amplification problem

Thread Starter

mah

Joined Mar 15, 2010
393
i want to amplify dc mv aignal from irradiation sensor using ua741 but i have single supply so i connected the negative supply to ground and i used virtual ground , but when i connect the sensor to the input of op amp and measure the signal when connected it gave .5 volt however it should be 5.5 mv when disconnected , i attached the connection drawing. please help
 

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LvW

Joined Jun 13, 2013
1,749
Are you aware that the dc reference Vcc/2 appears at the output as a dc level of G*Vcc/2 ? G is the non-inv. gain (1+R2/R1).
 

MrChips

Joined Oct 2, 2009
30,414
If Audioguru were still here he would promptly tell you that the 741 opamp is out of date.
Use a more modern rail-to-rail single supply opamp.
 

Thread Starter

mah

Joined Mar 15, 2010
393
Are you aware that the dc reference Vcc/2 appears at the output as a dc level of G*Vcc/2 ? G is the non-inv. gain (1+R2/R1).
if Vcc/2 appears in the output what will be the total output of the op amp?
 

LvW

Joined Jun 13, 2013
1,749
Please read my post again: The output is Vcc/2 multilplied with G.

It is normal practice for single-supply operation that the dc gain G(dc) is set to unity.
Only in this case the output dc voltage is identical to the reference voltage applied at the pos. input terminal.
Mostly, this is accomplished using an input capacitor in series with R1.
However, this does not work for dc signals.
Question: As you "want to amplify a dc mV signal" - do you intend to use a non-inverting or an inverting amplifier?
You should better specify your requirements.
 

Thread Starter

mah

Joined Mar 15, 2010
393
i need to use non inverting terminal , will it make difference ?
i know that i used inverting terminal here ,because of the virtual ground .
 

LvW

Joined Jun 13, 2013
1,749
i need to use non inverting terminal , will it make difference ?
i know that i used inverting terminal here ,because of the virtual ground .
In principle, you can use single supply operation (using virtual ground) for both schemes: inverting and non-inverting.
However, I repeat: What are your requirements in detail? In particular, what is the polarity of the dc input signal (pos, neg, both) and what gain is required?
 

GopherT

Joined Nov 23, 2012
8,009
Give a complete schematic with
- resistor values,
- power supply voltage
- expected input voltage range delivered to the op amp from your sensor.
 

WBahn

Joined Mar 31, 2012
29,823
i want to amplify dc mv aignal from irradiation sensor using ua741 but i have single supply so i connected the negative supply to ground and i used virtual ground , but when i connect the sensor to the input of op amp and measure the signal when connected it gave .5 volt however it should be 5.5 mv when disconnected , i attached the connection drawing. please help
The information you are providing makes little sense. You talk about what it does when the signal is connected and the indicate that this doesn't match your expectations of what it should do when disconnected.

How are we supposed to make heads or tails out of what you are saying?

As already pointed out, the 741 is a poor choice for this application. Not only because it is ancient, but because it is not intended for single supply operation and can only get within 2V or 3V of the supply rails. So you have to play games like providing a reference voltage. This is fine, but it has consequences. In particular, your mV scale input signal needs to be referenced to this same voltage, which it almost certainly is not.

Instead of discussing the particulars of one specific possible solution that you have latched onto, let's take a step back and talk about what you NEED in terms of the PROBLEM you are trying to solve. As others have already requested, this means telling us about the sensor you are using (what kind of signal does it provide, what is it's output impedance, it is a two-terminal device, etc) and what kind of signal you need once all is said and done.
 

Thread Starter

mah

Joined Mar 15, 2010
393
I have irradiation sensor and its output is ranging from 0 to 10 mv . it is two terminals positive and negative i need to amplify it to get 0- 10 volt using single supply op amp and it is preferred to be non inverting , so would you propose newer and suitable op amp.
 

Thread Starter

mah

Joined Mar 15, 2010
393
Do you have a make and model of the sensor. I'd like to look at its specs, if possible.
http://eppleylab.com/

  • Sensitivity: approx. 9 µV/Wm-2.
  • Impedance: approx. 650 Ohms.
  • Temperature Dependence: ±1% over ambient temperature range -20 to +40°C
  • (temperature compensation of sensitivity can be supplied over other ranges at additional charge).
  • Linearity: ±0.5% from 0 to 2800 Wm-2.
  • Response time: 1 second (1/e signal).
  • Cosine:
    • ±1% from normalization 0-70° zenith angle;
    • ±3% 70-80° zenith angle.
  • Mechanical Vibration: tested up to 20 g's without damage.
  • Calibration: integrating hemisphere.
  • Size: 5.75 inch diameter, 3.75 inches high.
  • Weight: 7 pounds.
  • Orientation: Performance is not affected by orientation or tilt.
 

Thread Starter

mah

Joined Mar 15, 2010
393
IS LM358 a good choice and how to connect it to perform as op-amp and differential amplifier , like traditional or it will need advanced settings?
 
Last edited:

WBahn

Joined Mar 31, 2012
29,823
http://eppleylab.com/

  • Sensitivity: approx. 9 µV/Wm-2.
  • Impedance: approx. 650 Ohms.
  • Temperature Dependence: ±1% over ambient temperature range -20 to +40°C
  • (temperature compensation of sensitivity can be supplied over other ranges at additional charge).
  • Linearity: ±0.5% from 0 to 2800 Wm-2.
  • Response time: 1 second (1/e signal).
  • Cosine:
    • ±1% from normalization 0-70° zenith angle;
    • ±3% 70-80° zenith angle.
  • Mechanical Vibration: tested up to 20 g's without damage.
  • Calibration: integrating hemisphere.
  • Size: 5.75 inch diameter, 3.75 inches high.
  • Weight: 7 pounds.
  • Orientation: Performance is not affected by orientation or tilt.
Which model? The best I can tell is Model PSP.

I was hoping their site had some reference designs, but they don't.

This appears to be a thermopile that can probably be modeled as an ideal voltage source in series with a 650Ω resistor.

A quick Google search for "thermopile amplifier circuit" turned up the following:

http://www.analog.com/en/content/using_thermopile_sensor_in_ir_digital_thermometers/fca.html

I'd suggest starting there.
 

Thread Starter

mah

Joined Mar 15, 2010
393
It's perfect but SMD i need through hole mounting and i think it's a little expensive for my application which generate mv signal .
 
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