choosing instrumentation amplifier for milliohm meter build - Page 1
Forum section: electronics / beginners

author: nour
author_rank: Frequent Contributor
timestamp: November 29, 2016, 01:00:50 pm
content: Hi,I want to build a milliohm meter like this over hereThe one that he is using is INA106.It is expensive and also not available for me now. and I believe it is a DIFFERENTIAL AMPLIFIER not an instrumentation amplifier (which I am not sure is it better to use in that design or not!)He mentioned somewhere in the comments that ina131 is replacement put also it is a little bit expensive for my applicationI want to choose another one that can do the job and within a reasonable priceafter some search, I have chosen INA333I don't know if this is a good choose or notCan someone help with this!!What is the important parameter that I should look forIs it the low offset voltage or low noise or low drift or is it low input BIAS current ??I really don't know!!

author: nour
author_rank: Frequent Contributor
timestamp: November 29, 2016, 09:50:49 pm
content: Is this the wrong section in the forum for this topic ??

author: not1xor1
author_rank: Frequent Contributor
timestamp: November 30, 2016, 05:03:27 am
content: Quote from: nour on November 29, 2016, 01:00:50 pmINA333 is suitable for a milliohm meter, but not as an INA106 replacement in the original circuit.INA333 is low power with a maximum of 7V single supply voltage, so you would need a lower supply voltage (e.g. 4 AA cells) and no DC-DC split-supply converter.Besides that, the negative input voltage can no longer get to GND (you no longer have a negative supply voltage).You might use a single opamp as voltage follower to get a low impedance virtual ground and a precision resistor (i.e.low ppm/K, may be 0.1%) to set the INA gain (I would go for 100 with 10mA of test current and just use 4 1.5 volt alkaline or Ni-MH batteries or just one Li-ion cell).You might check if there is a more suitable schematic athttp://kripton2035.free.fr/continuity-repos.html

author: nour
author_rank: Frequent Contributor
timestamp: November 30, 2016, 08:13:36 am
content: Quote from: not1xor1 on November 30, 2016, 05:03:27 amThis particular instrument is rail-to-rail amplifier, I think I can make it work with +-2.5v and overcome this issueAlso, I want to know in this particular application which parameter is important, is it Input offset voltage or bias current ??because I have found other inst. amp. that has a very low bias current less than 10pA put their offset voltage can reach as high as 200-400 uV and with-in the acceptable price range (around 5-6$)Should I go for those very low bias current opamps ??

author: Kleinstein
author_rank: Super Contributor
timestamp: November 30, 2016, 05:34:03 pm
content: For a low ohm meter bias current is not a problem at all. So it is offset voltage, offset drift and maybe low frequency noise (e.g. 0.1-1 Hz range) that matters. So something like the INA333 or AD620 are good. Depending on the chip a lower voltage is needed - helps to keep power low.For a low cost solution you could even get away with just an OP, as there will not be much common mode voltage. Especially if the following ADC has differential input, one could use just a simple amplifier (one OP) and the differential input of the ADC.

author: not1xor1
author_rank: Frequent Contributor
timestamp: December 01, 2016, 08:18:07 am
content: Quote from: nour on November 30, 2016, 08:13:36 amas stated in the 1st page of the datasheet INA333 is rail-to-rail just on its output (and within 50mV of both rails).INA333 inputs cannot get closer to 100mV to both rails.According to datasheet, both offset error and input bias current are very low and are not likely to cause any problem. Low freq. noise is also low and probably would not affect the LSD of a 5/6digit DVM (no digit flickering).I would simplify the schematic and use just a current source, the INA and 3 1.5V cells:(I've not yet understood how to embed images)