LTC1043 Woes - Page 1
Forum section: electronics / projects

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 01:12:15 am
content: Well, after getting my LTC2400 to work, I decided to go ahead and purchase (not sample) 2 LTC1043 capacitive dividers from Digi-Key. I built them according to the /2 and /4 configs in the Datasheet, and neither work. I was able to get the 5v in to drop down to 2.8v on the /2, but that was inaccurate, and the /4 outputs the same as the input. Any suggestions would be appreciated.

author: dannyf
author_rank: Super Contributor
timestamp: November 18, 2013, 01:17:01 am
content: I think you used the wrong C2 and R3 values. I would increase them a little bit.I would also separate the ground and do away with the thin copper traces between C5 and R2/R7.The decoupler on IC4 could be a little higher too and tilt more to the left.Hope it helps.

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 01:32:15 am
content: I don't believe there are any resistors...

author: wraper
author_rank: Unknown
timestamp: November 18, 2013, 02:53:03 am
content: Stupid question, but have you connected pin 4 to the + rail?

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 02:59:54 am
content: Yes... tried w/ and w/o. WIth, on the /4 gives me 0.2v out, w/o gives me 8v out when feeding in 8v

author: BravoV
author_rank: Super Contributor
timestamp: November 18, 2013, 03:08:49 am
content: Not sure if its related, I have 10 ordered while ago, two of them are toasted, although I'm not sure whether they're DOA, or I ruined them my self because of my noobness while back when handling this cmos chips.

author: wraper
author_rank: Unknown
timestamp: November 18, 2013, 03:09:41 am
content: How you can try without it if it is IC's power pin => no voltage, IC does not work. You also can't input higher voltage than on power pin, look what is written on the bottom of the picture you added. Probably you already destroyed IC such a way. ABSOLUTE MAXIMUM RATIGS Input Voltage at Any Pin .......... –0.3V <= VIN <= V+ +0.3V

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 03:12:34 am
content: Vin must be less than or equal to V+ must be less than equal to 18v. I am feeding both off of the same power source...

author: wraper
author_rank: Unknown
timestamp: November 18, 2013, 03:17:03 am
content: You wrote that tried with it being disconnected.

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 03:21:35 am
content: Oops... I will try it with a different LTC1043. Good thing I bought 2...

author: wraper
author_rank: Unknown
timestamp: November 18, 2013, 03:24:18 am
content: Another even more stupid question, did you connect 1uF capacitor between pin 11 and 12, that which is drawn inside the IC?

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 03:25:21 am
content: Yes.

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 03:28:35 am
content: Swapped in a new one, and still the 0.25v output. grrr...

author: wraper
author_rank: Unknown
timestamp: November 18, 2013, 03:31:09 am
content: Then I don't know, nothing else there, must work if IC is alive. At least if you are not connecting any low resistance load to the output.

author: BravoV
author_rank: Super Contributor
timestamp: November 18, 2013, 03:31:24 am
content: I think its time to gear up your scope and start probing to see if its "switching" or just staying quite & idling.

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 03:34:08 am
content: Sadly, I just sold my scope. Good news is, my DS1074Z is going to come in a day or 2

author: Andreas
author_rank: Super Contributor
timestamp: November 18, 2013, 07:01:31 am
content: Hello,checklist:- 10nF connected to clock pin? (most likely that the cap is cracked or not connected)- do you have a voltage follower (LTC1050 / LTC2057) at the output in front of the ADC?(otherwise the input of the LTC2400 will draw all voltages towards VREF/2.)- do you use film capacitors? (but a accuracy deviation of more than 100 ppm should have a other explanation)With best regardsAndreas

author: echen1024
author_rank: Super Contributor
timestamp: November 18, 2013, 10:01:44 pm
content: I am using cermaics.

author: branadic
author_rank: Super Contributor
timestamp: March 13, 2014, 10:34:20 pm
content: Hi all,any suggestions to correct guarding and shielding of the chip?Page 5 Test Circuit 4. CMMR Test of the datasheet shows pin 10 (or 1) used as a guard for the sample cap and pin 11 (or 2). This is shown in most of the design notes of LTC1043+LTC2400.Page 7 Figure 5 of the datasheet shows a shielding plane connected to pin 3 (or 12) instead, no connection at pin 1 (or 10).So what is best practice? I couldn't find any reference design or functional pcb layoutin the web to start with. I appreaciate some help.

author: Niklas
author_rank: Frequent Contributor
timestamp: March 14, 2014, 10:40:48 am
content: Quote from: echen1024 on November 18, 2013, 10:01:44 pmCan you be a bit more specific, like ordering codes? Dielectrica, voltage rating, package size etc could be involved...

author: Andreas
author_rank: Super Contributor
timestamp: March 16, 2014, 12:04:48 pm
content: Quote from: branadic on March 13, 2014, 10:34:20 pmSince pin 1 + 3 (or 10 + 12) are connected together internally the cirquits are equivalent.Best would be to use a good (old) foil capacitor (polypropylene) with the outside foil connected to pin 3.But since today these (outside foil) are not easy to get you might try to create your own shield around the cap.For my applications (space restricted) I use 1uF MKS-02 (mylar) foil capacitors which are mounted SMD-style on a guard area on the PCB. I know that they are only 3rd choice but from theoretical calculation I should get only about 1.3 ppm (typical) additional error.I would never use ceramics. Neither for the 10 nF (I use SMD PPS type WIMA SMD 1812) nor for the 1uF.And: I always use selected pairs of 1uF capacitors which are matched to less than 1% to minimize settling time of the output.with best regardsAndreas