current sensor with op amp

[rares.tohanean]

I have a problem with this schematic:
I want to measure the current through a conductor with TALEMA AC1015 sensor. The signal from the sensor is amplified with op amp MC1458 in an inversor configuration.

The schematic works like this:
current: 3.8A a.c. 50Hz output: 1.93V a.c.
current: 7.4A a.c. 50Hz output: 3.4V a.c.

The problem is that at the output terminal I have always a continuum component of 10V d.c..

Why is this and how I can get rid of it?

Thank you!
raresh.

 

[FvM]

The circuit has two errors.

- LM1458 is no rail-to-rail OP. It needs a negative supply voltage to allow zero common mode voltage.

- Talema AC1015 is a current transformer. The output voltage is a bipolar AC voltage. Thus the amplifier output is also AC voltage. A negative OP supply is needed for it.

 

[cdh7]

You could switch to a noninverting configuration and implement a DC bias network on the amplifier's input if you want to avoid a negative power supply, as long as you don't mind a DC offset on the output. In that case, you would also need to AC couple the transformer through a capacitor to avoid injecting DC current.

Also, for the input/output relationship you posted, it looks like the transformer needs a 100 ohm load, but your schematic shows a 200||1k =167 ohm load. You could change the feedback resistors from 1k,5k to 200,1k, or maybe use a voltage divider to bias your amplifier and change the input impedance at the same time.

Regards,
Chris

 

[rares.tohanean]

Thank you!

I will implement your advices and I'll come back with the results.

Sory form my lack of details (I see now).
I don't want to amplify the negative alternance because the ADC that follows doesn't need it. The software can cope with this situation.

raresh.