how to get the stable amplified output for strain gauge

Thread Starter

rajitpk

Joined Jan 30, 2018
108
my design is 350 ohm half bridge strain gauge with 5v as an input.
i used the following amplifier circuits. still now i get unstable voltage at output.
i designed the amplifier circuit with 100 gain, using ic 741 as an opamp
Rf=100K; Rg=10K; R1=R2= 1M; R4=R3= 4.7M
so suggest me what do to scale 1mv to 1v
 

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ericgibbs

Joined Jan 29, 2010
9,338
hi rajit,
Welcome to AAC.
Do have the option of using a OPA with an improved specification.?
The 741 OPA is not the best choice for a Differential amplifier.
E
 

Thread Starter

rajitpk

Joined Jan 30, 2018
108
hi ericgibbs,

i can change the OPA, can you suggest the suitable one?
also, is that my circuit is correct for my requirement?
 

ericgibbs

Joined Jan 29, 2010
9,338
hi,
I use the MCP6002 series, they operate from a 5V supply and are rail to rail output.
The 6002 is a dual but there is also the MCP6001 single OPA, not too expensive.

E
 

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Thread Starter

rajitpk

Joined Jan 30, 2018
108
hi,
I use the MCP6002 series, they operate from a 5V supply and are rail to rail output.
The 6002 is a dual but there is also the MCP6001 single OPA, not too expensive.

E
thanks ericgibbs,
i will use it on my circuit..
is there is any need to make a correction on my circuit ?
 

ericgibbs

Joined Jan 29, 2010
9,338
hi,
I would suggest a circuit like this would be suitable.
1mV thru 10mV input, 0v thru +5V output.
Make the RF adjustable.
E

EDIT:

The 2nd image shows the bridge connection.
Use at least 0.1% tolerance resistors throughout the circuit.
 

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Last edited:

Thread Starter

rajitpk

Joined Jan 30, 2018
108
hi,
I would suggest a circuit like this would be suitable.
1mV thru 10mV input, 0v thru +5V output.
Make the RF adjustable.
E

EDIT:

The 2nd image shows the bridge connection.
Use at least 0.1% tolerance resistors throughout the circuit.
okay
hi,
I would suggest a circuit like this would be suitable.
1mV thru 10mV input, 0v thru +5V output.
Make the RF adjustable.
E

EDIT:

The 2nd image shows the bridge connection.
Use at least 0.1% tolerance resistors throughout the circuit.
okay ericgibbs. thank you
 

ebp

Joined Feb 8, 2018
2,332
What do you mean by "unstable"?

Do you understand that the performance of that amplifier circuit is very dependent on precise matching of the resistors?

What are you using for bridge excitation?
 

Thread Starter

rajitpk

Joined Jan 30, 2018
108
What do you mean by "unstable"?

Do you understand that the performance of that amplifier circuit is very dependent on precise matching of the resistors?

What are you using for bridge excitation?
hi,

bridge excitation is 5v

at the output of the amplifier i get output voltage varies from 3.4 to 3.6 v
 

ebp

Joined Feb 8, 2018
2,332
and 5 volts from where?
Details are important, otherwise we are just left trying to guess what the circuit is.
 

OBW0549

Joined Mar 2, 2015
3,143
Rather than trying to build your own bridge amplifier from op amps and discrete resistors, you'll get much better results (and a lot less frustration) using one of the many commercially available instrumentation amplifier chips. Linear Technology has several; I've used both the LT1789 and LT1167. Texas Instruments also has a good selection of IA's.
 

Thread Starter

rajitpk

Joined Jan 30, 2018
108
and 5 volts from where?
Details are important, otherwise we are just left trying to guess what the circuit is.
half bridge circuit excitation input voltage is 5v and opamp input 12v
the output shows 3.4 to 3.6 v( not stable) at the same time not a slow or fast
 

ericgibbs

Joined Jan 29, 2010
9,338
hi rajit,
As a general rule I use a 'true' Instrumentation amplifier for low level signals, the type shown post #15 by @OBW0549 are OK.
I also use the AD and INA series.

E
 

Janis59

Joined Aug 21, 2017
969
The many novices are making one and the same but very huge mistake - they mount that wonderful circuit JUST from the resistors and opamps. Thus it not work well and cannot even theoretically. Because all those resistor values must be passed with ca 0,01% accuracy. Be sure that all them You measure before soldering and if there is written 10,01 kOhm it means that 10,0 and 10,02 are not tolerated. That is specifism of this particularly circuit.
 

ebp

Joined Feb 8, 2018
2,332
One more time:

WHERE does the excitation come from? Not the magnitude. WHERE? Is it a precision-regulated clean, noise-free supply?
You get out of a bridge what you put in. If you put in trash you get out trash.
 
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