EEVblog 1667 - Reverse Engineering the Brymen BM2257 Multimeter

[EEVblog]

Reverse Engineering the Brymen BM2257 Multimeter front end to understand what changes have been made from the BM235 in the Low Z mode. DaveCAD time. Also some testing of the LowZ mode.

[SiliconWizard]

What a bobby-dazzler!

[Kleinstein]

Nice video. I think the ohms mode is reading back the actual voltage not via the 10 M resistor, but though a path (still a few parts missing) from the very bottom Q1+Q8 transitor clamp. They need the 2 PTC paths separate: one to drive a current and the other to read back the voltage. One can not measure resistance with just 1 PTC in series.

[EEVblog]

Nice video. I think the ohms mode is reading back the actual voltage not via the 10 M resistor, but though a path (still a few parts missing) from the very bottom Q1+Q8 transitor clamp. They need the 2 PTC paths separate: one to drive a current and the other to read back the voltage. One can not measure resistance with just 1 PTC in series.

Quite possible.

[SteveLTN]

Ha I always thought the 10M long resistor was the hybrid divider network. I guess such a resistor network isn't required in a 0.2% meter? Could anyone point to me which ones are the alternative divider resistors? I assume they are standalone ones?

[EEVblog]

Ha I always thought the 10M long resistor was the hybrid divider network. I guess such a resistor network isn't required in a 0.2% meter? Could anyone point to me which ones are the alternative divider resistors? I assume they are standalone ones?

Yes, just normal (high spec) SMD resistors for the bottom dividers. Hardly any 6000 count meters will use a ceramic hybrid resistor divider. It's not woth the extra expense at that price point and resolution.

[thm_w]

Ha I always thought the 10M long resistor was the hybrid divider network. I guess such a resistor network isn't required in a 0.2% meter? Could anyone point to me which ones are the alternative divider resistors? I assume they are standalone ones?

This confused me a bit too, dave had the 10M going into the ADC on the diagram for simplicity, looks like its usually switched to various discrete resistors: https://lygte-info.dk/info/DMMDesign%20UK.html

[EEVblog]

Ha I always thought the 10M long resistor was the hybrid divider network. I guess such a resistor network isn't required in a 0.2% meter? Could anyone point to me which ones are the alternative divider resistors? I assume they are standalone ones?

This confused me a bit too, dave had the 10M going into the ADC on the diagram for simplicity, looks like its usually switched to various discrete resistors: https://lygte-info.dk/info/DMMDesign%20UK.html

Yes, as I said, it's simplified.
This is why input impedances are often 11Mohms vs 10Mohms
On a higher spec meter all the divider resistors are on the same ceramic hybrid base so they have the exact same tempco.

[Kleinstein]

....
This confused me a bit too, dave had the 10M going into the ADC on the diagram for simplicity, looks like its usually switched to various discrete resistors: https://lygte-info.dk/info/DMMDesign%20UK.html

There is another possible input configuration inside the DMM chip sets. This is an inverting amplifier with switching the resistor in the feedback. This curcuit is also called active divider. This way the input resistance would be constant at 10 M.
The active divider is quite common for the AC input in higher grade meters.