CADDOCK 1776-C48 odd resistor dividor?

[Felicitus]

I have watched Dave's Review of the Agilent U1253A OLED Multimeter (EEVblog #56), and I had a look at the resistor divider they used (CADDOCK 1776-C48). When I calculate the division factors, I get a result of like 9.000090001, which sounds rather odd. Can anybody shed some light on what's going on and why Agilent decided not to use the traditional 9M/900K/90K/9K/1K divider?

[Rerouter]

it may be to cancel out the resistance to ground of the adc sample and hold circuit,

[Jay_Diddy_B]

Hi,

Those resistors values can be used to make divide by 10, 100, 1000 and 10000 with switches to ground:



The conventional divider requires floating switches.


So the Caddock 1776-C48 is probably easier to use in auto-ranging DMMs.

Jay_Diddy_B

[ludzinc]

Hey Felicitus

Thanks for asking this question - I've been also trying a DIY auto-ranging circuit based on the traditional divider tree and also was stumped with the whole 'not blowing up your switch' issue.

Suggestions about using floating FETs are fine, but I couldn't work out how to drive the FET.  This approach is awesome in it's simplicity!

[Felicitus]

So the Caddock 1776-C48 is probably easier to use in auto-ranging DMMs.

You are my personal hero of the day! Not only have you found the answer, but also answered my other question as well! I just simulated this with a diode clamping circuit, and it works very nicely! Thank you so much, I was banging my head against the wall for like a week or so!

cheers
Feli

[capnregex]

I know this thread might be a little old, but now I'm wondering what to use to switch the divider with?

[Zero999]

I know this thread might be a little old, but now I'm wondering what to use to switch the divider with?

A MOSFET with low leakage current and high enough voltage rating to handle the open circuit voltage, would be ideal.

[Rerouter]

In this case it only has to withstand the clamping voltage, allowing you to use some extremely low leakage ones,

[David Hess]

So the input resistance varies depending on the range selected?

I thought it was bad enough when multimeters switch to high input resistance on low voltage ranges without notification.  I assume for the intended use this annoying property is irrelevant.

[leonerd]

I know this thread might be a little old, but now I'm wondering what to use to switch the divider with?

I am wondering this too. In fact I started a new thread on the subject; https://www.eevblog.com/forum/projects/analog-switches-for-volt-meter-range-switching/msg1261795/#msg1261795