LTC2400 - Page 1
Forum section: electronics / projects

author: pallav.aggarwal@gmail.com
author_rank: Contributor
timestamp: February 07, 2016, 06:30:20 pm
content: Hello All,I need help using LTC2400:I am using LTC2400 Evaluation board and using ST micro to interface it over SPI.I even used on board Voltage reference Lt1236(5V) to power LTC2400 for better accuracy as suggested in the demo board documentation.When I am using LTC2400 board directly without any voltage divider, accuracy is very good, reading matching till 4-5 digits.But when I use voltage divider (25Kand 1K) so that I can get 1/26th Vin so that I can read voltage up to 65Volts, I see accuracy is affected and now its 0.1% - 0.5% accurate and if I go below 3V (VIN before voltage divider) accuracy is 1.5%. I even tried putting 0.1uF and 0.01uF caps close to ADC input so that if noise is affecting might improve but there is no noticeable improvement.Could you please help me where I might be going wrong?Thank you.Best Regards,Pallav Aggarwal

author: awallin
author_rank: Frequent Contributor
timestamp: February 07, 2016, 06:57:06 pm
content: It says here the input impedance is 1.66 MOhm:http://cds.linear.com/docs/en/application-note/an80.pdfcould you post your schematic and a plot of your measurements vs. what you expect with your divider + the input impedance?

author: Marco
author_rank: Super Contributor
timestamp: February 07, 2016, 07:02:27 pm
content: What are you comparing the LTC2400 against?

author: pallav.aggarwal@gmail.com
author_rank: Contributor
timestamp: February 07, 2016, 07:18:59 pm
content: 1st of all thanks for your reply and taking time to help.I used LTC2400 demo board as here (http://www.linear.com/solutions/2827)Please find attached xls sheet with readings & hand drawn schematic/block diagram for better understanding the setup.I am using Fluke 179 (0.09% and calibrated), even if my meter is not good enough but i could not digest the non linearity.Thanks again.

author: Marco
author_rank: Super Contributor
timestamp: February 07, 2016, 07:52:37 pm
content: You are putting around 0.15W into the resistor divider at the high end BTW.I'd check how well the LTC2400 matches with the Fluke if you put the Fluke behind the divider as well.

author: chris_11
author_rank: Contributor
timestamp: February 07, 2016, 08:18:22 pm
content: Input bias current of the LTC2400 is code/voltage dependent. Use a LTC2492 or similar or use a low impedance buffer.BRChristian

author: Cerebus
author_rank: Super Contributor
timestamp: February 07, 2016, 08:18:30 pm
content: I'd advise going back and *carefully* reading the LTC2400 datasheet again. There is a quite lengthy section about how source impedance and capacitance affect the accuracy of the result. I strongly suspect you'll find the answer there.The LTC2400 is capable of some amazing good results but only if you pay attention to its proper care and feeding. Things to watch out for are:Source impedance and capacitance as mentioned, and discussed at length in the datasheet.Grounds, as in all precision analogue work. Star grounding is amustwhen you're looking at sub part per million accuracy.Noise from other SPI devices. It's best to use the LTC2400 on its own SPI bus. I've had problems when it shared a bus with with an SPI LCD that went away when it wasn't sharing SCK and MISO with another device.Noise injected into your measurement source by the LTC2400. I haven't quantified the noise but to give you an idea it managed to make a right mess when I tried to use it directly (i.e. unbuffered) to measure the compliance voltage of a 1 mA current source.Proper decoupling, as if it needs mentioning.Ian

author: pallav.aggarwal@gmail.com
author_rank: Contributor
timestamp: February 08, 2016, 07:39:31 am
content: Thank you all for your replies and suggestions.@MacroSure, I am aware of 0.15w, as this was setup for quick test i used it with 1/4W resistors, in final design i will use better resister wattage, also i will check how LTC and Fluke shows after voltage divider.@ Ian/CerebusI will read datasheet again about Input impedance and capacitance and see if i made a mistake.Ground connections are star connected.SPI is independent.Decoupling is as recommended as i am using LTC evaluation board.Need to see other noise sources if any.Thank you.Best Regards / Pallav