# Amplifying a 50KHz current sense resistor voltage

Status
Not open for further replies.

#### treez

Hello,

I have a current sense resistor which has sinusoidal 50KHz AC current in it, as follows.....

http://i46.tinypic.com/xeqyl1.jpg

The voltage across the sense resistor is +/30mV.

The task is simply to amplify it to a voltage above 1.2VDC (1.2V is the trip level of the follow-on comparator).

.....The first opamp doesnt give much amplification due to the GBW.

However, i then add the RC filter, and a second amplfier which amplifies it to above 1.2V.

Do you know how i can work out the peak output voltage of the first (LHS) opamp?

Also, do you think this is the best way to do this?

.......basically, the sense resistor will either have current in it (50KHz AC) or zero current, and i simply have to flag that up.

LT1006 DATASHEET:
http://cds.linear.com/docs/Datasheet/1006fa.pdf

#### FvM

##### Super Moderator
Staff member
The description isn't very clear. Do you mean to operate the first amplifier stage as half wave rectifier, utilizing output saturation with single supply? This might work, but you should check the detail behaviour of the chosen amplifier IC in a simulation.

P.S.: It looks like the circuit is operating at the AC gain and slew rate limits. A faster OP may be a better choice. You should also check the effect of placing a Rset to increase the slew rate.

Last edited:
treez

### treez

points: 2

#### treez

FvM
Do you mean to operate the first amplifier stage as half wave rectifier, utilizing output saturation with single supply?
..yes thats right.

I hope i made it clear that my only goal is to detect whether current is flowing , or is not.

When current is flowing, it will be 300mA peak AC sine.
When current is not flowing, it will literally be zero Amps.

#### picgak

Treez first you have to use a precision rectifier using an opamp and then amplify to the required level
regards ani

treez

### treez

points: 2

#### treez

thanks picgak, but on the simulator, it works fine like as in the above.........and as FvM said, i am doing the following........

operate the first amplifier stage as half wave rectifier
so surely i dont need a precision rectifier?

i cant understand why i need a precision rectifier, when all i have to do is tell if their is current flowing or not....there is no precision needed.

#### FvM

##### Super Moderator
Staff member
i cant understand why i need a precision rectifier
Why do you ask? You've already seen that the circuit basically works. If the real chips behaves slightly different than LTSpice model, you can still adjust resistors to achieve the intended behaviour.

treez

points: 2