Analog signal amplified.

[Gildardo]

Hi there. I’m working with a strain gage which gives no more than 10 mV. Given that the Picaxe analog input receives voltage around [0-5v]. I need to get a small this signal amplified from [0-10mV] to [0-5v]
I don’t want to use transistor configurations because the amount of elements needed and the inexact signal obtained with it. I neither want to use op-amps because a (-) alimentation font would be needed to feed the op-amp.
Any of you guys know a high gain amplifier C.I. which works with low energy (5-12V)?

 

[moxhamj]

I am not sure what a negative alimentation font is, but if you mean negative power supply, it is quite possible to use an op amp with a single 5V supply. A differential amplifier with a gain of 300 uses 4 resistors. You will also need two more resistors to create a 1.5V virtual earth via a voltage follower. That is a total of one chip (LM324) and 6 resistors.

If an op amp solution is ok then pls say so and I can post a circuit.

 

[womai]

The best choice for your application is a so-called instrumentation amplifier - which is just a normal op-amp but optimized for excellent common mode rejection ratio (CMRR) - In simple terms, it is very immune to large offsets in the input voltage and really only amplifies the difference between + and - input. Texas Instruments has a good selection, as does Analog Devices and Maxim.

Wolfgang

 

[Jeremy Leach]

Or could use a precision Voltage-To-Frequency converter such as the LM331. This might offer advantages over op-amp solution due to:

1. 8 pin , low cost package

2. Using Count command it is possible to read to high resolution, and the reading will automatically be averaged.

Edited by - jeremy leach on 23/01/2007 08:22:47

 

[Dippy]

I'll go along with The Count on that one, except with some single rail types (eg TS942) you can get good gain without virtual earths. Try it. TS942 also is micropower but only any good gain at low Fs.

Or with a deep wallet you could go for a 1B31AN strain gauge conditioner ic. I just love the way the Data Sheet says "Low Cost"!! It must have been written for Bill Gates.

There are probably numerous others that are suitable. Many will have a 'My Favourie Op Amp'. So check out 'Single Rail' typ op-amps before dismissing them.

Edited by - Dippy on 22/01/2007 09:16:48

 

[Gildardo]

Thanks for your help guys. I'll check the data sheets of the CIs you mentioned.

Dr.Acula...could you post the circuit with the op-amp solution please?

 

[moxhamj]

Schematic below. The strain guage/pressure sensor or whatever is a bridge - maybe with a 10k resistance or similar. This goes through two voltage followers to match impedences, then into a differential amplifier. The 4th op amp set a 1.5V reference. If the output goes down instead of up when a strain is applied then swap the strain guage outputs around.
The 324 can only swing to within 1.5V of the power supply, ie between 1.5 and 3.5V. So with a 10mV input, a gain of 180 will give 1.8V, so the output will go from 1.5V to 3.3V.

<code><pre><font size=2 face='Courier'>


o---o-------------------------------5V---------o------------------------------------------
| | ___ 1.8M
| Strain guage .---------. | .--|___|---.
o--------. | | | 10k | |
| | | | | ___ | 13|\ |
.-. .-. | 2|\ | | .-----|___|----o---|-\ 14 |
| | | | '--|-\ 1 | | | ___ | &gt;---o------
| | | | | &gt;---o--------------)-----' -|___|-----o--|+/ To picaxe
'-' '-' '------|+/ .---)---------' 10k |12|/
| | | 3 |/ | | .---------. .-.
o--------)----' | | | | | | 1.8M
| o----. -------------. | .-. | | | |
| | | | | | |33k | 9|\ | '-'
.-. .-. | | 6|\ 7 | | | | '--|-\ 8 | |
| | | | | '---|-\ | | '-' 10| &gt;---o------'
| | | | | | &gt;-----o--------' o-------|+/
'-' '-' '------|+/ | |/ 1.5V
| | 5|/ .-.
.--------' | |
| | | 15k
| '-'
| |
o----o-----------------------------0V-----------o---------------------------------------------

LM 324 pin 4=5V, pin 11=0V

Gain =1.8M/10k=180
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
</font></pre></code>