Problem with an opamp circuit picking up interference

[ljbdragon]

I'm designing a circuit which amplifies a small AC signal generated by a current transformer and converts it into a higher DC voltage which will eventually go into a data logger. However my problem is that the circuit is very sensitive to external electrical noise i.e switching a power tool on and off can be seen on the output of my circuit in the form a spike. As far as I can tell most of the interference is getting into the circuit via the current transformer, although some is getting in via the power supply.
My question is how can I clean up the ac signal before it enters the opamps, and secondly is there anything I can do to the power supply to prevent mains born interference from getting through to the main circuit.
The ac signal is 15mV up to 500mV, and the power supply is a basic dual rail type +5 and -5

Any advice would be very welcome.

 

[RoboWanabe]

I think you can perform whats called a Common Mode Rejection Ration (CMRR) which gives a measure of how good the op-amp is at rejecting interfering signals with respect to the requierd signal?

 

[ljbdragon]

Thanks RoboWanabe, I see what you mean, I'm using a LM348 quad opamp with 90db of CMRR, maybe I need something with a much higher CMRR.

 

[RoboWanabe]

No problem yea i think the higher the CMMR the better but 90 db seems pretty high still? .unfortunatly im not to sure of how to suppress it befor the op-amp, maybe use a bandpass filter if you know the range of frequencies you want through? please bear in mind i am kind of a noob at all this

 

[ChrisP58]

Can you post your schematic? That would make it easier to help.

 

[ljbdragon]

I've attached a schematic of my circuit.

 

[RCinFLA]

That circuit is not employing common mode rejection. If you need multiple stage gain then the early stages should also have balanced outputs feed the next balanced stage input.

 

[ronv]

What frequencies do you expect to see from the current transformer.
How far away from the circuit is the transformer?
A filter may be in order.

A .1 Ufd. ceramic capacitor and say a 10 Ufd. electrolytic from both the positive and negitive supply to ground would be a good idea.

 

[crutschow]

I think you can perform whats called a Common Mode Rejection Ration (CMRR) which gives a measure of how good the op-amp is at rejecting interfering signals with respect to the requierd signal?

CMRR rejects noise that is common to both inputs of the op amp. Since the op amp has one input to common the CMRR has no effect on any noise in the system.

If you only want to be sensitive to the average current of the fundamental mains frequency then you can filter the signal. For example adding a 0.16μF capacitor across R8 will roll off the frequency response for any noise above 100Hz.

 

[Roff]

When the power tool puts noise on the line, that noise will cause current fluctuations in the load you are measuring with the current transformer. In addition to post-sensor filtering, you might need to put a common-mode choke on the lines feeding the load under test.

 

[rumpfy]

LJ ; did look at your cct.
For your application you need to use balanced input arrangement; like a twisted pair cable as per the telephone system.
Your circuit is refenced to ground; you need to balance the current transformer coil and use the + and - inputs of the op amp. I think LM 1489 is used for balanced input. The opamp you have is possibly ok but have a look at some examples around the line receiver arrangements.

 

[WTP Pepper]

As previously mentioned, having an op-amp with high CMRR is irrelevant if you are only using it as a single ended input.
1. Use a differential amplifier on the input.
2. Wire the input using a twisted pair so your common mode rejection starts even before it gets to the amplifier i.e. any electrical noise induced on the input wiring is induced in both wires that hopefully the op-amp will remove.
3. Don't try and put all your gain in a single stage amplifier. Don't over spec the amplifier in terms of a super high bandwidth op-amp if you don't need it as it will amplify wideband noise present whether you want to or not.
4. Make sure your current transformer is shielded otherwise it will just pick anything that can be induced in it i.e. LW radio.
5. As for the power supply, using good quality linear regulators (LM7805/LM7905) should be fine with decent suppressing caps on both input and output.
6. If you are using Veroboard/strip board, isolate copper tracks were there are signals that don't run the entire length of the board. These can act as aerials.

You could also add a filter on the mains side. The old cooker filters (Belling Lee type in the UK) are pretty good.

You may also consider an op-amp with high PSRR. Power supply rejection ratio. These op-amps will reject to a certain effect and mush on the power lines that if you have designed to PSU correctly should be minute anyway.

 

[RoboWanabe]

Ahh yes for CMRR to work the interfering signal needs to be at both inputs and the requierd signal on one input and then the op-amp can reject it. but i am not sure of how to implement it sorry. If some one could show an example that would be very helpfull

 

[ljbdragon]

Many thanks for all the suggestions posted so far, I've tried forums before in the past, but have never had such an outstanding response as I've received here. As a result I'm now attempting to redesign my project taking into account all your suggestions and ideas.

 

[Roff]

For optimum common mode rejection, use an instrumentation amplifier.

 

[WTP Pepper]

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar6.html

3rd hit on Google "Differential amplifier" . I assume you know how to use Google?

 

[ljbdragon]

Thanks, I'll take a look

 

[Roff]

http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar6.html

3rd hit on Google "Differential amplifier" . I assume you know how to use Google?

To get really good CMR, you either need an instrumentation amp, or, if you use the diff amp in the link, a pot to adjust the CMRR.

 

[ljbdragon]

To get really good CMR, you either need an instrumentation amp, or, if you use the diff amp in the link, a pot to adjust the CMRR.

Is it possible to create a reliable instrumentation amp from ordinary amps like 741 or LM348, or is there a particular chip you would recommend.

 

[Roff]

I think I would use INA126 from TI. Mouser has them for US $2.46 (are you in the US? It helps if you list your location).
You can make a diff amp or an inamp from op amp(s) and resistors, but getting the CMRR and the gain exactly as you want them is a pain.