Fluke 752A 10:1 Input Resistance

[LKM]

Hi All,

    Does anyone know why the input resistance of the 10:1 divider is 380kOhm and not the expected 400k?

    In the 100:1 divider the input resistance is 2M instead of the expected 4M because there is a 4M divider chain in parallel to guard each of the 30R strings which are in metal cans.

    I'm assuming a similar situation is present in the 360k divider string of the 10:1 but so far I'm not seeing it described in the manual or schematic. There is mention of a passive guard but no resistor divider for the 10:1 section.

    I calculate 6.12Mohm in parallel with the 360k to give 340k but I'm not sure from where.

Thanks!

[dacman]

Because the 100:1 divider resistors are still in the circuit.  Above the 10:1 divider is 3.6 MOhm from the 100:1 divider which is now in series with the 4 MOhm of guard resistors.  That is 7.6 MOhm in parallel with 400 kOhm, which is 380 kOhm.  This value is in a troubleshooting chart in the back of the manual as the correct value.

[seoma]

Hello dacman
I think your studies on electronics are very high
Thank you for answering the question
Give more guidance
Thanks

[LKM]

Found these notes cleaning my desk, figured I'd post for posterity. dacman's info checks out