Principle of operation datasheet error ?? (differential ADC)  - Page 1
Forum section: electronics / beginners

author: AwArD_RzD
author_rank: Regular Contributor
timestamp: May 08, 2014, 02:24:37 am
content: Hey everyoneI was checking many data sheet today about some differential op amp and adc, and this one intrigue mehttp://www.mouser.com/ds/2/405/slas512b-116272.pdf. It's a ADS8422 from TI. I never used differential stuff for now but i understand that the ADC take the +IN minus the -IN in a common mode and give you the full amplitude, but when you look at the page 16 to 19 all the ADC are not wired correctly, -IN + +IN should cancel the signal right ?I looked for the Data sheet of the op amp used in the examplehttp://www.ti.com/lit/ds/symlink/ths4131.pdfand at the page 16 it's not wired like the Principle of operation of the ADS8422 data sheet. Same op amp, same kind of application so it is 2 way of doing it or i just found a trap for young player who started reading about diff amp application and got a error in a data sheet.I planned to try the single end to differential (page 17 of the ADS8422) so If someone can help me understand what's going on it will be greatly appreciated.Thanks !!

author: ejeffrey
author_rank: Super Contributor
timestamp: May 08, 2014, 02:48:30 am
content: Quote from: AwArD_RzD on May 08, 2014, 02:24:37 amI don't quite understand what you mean, but the figures in the data sheet look correct to me. The measured signal is (Vin+ - Vin-). They use a fully differential amplifier to drive the differential input of the ADC.

author: Teledog
author_rank: Regular Contributor
timestamp: May 08, 2014, 03:03:02 am
content: A little off topic,but this is a datasheet we had in one of our textbooks;http://www.repeater-builder.com/molotora/gontor/25120-bw.pdf

author: AwArD_RzD
author_rank: Regular Contributor
timestamp: May 08, 2014, 12:06:55 pm
content: Quote from: ejeffrey on May 08, 2014, 02:48:30 amI just tried to simulate in Multisim and well any combination give the correct result, i don't know why i was thinking that will cancel each other in the ADC process.So i assume they just flipped the op amp to put the single ended input at the top for convenience of the schematic.

author: AwArD_RzD
author_rank: Regular Contributor
timestamp: May 08, 2014, 08:13:49 pm
content: Quote from: Teledog on May 08, 2014, 03:03:02 amHaha nice one!

author: AwArD_RzD
author_rank: Regular Contributor
timestamp: May 08, 2014, 08:41:03 pm
content: Well i did more experiment in Multisim and there my setup for the page 17 of the AD8422 adcChannel one act like the ADC +IN (Yellow)Channel two act like the ADC -IN (Blue)Channel three is the original signal (Pink)The Red signal is the Math function CH1 - CH2The common mode is at 0VIn the first picture you see that the math function will give you the inverted value from the single ended input. When connected like the application exampleIn the second picture i wired the circuit with the OUT+ => IN+, OUT- => IN- (like any other example) , and this give me the correct reading with the Pink and Red signal at the same amplitude and phase.So why they show all the example in the data sheet with the signal inverted to drive the ADC ?? is there any benefit to drive the ADC like that.