Op Amp - inputs higher than rail - output less than rail? - Page 1
Forum section: electronics / beginners

author: Plasmateur
author_rank: Regular Contributor
timestamp: March 15, 2017, 01:37:24 pm
content: Hello,I'm trying to make a current measurement with an op amp. There will be 50V going into a resistor to make the measurement. The differential reading between the inverting input and the non inverting input will give an expected voltage less than the rail voltage which will be my current reading. This is for a relatively higher frequency application - 1MHZHowever each voltage on the inputs with respect to the rail will be higher than than the rail.Are there op amps that can do this, and, are there fully differential op amps that can do this?

author: amspire
author_rank: Super Contributor
timestamp: March 15, 2017, 01:48:17 pm
content: Dave discussed this kind of problem in his videos on the LT3080 power supply design - around about video 225. Gave a couple of solutions.

author: Plasmateur
author_rank: Regular Contributor
timestamp: March 15, 2017, 01:54:33 pm
content: Quote from: amspire on March 15, 2017, 01:48:17 pmI will try to find the video. Thank you for the response.

author: BrianHG
author_rank: Super Contributor
timestamp: March 15, 2017, 01:58:35 pm
content: Here is the part you are looking for:http://www.linear.com/product/LT1997-3

author: BrianHG
author_rank: Super Contributor
timestamp: March 15, 2017, 02:06:21 pm
content: Quote from: BrianHG on March 15, 2017, 01:58:35 pmNow, this is no more than a regular opamp inside with resistor dividers for the inputs. You can copy the illustration an use a normal opamp.Also, there exists cheap 140v opamps with R-R in and out if that may suit you needs.

author: Plasmateur
author_rank: Regular Contributor
timestamp: March 15, 2017, 02:22:29 pm
content: Quote from: BrianHG on March 15, 2017, 02:06:21 pmOh wow. This is nice. Thank you so much.So, is the common mode voltage range what I need to consider for higher voltage inputs? That is, because the common mode voltage range on this particular op amp is so high (+/-160V), it should be able to handle the 50V on the inputs? Just curious as I go to select an op amp with a slightly higher -3dB bandwidth.I'm not sure if these are available in fully differential op amps, but if not, then I'll just use two op amps!

author: amspire
author_rank: Super Contributor
timestamp: March 15, 2017, 02:26:09 pm
content: There are a surprising number of high side current monitor chips from companies likeAD, Maxim, LT and TI. Just saw a part I had never seen before from Zetex that can do the voltage, but probably not the bandwidth:http://www.icpdf.com/ZETEX_datasheet/ZXCT1080_pdf_4710628/Looks like the INA168/169/170 can probably do the job. 60V and can manage 1MHz bandwidth if you use 0dB of gain.http://www.ti.com/lit/ds/symlink/ina168.pdfI think in his video, Dave looked at one of the IC's in this family - maybe the lower voltage INA138.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: March 15, 2017, 03:36:33 pm
content: Hi,The LT1999 is a good part for this application.Link:http://www.linear.com/product/LT1999It has a common mode range on the inputs from -5 to +80V and is available with gains of 10, 20 and 50.Regards,Jay_Diddy_B

author: BrianHG
author_rank: Super Contributor
timestamp: March 15, 2017, 03:39:50 pm
content: For high frequency, if you can give the op-amp a higher supply voltage, I would use this guy:LTC6090CS8E-5Up to 140v, 24Mhz.Do you have a diagram of how you are measuring your current? Sometimes there is a solution you missed which would allow a simple low voltage differential op-amp.