Autorange resistance measurement - Page 1
Forum section: electronics / projects

author: alex.m
author_rank: Contributor
timestamp: January 10, 2014, 03:37:19 pm
content: I'm trying to understand how digital multimeters with autorange measure resistance with sub 1% precision and how the circuit is actually built. After some research I did not find a definitive answer.The measurement is accomplished by injecting a known current and measuring the voltage develop across the unknown resistance.When looking for precision current sources that use a single supply I frequently encounter this circuit (fig1).However with several ranges of measurement the current source must be variable.For example for these ranges with a voltage reference of 2V and an ADC also with a 2V voltage reference the current sourced have these values:200 ohm -> 10mA2 kohm -> 1mA20 kohm -> 100uA200 kohm -> 10 uA2 Mohm -> 1uAIn order to change the value of the current in circuit we can have several precision resistances for Rset and switch between them. (fig2)In multimeters that have a range selector I believe that the contacts in the wheel close the circuit according to the range selected.However in an autorange multimeter the switching must be done digitally by a switch IC or by relays.Relays seems strange but I did saw them in a design in MAX1492 datasheet.Using a IC switch is good for the large ranges however in the 200 ohm a 2 ohm Ron on the switch means a instant 1% error. It is possible to correct this offset?It is also possible to not use the 200 ohm resistor and using a PGA with 10 times gain for measuring the voltage.In Analog Devices page of DMM design in the block diagram they use a Programmable current source with DAC however I could not find any reference design that can generate precise current from 1uA to 10mA.http://www.analog.com/en/digital-multi-meter/topic.htmlI'm certainly failing somewhere in my reasoning but is strange the lack of info in the topic.What stategy for this problem is used on good multimeters?Any help is welcomeBest regards.

author: dannyf
author_rank: Super Contributor
timestamp: January 10, 2014, 04:40:57 pm
content: QuoteTry HP.Generally, it is far easier to drive the dut through a known precision resistor and measure the voltage drops. You need two measurements to calculate the dut's resistance.You can switch in the right resistors to "auto-range".

author: w2aew
author_rank: Super Contributor
timestamp: January 10, 2014, 04:42:06 pm
content: Not sure of exactly how most DMMs do it, but there are a myriad of ways, such as...- measuring actual current and voltage- using programmable current source with DAC and a single reference (easy on an IC)- etc.

author: Dr. Frank
author_rank: Super Contributor
timestamp: January 10, 2014, 05:30:21 pm
content: Download the Service Manual for the HP34401A from the agilent site, for example. Schematics should be included. Very instructive.Yep, it's page 9-10 or p. 157.The circuitry creates constant currents from 0.5µA to 1mA.The HP current sources look differently to yours, they are floating, not fixed to GND.They are also intended for 4 wire Ohm, where current and voltage paths are separated.The switching of the different currents, and also the switching of the voltage channel are done by FET transistors. The resistance of those do not influence the measurement.Only the very high isolation in off-state is important.So, FETs can easily and precisely switch small currents and also bigger currents.10mA are not needed, as 200 Ohm * 1mA = 200mV, which can precisely be measured.Also, the voltage burden on the resistor is kept small, to avoid heating.FET switches can easily be steered by logic circuitry, and decouple galvanically the analogue from the digital steering circuit.That's the whole secret.Frank