Help with Voltage amplifier (mV)

[rtronic]

Hi, I'm using a MPX2010DP pressure sensor to build a differential manometer, the output voltage of the sensor is between 0 - 20 mV and I need to send this voltage to an ADC converter, I need a gain close to 100 to watch the results in kPa (the units I need), I'm trying with a differential amplifier with LM741 but when I simulate the circuit in Proteus the output voltage never exceed 1 V no matter the resistors I'm using the configuration in this page http://www.electronics-tutorials.ws/opamp/opamp_5.html but I'm not sure what I'm doing wrong, any suggestions?

 

[(*steve*)]

The input offset voltage of the 741 can be 5mV, so beware of that.

Are you using a double-ended power supply?

 

[Harald Kapp]

Can you supply your schematic?

Apart from the offset voltage of the 741 there may be an issue with the sensors's output impedance. Worst case it is 3 kOhm which may seriously influence your amplifier circuit. I suggest you use the instrumentation amplifier configuration from the same tutorial cited in your post.

You can find low offset OpAmps from many manufacturers. One example: http://para.maxim-ic.com/en/search.mvp?fam=op_amp&480=200&tree=amps
Select from the table according to your requirements.
Other places you may look at are Texas Instruments, Linear Technology, Analog device, to name only a few.

Most of these manufacturers (if not all) offer ready-made instrumentational amplifiers which have the added benefit of better metched components compared to a self-made construction.

Regards,
Harald

 

[TedA]

rtronic,

I agree with the previous posts.

We need your schematic, in order to be of much help to you. ( Specifically, the schematic of the circuit being simulated.)

The cautions about the limitations of the 741 and power supplies are on the mark.

Do you know your way around the Proteus simulator? It's possible that your problems are really with the use of the simulator, rather than with the circuit itself.

What ADC are you driving? What input voltage range?

Have you looked at the data sheet and app notes for the MPX2010 ? See Freescale Semi at:

http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPXx2010

Be sure to see AN1097 and AN4010. These give good detail about applying the device.

As already suggested, a ready made instrumentation amp IC can solve many problems for you.

If you are mainly trying to learn about instrumentation amplifiers, I can see that you might want to make your own, using separate op-amps. In this case, the 741 can be made to do the job, particularly if the digital system can be used to correct for offset and gain errors.

Ted