Continuity testing with IC's having balanced outputs - Page 1
Forum section: electronics / projects

author: Aakushti
author_rank: Newbie
timestamp: April 26, 2017, 11:25:06 am
content: HelloI was inspecting my FFT plot and I noticed that the spectrum hardly changed , I suspected that somewhere the differential signals IFON(pin 21) and IFOP(pin 20) going to my ADC were shorting and hence the FFT was merely plotting the noise due to the nearby power rail since the amplitude in the FFT plot was extremely small . I checked for continuity and my DIMM started beeping when I tested between the IFOP and IFON pins . Is this normal while checking for continuity between differential/balanced signals or is there a solder bridge between the pins ?Thanking YouRegards

author: sanwal209
author_rank: Regular Contributor
timestamp: April 26, 2017, 12:16:06 pm
content: From the schematic there are two series capacitors 10uF between Impedance transforming network. So We can eliminate the Transforming network from the cause of short circuit. But there is an inductor parallel to IFOP and IFON in application schematic drawn in datasheet so probably that can be the cause if you have an inductor in your design.Check the continuity threshold of your Multimeter. According to datasheet differential impedance is 200 ohm. I would suggest to check the threshold impedance of continuity in your multi meter or measure the resistance between these two pins.

author: Aakushti
author_rank: Newbie
timestamp: April 26, 2017, 12:22:10 pm
content: Barebone IC gives 250 ohm between the IFOP and IFON but in the PCB it gives 70ohm .

author: MagicSmoker
author_rank: Super Contributor
timestamp: April 26, 2017, 12:23:43 pm
content: Well, there are 50R resistors from each IFOx output pin to the positive rail...

author: sanwal209
author_rank: Regular Contributor
timestamp: April 26, 2017, 12:28:49 pm
content: If its 250R then your IC is fine and problem is somewhere else. The schematic you have posted is different from the application schematic in datasheet. Which on are you using?

author: Aakushti
author_rank: Newbie
timestamp: April 26, 2017, 12:37:20 pm
content: The IFOP and IFON from ADL5801 is passed to a high pass filter . There is no short after C56 and C57 . The short appears before that . The IC is fine but when I measure impedance between IFOP and IFON pins in the pcb the impedance is 70ohm , I think there might be a solder bridge

author: sanwal209
author_rank: Regular Contributor
timestamp: April 26, 2017, 12:41:37 pm
content: Can be solder bridge or shorted C55. Check the Capacitor

author: dmills
author_rank: Super Contributor
timestamp: April 26, 2017, 12:46:43 pm
content: The two 50 ohm pull ups to the 5V rail gives you 100 ohms across those two pins even before you consider anything else.If the chip is good for 250 ohms then (100 * 250) / (100 + 250) = 71 ohms, so that is what you would expect to measure there, 70 ohms is close enough.Is the LO present and correct?Regards, Dan.

author: sanwal209
author_rank: Regular Contributor
timestamp: April 26, 2017, 12:48:42 pm
content: Oh wait everything is fine in your circuit. I just looked at R3 and R20. There are connected in series to eachother but parallel to IC. So if you calculate 100R parallel to 250R its almost 70 ohm.There is not short on your PCB.50R--------TP1----/\/\/\-----250R |---- 5v--------TP2----/\/\/\-----50RIf you measure resistance on TP1 and TP2 it should be almost 70R. For me your Hardware is fine.

author: Aakushti
author_rank: Newbie
timestamp: April 26, 2017, 12:53:56 pm
content: I haven't checked the LO yet . Will be getting access to spectrum analyzer tomorrow .

author: Aakushti
author_rank: Newbie
timestamp: April 27, 2017, 01:46:17 am
content: I was testing the impedances and I found out that between RFIP(16) and RFIN(15) the impedance should be 50ohm but its showing the value in mega ohm .

author: sanwal209
author_rank: Regular Contributor
timestamp: April 27, 2017, 07:23:59 am
content: In PCB or barebone?

author: Aakushti
author_rank: Newbie
timestamp: April 27, 2017, 11:51:10 am
content: Both actually . I had a barebone spare and one on the PCB . Both are showing megaohms