op07 as current sense amps - Page 1
Forum section: electronics / projects

author: electros6
author_rank: Regular Contributor
timestamp: January 11, 2014, 01:24:06 pm
content: Hi guys ,I used OP07 first time as a current sense amplifier for high side sensing 0 to 4A via a 0.1 ohm shunt. when I connect OP07 as differential amplifier , I found that there is around 1.2V seen at the ouput even when no current is flowing.Please somebody clarify.I tried 3 opamps but ended in same result.somebody post me schematic to use op07 as differential amplifier

author: sync
author_rank: Frequent Contributor
timestamp: January 11, 2014, 01:44:03 pm
content: Please post your schematic.

author: electros6
author_rank: Regular Contributor
timestamp: January 11, 2014, 02:17:24 pm
content: schematic attached

author: electros6
author_rank: Regular Contributor
timestamp: January 11, 2014, 02:18:07 pm
content: schematic attached

author: dannyf
author_rank: Super Contributor
timestamp: January 11, 2014, 02:29:52 pm
content: QuoteIt needs very high common mode voltage range.

author: sync
author_rank: Frequent Contributor
timestamp: January 11, 2014, 02:41:02 pm
content: It should work. It's important that the OP07 is powered with positive and negative supply voltages. It's not a rail to rail op amp. But you use a +/-10V supply. That's ok.But the MOS-FET always conducts through it's body diode. Probably not what you want.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 11, 2014, 03:01:42 pm
content: Hi,It is certainly not a common mode range problem for the op-amp. Both input are at 6V with respect to ground. The op-amp has +/-10V supplies. It is not a rail-to-rail issue.There is a small mistake in the OPs schematic. The top of R4 should be connected to ground.We then have this:Which works reasonably well at face value.The issue with using a differential configuration, for measuring small voltage, is that you need really good resistor matching to obtain a high common mode rejection ratio.If I change one of the strings to 1% resistors (worst case) I get:If I change them the other way The output goes in the opposite direction:I suggest using a current sense amplifier.The zip file contains the LTspice modelJay_Diddy_B

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 11, 2014, 03:40:36 pm
content: Hi,This can be taken a step further be introducing Monte Carlo analysis. Monte Carlo analysis is named after the casino. Parts will vary in value randomly within the specified tolerance. The distribution of values is equal.In this example the simulation is run 50 times using a dummy variable in a .step command.The example uses 5% resistors.Output voltage variation:Jay_Diddy_B

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 11, 2014, 03:53:32 pm
content: Quote from: sync on January 11, 2014, 02:41:02 pmYes, MOSFET is "upside down".

author: underwurlde
author_rank: Newbie
timestamp: January 11, 2014, 09:09:48 pm
content: Look at using INA138 or INA168 devices instead?What bandwidth do you need BTW?A

author: BravoV
author_rank: Super Contributor
timestamp: January 12, 2014, 09:43:10 am
content: Quote from: Jay_Diddy_B on January 11, 2014, 03:40:36 pmJay_Diddy_B, let alone the topic, I always enjoy your posts throughout the forum on utilizing the LTSpice as examples.Definitely your LTSpice kung-fu there helps a noob like me, a lot, thanks.