multimeter autoranging circuit - Page 1
Forum section: electronics / projects

author: House91320
author_rank: Regular Contributor
timestamp: June 12, 2011, 05:57:59 pm
content: what type of silicon dose a autoranging circuit use.

author: Simon
author_rank: Unknown
timestamp: June 12, 2011, 06:00:19 pm
content: same as any other IC !

author: House91320
author_rank: Regular Contributor
timestamp: June 12, 2011, 07:55:35 pm
content: scuse me i didnt mean they material itself i ment the device like a mosfet or a bjt.

author: Simon
author_rank: Unknown
timestamp: June 12, 2011, 08:21:08 pm
content: well I'm sure there are solutions in either and possibly hybrid solutions. Why ?

author: Zero999
author_rank: Super Contributor
timestamp: June 12, 2011, 08:34:51 pm
content: Why not just Google it?http://www.google.co.uk/search?um=1&hl=en&client=opera&hs=OGs&rls=en&channel=suggest&tbm=isch&sa=X&ei=jyL1Tab8H4uf-QbPzrnOBg&ved=0CDMQvwUoAQ&q=multimeter+autoranging+schematic&spell=1&biw=1920&bih=919http://schematicwiring.com/simple-electronic-applications/autoranging-circuit-schematic-diagram/http://www.hobby-hour.com/electronics/my68.php

author: Wartex
author_rank: Frequent Contributor
timestamp: June 12, 2011, 09:12:25 pm
content: Hero999, get out with your common sense and logic!

author: House91320
author_rank: Regular Contributor
timestamp: June 12, 2011, 10:19:45 pm
content: could it be done with some very low resistance mosfet

author: ejeffrey
author_rank: Super Contributor
timestamp: June 13, 2011, 09:35:15 am
content: You can do it with a MOSFET, but generally you would use an analog switch IC or transmission gate. The problem with a single MOSFET is that the resistance varies with the signal voltage, eventually cutting off when the signal gets to within a few volts of the gate voltage. Thus, a single MOSFET makes a great current switch (say, feeding into an op-amp virtual ground) where the voltage is nearly constant, but not a very good voltage switch unless you signal is confined to a rather limited range.A transmission gate solves this problem by having both a P-channel and an N-channel MOSFET in parallel. For voltages close to the negative rail, conduction is via the N-channel device, and for voltages close to the positive rail conduction is via the P-channel device. At mid-rail, both devices conduct.IC analog switches typically have a higher on resistance than discrete MOSFETs, from a few ohms to a few hundred ohms. The on-resistance also still varies somewhat with the signal level. For this reason it is usually preferred to use them in a configuration that doesn't conduct much current. A classic example is a non-inverting op-amp. The obvious solution is to switch the gain by switching resistors in and out of the feedback path. The clever solution is to use a resistor ladder from the output to ground and use analog switches (often available as 4x1 or 8x1 multiplexers) to connect one of the nodes to the inverting input. In that way, gain does not depend on the on resistance.

author: Zero999
author_rank: Super Contributor
timestamp: June 13, 2011, 08:21:09 pm
content: Quote from: House91320 on June 12, 2011, 10:19:45 pmWhy not read some of the sites I linked to? Then if they're bits you don't understand, do more Googling, then if you still have questions post them here.

author: Zad
author_rank: Super Contributor
timestamp: June 13, 2011, 10:49:43 pm
content: For my LCR meter design, I am using a couple of Analog Devices 16-channel ADG706 multiplexers. "On" resistance is 2.5 Ohms, with a 0.5 Ohm flatness.

author: Neilm
author_rank: Super Contributor
timestamp: June 14, 2011, 08:01:40 pm
content: A good multimeter will also have protection on the inputs in case the user sets the instrument to ohms and connects it to a live circuit.Neil