Amplifying 75mV signal to 5V using IC741

[ankit-max]

I've a shunt resistor which creates a voltage drop with reference to current. The maximum voltage provide by the sensor is 75 mV.

I want to amplify the voltage 75mv to 5v and provide it to the micro controller port pin for further operation. I am using IC 741 for non-inverting amplifier.

I got the relationship Rf=65.666*R1. What values of Rf and R1 should I use?

Thanks.

 

[barry]

There are a LOT better opamps than a 40-year-old 741. Regardless, you are not going to get EXACTLY the scaling you specified, but you can get close. Is this going into an analog-to-digital converter, or are you using a digital input? If you're using an ADC, then I don't think you want exactly 5V, you need some headroom; maybe scale it to about 4.5 volts. Pick R1 to be in the range 1K to 10K.

 

[crutschow]

Is the current shunt in the common (return) line or the power line?

 

[ankit-max]

Hi Barry,

Yes, The amplified output from the op-amp will be going to 8-bit ADC of PIC16F72 micro-controller. Yeah, I don't need exactly 5 V, but I want to prevent the pin of the micro getting more than 5V (maybe using a zener didoe to prevent this?)

Okay, I will try with R1 of that range and will let you know how it goes.

Thank you for your response. Appreciate it!

- - - Updated - - -

Is the current shunt in the common (return) line or the power line?

Hello, crutschow

The current shunt is used in series with battery charger and it's output will be going to the non-inverting pin of the 741

 

[barry]

Can you explain your circuit? If your sense resistor is in series with the battery charger output, you can't just look at one side of the resistor, you need to use a differential arrangement.

 

[ankit-max]

Can you explain your circuit? If your sense resistor is in series with the battery charger output, you can't just look at one side of the resistor, you need to use a differential arrangement.

Here is the circuit diagram:-



Sorry for the late response.

 

[betwixt]

Alternatively, you could try putting the resistor in series with the battery negative side. As long as you are careful with the sense wire placement, you should be able to measure the voltage drop relative to ground.

Brian.

 

[ankit-max]

Alternatively, you could try putting the resistor in series with the battery negative side. As long as you are careful with the sense wire placement, you should be able to measure the voltage drop relative to ground.

Brian.

Thank you for suggestion. Can I get the linear relationship between output voltage of the op-amp (which I will feed to the ADC of micro) and the input voltage of the op-amp, i.e. o/p of current sesnor-75 mV with LM 358 or 741 IC?

Like, 75 mv - 5 V

so for 1 mv - (5/75) V

10 mV - (5/75) x 10 V

 

[betwixt]

Yes, as long as the amplifier is linear it will give a magnified but proportional output voltage. A voltage gain of 66.66 will give you 5V out for 75mV input.

As mentioned earlier, you should be looking a better op-amps for this, in particular ones with a low input-offset voltage, otherwise your output could be seriously inaccurate. Consider that an unadjusted LM741 when fed from a low impedance can still produce an error of 7.5mV or 10% of your total reading. The 358 is only marginally better, it is possible to trim (null) the offset of a 741 but not a 358. Also check that your op-amp output can reach down to zero if you are using a single rail supply.

Brian.

 

[ankit-max]

Thank you for the info, Brian. I am having one more complication which is:- I want to block the voltage > 5 V going to the micro pin coming from the op-amp.

Should I use 4.1v Zener diode or there are other better options?

Thank you.

 

[betwixt]

You have conflicting requirements so a compromise has to be made.

Yes, a Zener diode across the ADC input is the simplest solution and as a bonus, it will protect against negative voltages which could damage the ADC.
The conflict comes from the need to protect the amplifier as well. A 4.7V Zener shold hold the ADC voltage below the 5V VDD and still give you most measurement range but it will also sink op-amp current if it tries to go over 4.7V at it's output. The cure, is to fit a current limiting resistor in series with the op-amp output but this also introduces side effects, one is to limit the ADC input current so you may have to wait slightly longer before measuring if you are switching between other ADC channels, the other is it will create distortion in the highest measurments as the Zener starts to conduct. A Zener doesn't have a perfect knee point, it starts to conduct just before it's rated voltage and continues to conduct more to just past it. It means as you approach 4.7V some extra voltage will be dropped in the resistor and make the reading slightly lower than it should be. This effect will only happen near to 4.7V though so it may not cause you a problem.

Another option is to keep the series resistor but instead of a Zener diode, use two normal diodes, one to ground and one to VDD. Connect both with cathode ends toward VDD. These will conduct if the voltage goes Vf above VDD or Vf below VSS wher Vf is the diode forward voltage drop. Technically, this puts the range outside of safe limits but if you use Schottky diodes or types with low Vf it should be OK. For example a BAT85 diode has a Vf of around 0.3V compared with a normal 1N4148 type which is around 0.6V.

Brian.

 

[ankit-max]

That's a good info. I will be trying both options, diode one looks simple just that I will have to do changes in the programming because of the voltage drop of the diode.

I will experiment it and let yo know how it goes.

Thank you.