instrumentation Amplifier : single supply vs dual supply - Page 1
Forum section: electronics / beginners

author: alireza7
author_rank: Regular Contributor
timestamp: February 11, 2018, 05:49:48 pm
content: hii want to use AD8220 which is an JFET Input Instrumentation Amplifier .in its datasheet it say that :4.5 V to 36 V single supply±2.25 V to ±18 V dual supplyin my application i have a signal from my primary sensor in the range of 100 uv up to 5mv and i want to make it 1000x amplify then read it with an ADC.its an Instrumentation amplifier and i want to use it in an measuring instrument and it's essential for my circuit to be low noise and low offset and low drift and high CMRR and .....at the first stage power supply of Instrumentation Amplifier is my concern.is it better to choose a single supply configuration or dual supply (especially with noise considerations )?if you suggest me to choose a dual supply configuration since my circuit have just a 12v supply , how should i make the negative voltage ? can you suggest me any ic? As far as i know i should use a dc to dc converter for this application but i think they are switching and produce too much noise .in the case you suggest me single supply how should i make reference voltage or virtual ground?i hear that making a low noise reference voltage is still a problem.

author: jaycee
author_rank: Regular Contributor
timestamp: February 12, 2018, 01:16:31 am
content: The important criteria here is that you have a very low bottom range requirement of 100uV. Does the opamp handle inputs that close to 0 if it is used as a single rail device ?According to the data sheet, it does...In that case, a single rail is fine.

author: ogden
author_rank: Super Contributor
timestamp: February 12, 2018, 03:02:47 am
content: If your sensor is not "tied" to GND - indeed use single supply. Delta-sigma ADC with differential input is preferable in your case. You shall check thermocouple ADC circuits. Example:http://www.analog.com/en/design-center/reference-designs/hardware-reference-design/circuits-from-the-lab/cn0271.html#rd-overviewhttp://ww1.microchip.com/downloads/en/AppNotes/00842a.pdf

author: alireza7
author_rank: Regular Contributor
timestamp: February 15, 2018, 08:12:52 pm
content: actually my sensor have two prob and each of them should be connected to one differential input of my instrumentation amplifier .as ogden said i should not tie my sensor to GND in single supply configuration.should i tie one of my probs to a specific voltage like 0.5v 1.5v vcc/2 or my virtual ground?

author: danadak
author_rank: Super Contributor
timestamp: February 15, 2018, 08:46:33 pm
content: The 8220 will handle single supply as long as youmeet the common mode allowed input voltage rangefor the single supply use. There are graphs coveringthis in datasheet, and specs.Page 13, 14....http://www.analog.com/media/en/technical-documentation/data-sheets/AD8220.pdfAs an aside PSOC 5LP UP can handle this with its 20 bit differential A/D and inputrail to rail capability. Again as long as common mode put out by the sensormeets its allowed range. Plus you get a digital filter should you need to postprocess the sensor signal path. This shows single ended example, differentialmode a trivial setting. This is all onchip + reference + arm core + a whole lotmore. In fact the PSOC A/D will common mode outside its supply rails by 100mV, not much but solves many application problems, like high side sense forexample.Regards, Dana.

author: alireza7
author_rank: Regular Contributor
timestamp: March 29, 2018, 10:05:17 pm
content: Quote from: ogden on February 12, 2018, 03:02:47 ambut there is a picture in ad8220 data sheet which have contradiction with your statement "If your sensor is not "tied" to GND ..."Figure 63 in page 22 , one terminal of the sensor tied to ground and an other terminal is float.in the case that our sensor have two probes and we want to amplify and read induced voltage between probes ,shouldn't we tie one of the probes to the ground in the case of single supply?i'm not sure but it seems to me it's better to do this in both single supply or dual supply casedoes doing this help with common mode reduction ? or it has any negative effects that i don't consider ?

author: ogden
author_rank: Super Contributor
timestamp: March 29, 2018, 10:18:57 pm
content: Quote from: alireza7 on March 29, 2018, 10:05:17 pmRight. Sorry, I missed to study AD8820 datasheet, regarding it's REF pin.It appears that you can use single supply as well.QuoteNot always input voltage range of amplifier includes negative supply rail voltage.

author: alireza7
author_rank: Regular Contributor
timestamp: March 29, 2018, 10:26:59 pm
content: Quote from: ogden on March 29, 2018, 10:18:57 pmyes that's right it's not true in general but i mean in the ad8220 case or any other amplifier which includes negative rail .