I was wondering if you guys can help. I was building a weghing scale using some strain gauges and an A-D going into a Pi. I hvae used a 741 op amp with the offset nulling pot. Without having to use 2 batteries to create a negative polarity to feed into the offet nulling part of the cct. can this be done by using just 1 battery and using the ground begating the need for -ve power source ?

Thanks for your help in advance

 

hi Shak,
Welcome to AAC.
A 741 OPA is a very poor choice for this type of project are you able to acquire a modern single supply OPA and even better still a AD623 or similar.?
E

 

hi Shak,
Welcome to AAC.
A 741 OPA is a very poor choice for this type of project are you able to acquire a modern single supply OPA and even better still a AD623 or similar.?
E

Thank you very much for getting back so quickly. The reason i chose this is the differential output from the guages was quite large and having the nulling circuit meant i could remove the voltage so that i can get the biggest possible swing from my strain measurement. ~10mA
The A-D that i am usiny only allows a max of 3.6V hence the offset. (plus it was something i have worked with before)
I have never used or heard of the AD623 unitl you have just mentioned this.

 

hi SJ,
OK, do you have a circuit diagram of your circuit using the 741, post it and we can check it out.
E

 

This is the circuit that i have built it has a gain of 470

 

hi,
The 741 is designed for a dual supply, I guess you know that the Vout swing will limit at about 2V below the supply voltage.
What are the resistor values on your circuit and what is the supply voltage to the 741 and the Bridge.?
E

 

hi,
This circuit is typical of what the Vout will be when using a single supply voltage.
Note the +0.8V minimum Vout level.
E

 

The design of a lousy old 741 opamp is 52 years old. It was designed to use only a +15V and -15V supply but some might work with +6V and -6V. It is too noisy to use a gain of 470 times.

Since your ADC does not accept negative input voltages then use a modern low noise rail-to-rail opamp without a negative supply.

 

hi,
The 741 is designed for a dual supply, I guess you know that the Vout swing will limit at about 2V below the supply voltage.
What are the resistor values on your circuit and what is the supply voltage to the 741 and the Bridge.?
E

The supply to the bridge is quite high, as i am using 4 x 350 ohm gauges in a 1/4 bridge i am having to run 24 v though the brige to get the 10mA of current draw otherwise i was seeing big temperature drifts.
Ive got a 100 ohm and 47K resistors to get the 470 gain factor. It doesnt really matter if the system is noisy, i dont need to sample quickly. once every 10 mins is fine. I can filter and average the data coming from the unit.
I have been using the same 2 12v batteries to supply the plus minus voltage to the chip. The datasheet says that it can onlt take a max of 22Vdc so i used a potential divider to take the voltage doen to plus minus 10VDC

 

The design of a lousy old 741 opamp is 52 years old. It was designed to use only a +15V and -15V supply but some might work with +6V and -6V. It is too noisy to use a gain of 470 times.

Since your ADC does not accept negative input voltages then use a modern low noise rail-to-rail opamp without a negative supply.

The biggest issue that i had with this unit was that i was getting a differntial voltage of around 10mV. As soon as i come to aplify this the voltage was exceeding the limits of the ADC 0-3.6V.
So, i needed to use some sort of ofset nulling to negate the offset voltage hence going down the route of the 741OPA. As mentioned in my previous post to Eric, i am not overly concerned with the noise as i only really need a reading once every 10 mins or so. I can look into filtering or averaging the values to negate the noise from the unit.

 

hi Shak,
If you try to drive the bridge with high currents you could see changes in output due to self heating of the strain elements.
As you are using +/-12V supply to the 741, ensure that you do not over voltage the ADC input.

Is this LTSpice image close to your circuit design.?
E

 

hi Shak,
If you try to drive the bridge with high currents you could see changes in output due to self heating of the strain elements.
As you are using +/-12V supply to the 741, ensure that you do not over voltage the ADC input.

Is this LTSpice image close to your circuit design.?
E

Yes, the spice design is close to what i am building.
I was trying to use the offset null to not over voltage the ADC.
The differential strain output is very small. Even eith these gains i am seeing only a few mV of change from a unloaded to a fully loaded system.
I wanted to only get a couple of volts of change from load to unload to not be in the noise floor of the ADC (if all of that makes sense)

I appreciate all of your help E. Very insghtful !

 

As already mentioned, 741 op-amp is 50-year old design. There are much better op-amps to choose.

You can use a single supply op-amp or you can use a split-rail op-amp with a single power supply if you create a pseudo-ground by splitting the power supply rails.

If you are using a bridge circuit for input you may be ok with just a single supply op-amp circuit.

In any case, you need to remove the DC offset. There are ways to do this.
1) You can balance the strain gauge bridge with a trimmer resistor.
2) You can balance the op-amp input offset with the op-amp null inputs.
3) You can subtract the offset with a trimmer connected to the op-amp inputs.

I would split the high gain requirements into two op-amp stages.

 

hi,
Measure the bridge millivolts out when the bridge is carrying its maximum load.
Let us know the mV
E

 

hi,
Measure the bridge millivolts out when the bridge is carrying its maximum load.
Let us know the mV
E

So, the differenial output is 10mV and then once full load is applied we get a 12mV output. This is with no aplification.

Cheers,

 

As already mentioned, 741 op-amp is 50-year old design. There are much better op-amps to choose.

You can use a single supply op-amp or you can use a split-rail op-amp with a single power supply if you create a pseudo-ground by splitting the power supply rails.

If you are using a bridge circuit for input you may be ok with just a single supply op-amp circuit.

In any case, you need to remove the DC offset. There are ways to do this.
1) You can balance the strain gauge bridge with a trimmer resistor.
2) You can balance the op-amp input offset with the op-amp input null inputs.
3) You can subtract the offset with a trimmer connected to the op-amp inputs.

I would split the high gain requirements into two op-amp stages.

I do have a couple of different OPA i have the MCP6004 and the 741.
The trimmer resistor idea is a good one. This will balance the bridge and negate the need for the offset nulling.

Then a sigle polarity supply can be used.

 

hi SJ,
The MCP6004 would be a good choice.
E

 

hi SJ,
The MCP6004 would be a good choice.
E

Thats great thank you for your time and effort E!

I will use a trimmer resistor to offset the DC by running it paralell to one of the legs of the bridge. This should solve the issues i was seeing hopefully!

Thanks Again
SJ