayed on a scope as you'd expect but an an amplitude different than I get adding them all up. 

I then measured that signal with every DMM within reach and with the scope (SDS2354X-E) as well, using the StdDev function.  Here they are rounded to 100µV for consistency and suppression of irrelevant digits.  No surprises IMO.  On meters with separate mVAC and VAC ranges, I tested both as they did vary on some models.

Siglent SDS2354X-E:  310.5mV

Fluke 8506A, Hi-Accuracy mode: 310.6mV

HP 34401A: 307.6mV

Fluke 8808A: 307.5mV

Fluke 8842A: 307.1mV

Fluke 289:  307.0 (both mVAC and VAC)

Fluke 189:  297.2mV (mVAC), 0.3042V (VAC)

Fluke 116:  ~300mV (mVAC, very unstable), 0.225V (VAC)

Fairchild 7000A (AVG responding): 0.2832V

Fluke 27 (AVG responding): 280.0mV (mVAC), 0.282V (VAC)

and (I almost forgot it) my old friend the Fluke 98 Scopemeter, after some careful setup....310.4mV (multimeter mode, 100µs/div, 300mV range) w/ ~ 1.0mV peak noise.  It also displayed a reasonably stable 0.310V in the 1V range at the same timebase setting.

--- End quote ---
Swake:

--- Quote from: EPAIII on November 12, 2023, 07:32:06 am ---If you are going to compare the readings on two meters, they should both be connected to the test Voltage at the same time.
--- End quote ---
Absolutely not. JohanH just explained in the message previous to yours one of the principal reasons why not.
PlainName:

--- Quote from: Swake on November 20, 2023, 09:00:06 am ---
--- Quote from: EPAIII on November 12, 2023, 07:32:06 am ---If you are going to compare the readings on two meters, they should both be connected to the test Voltage at the same time.
--- End quote ---
Absolutely not. JohanH just explained in the message previous to yours one of the principal reasons why not.

--- End quote ---

Johan explains why the test voltage may differ depending on instrument. EPAIII explains how to circumvent that - the absolute value isn't relevant, only the difference between the two displays.
Navigation
Message Index
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod