Current measurement IC ( Op Amp ) for hight voltage versus Max Supply voltage  - Page 1
Forum section: electronics / projects

author: Maxoverdrive
author_rank: Contributor
timestamp: August 17, 2020, 07:12:30 am
content: HelloI have a little electronic knowledge but not in this domainI work on a Ion probe system , I need to make an accurate measurement of the current peak which is of few milliamp , but the main line voltage is 90 V DC , the peak current is very narrow as a pulse so I need hight sampling rate sampling at least 100 KhzI have read a lot and I dont undertand one pointAs the IC Op Amp is not feed by the main line ( in this case 90V ) why there is an IC voltage limitationexample : TiTechnote SBOA298 : the OPA354 is feed by the same voltage as the main line so OK main line voltage is a limitationbutexample : INA293 the IC is feed with 22V DC max ( separate voltage source ) but the maximum line voltage is 110V maxIn a current sense measurement the voltage is measured at the resistor terminals so the voltage is low and far lower than teh main line voltageSo my question is : If I feed the IC Op Amp with an isolated PSU to be separate it from the DC main line can I use a high performance IC low noise / hight frequency / acceptable slew rate as the OPA354 or other as a Op Amp +/- 10 V ou 15 +/- VThanks for your answerMO

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2020, 08:32:49 am
content: Yes, the op-amp used to measure the current, can be powered off a lower voltage.

author: Maxoverdrive
author_rank: Contributor
timestamp: August 17, 2020, 08:40:29 am
content: Dear Zero999ok thanks that confirm my point of view , IC are cheap but I want to avoid to burst the oscilloscopeDo you know why some circuit have a isolation amplifier and some other no , if use a separate power supply to feed the Op Amp normally I do not need a isolation amplifier ?Some current sense IC as the Avago 7800 have an integrated isolation circuit some as the INA293 have any .RegardsMO

author: OM222O
author_rank: Frequent Contributor
timestamp: August 17, 2020, 09:19:00 am
content: Op amps have a rated common mode voltage, so for example if your shunt resistor is placed for high side current sensing, voltage on one point maybe 90v and on the other side of the resistor it can be 89.9V.There is 0.1V across the resistor but there is also 90V of common mode voltage! If you exceed the common mode voltage rating of the op amp, it'll be fried however if you use low side current measurment, you can use any op amp as there is 0v common mode voltage.

author: Zero999
author_rank: Super Contributor
timestamp: August 17, 2020, 09:42:28 am
content: Quote from: Maxoverdrive on August 17, 2020, 08:40:29 amThe application note, TI SBOA298 you referred to earlier is for low side current measurement. Assuming your circuit is measureing current on the low side, as in the PDF linked to below and the op-amp shares the same ground, as the current souce, the op-amp can run at a much lower voltage, without any isolation.https://www.ti.com/lit/an/sboa298a/sboa298a.pdf

author: Maxoverdrive
author_rank: Contributor
timestamp: August 17, 2020, 11:32:11 am
content: thnks for the repliesI am on the ''low side '' I confirm , all Ion probe current are done on the ''low side '' , why I don't know , but most lab setup use a Analog 5B40 isolation amp module , that a quite old and the performance is quite low regarding frequency and noise , that why I try to improve the circuit to record the best current peak possibleI have located this doc from Texas Ins with a diode circuitApplicationReportSLLA190–February2006to limit voltage , that confirm what OM2220 have wroteINA168 is an IC with affordable cost as the INA293 , that allow some experimentents , LT1995 is also a possible choice but it s look less easy to useI will progress on test circuits and put reports asapRegardsMO