Differential Adjustable gain Instrumentation amp

[aq_mishu]

+lm324 +differential amp

Hey guys,
I need an Instrumentation amp for measuring a differential voltage across a sensor of 10mV that have to be amplified to exactly 1v. Thus I need to tune the gain using a VR. Now I want to use lm324 with single supply. the output (amplified voltage) have to be with respect to the ground. So i can easily use a DVM/panel meter or so... any ideas?? please if possible provide me a schematic.

Note: The CMR will be 0

Mishu~

 

[kender]

lm324 instrumentation amp

Mishu,

One thing in your description seems confusing. If the input of the panel meter is always exactly 1V (i.e. it's constant), how the user will see the changes in the signal? Could you tell a little bit about the whole application? What's the sensor?

- Nick

 

[FvM]

variable resistor instrumentation amp

There is a standard 3 OP instrumentation amplifier circuit, that has a single gain determining resistor. You should be able to find the circuit easily, or look for the TI INA128 as a single chip version of this circuit.

Typically, an instrumentation amplifier is designed with fixed precision resistors (e. g. 1% or 0.1% accuracy) rather than variable resistors. With a LM324, the accuray would mainly suffer from offset voltages and offset drifts. Thus designig the instrumentation amplifier with 1% resistors provides a better accuracy than this OP could ever achieve.

 

[aq_mishu]

Mishu,
If the input of the panel meter is always exactly 1V (i.e. it's constant),
- Nick

That was an example... i mean if it is 10mV, it have to be amplified to exact 1V. Thus if 20mV, then 2V. Actuallyi will measure small differential voltage across a shunt. (mostly for current sense...)

To FVM=>

The TI INA128 may not be available in my country and may not be affordable in price. If, then i will use it. other wise i have to choose different way. Since i need a exact gain, thus that one single resistor should be variable for fine tune. Because most of the time, here 1% tolarence resistor is not that much available... I'll also look for the same too.... Also 324 was intended to be used because it can work with single supply (no +/- 12 V required) so far I know... because providing a a symmetric supply will bring some extra job for me.

Now to all==>
Advice...

 

[FvM]

I mainly quoted the INA128 as example for the suggested circuit.

If you can't use precision resistors, a variable resistor can be used with the circuit of course. Please notice, that resistor mismatch also affects the common mode rejection of the differential amplifier. With badly matched resistors, it may need adjustment too.

My doubts regarding LM324 performance continue. You can calculate the expectable offset and drift behaviour on your own. At least an offset adjustment would be needed, the drift may be tolerable for a limited temperature range.

 

[aq_mishu]

hmmm... i dont need very much precition at this stage... in case of real precetion, ihave no way but to use the 1% tolarance or even the INA128 or something like that. I already have made a prototype and i'll do some testing on it... will let you guys know... let's see...

 

[aq_mishu]

Hmmm... i did this... but output is zero... i mean when no input (input shorted), then around 0.6v and when input of around 0.45-0.5v, then also same thing.... The image is the schematic... the pot is the Rg. set around 3.33k

Now any suggestion??

Also let me know that will be ok if i use differential input in lm3914 after a normal inverting differential (or non inverting) amplification?? just one single op-amp... but i have to use single Vcc as 9V. no way to use +/- 9V.

Also in case of differential measurement, shouldn't i measure it from the op-amp output to IN-LOW of the amp input??

 

[FvM]

As any differential amp, the circuit has a restricted common mode range. With floating inputs, the OP inpt current is forcing reliably a violation of common mode range.

 

[aq_mishu]

so what is happening here?? man... all i need is to measure 0-.5v (floating) with LM3914 as 0.5v equivalant/amplified to 5v with respect to ground/floating. I need an amplifier with single supply with finetune for gain. Now the INA of such Instrumentation amp IC is not available in my country. Some even never heard of it (my luck!!). Thus i have to stick it more common items... please suggestion... also let me know if the schematic is ok and supply as i mentioned that single supply. input voltage is DC ofcourse... i will take it from a sensor.