CADDOCK 1776-C48 odd resistor dividor? - Page 1
Forum section: electronics / projects

author: Felicitus
author_rank: Regular Contributor
timestamp: February 12, 2014, 10:08:22 pm
content: 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?

author: Rerouter
author_rank: Super Contributor
timestamp: February 13, 2014, 02:00:26 am
content: it may be to cancel out the resistance to ground of the adc sample and hold circuit,

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 13, 2014, 03:09:54 am
content: 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

author: ludzinc
author_rank: Unknown
timestamp: February 13, 2014, 03:59:06 am
content: Hey FelicitusThanks 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!

author: Felicitus
author_rank: Regular Contributor
timestamp: February 13, 2014, 04:02:26 am
content: Quote from: Jay_Diddy_B on February 13, 2014, 03:09:54 amYou are my personal hero of the day! Not only have you found the answer, but also answered myother questionas 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!cheersFeli

author: capnregex
author_rank: Newbie
timestamp: April 14, 2014, 02:58:42 pm
content: I know this thread might be a little old, but now I'm wondering what to use to switch the divider with?

author: Zero999
author_rank: Super Contributor
timestamp: April 14, 2014, 10:18:49 pm
content: Quote from: capnregex on April 14, 2014, 02:58:42 pmA MOSFET with low leakage current and high enough voltage rating to handle the open circuit voltage, would be ideal.

author: Rerouter
author_rank: Super Contributor
timestamp: April 15, 2014, 02:56:18 am
content: In this case it only has to withstand the clamping voltage, allowing you to use some extremely low leakage ones,

author: David Hess
author_rank: Super Contributor
timestamp: April 19, 2014, 10:56:48 pm
content: 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.

author: leonerd
author_rank: Regular Contributor
timestamp: July 21, 2017, 11:48:07 am
content: Quote from: capnregex on April 14, 2014, 02:58:42 pmI 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