Instrumentation amplifier... Common mode

[SimonTHK]

In this video: YouTube - Lecture - 11 Instrumentation Amplifiers
In the first 5 min he talks about the instrumentation amplifier.

He says that V1 + V2 / 2 = Vci Where Vci is common mode voltage.
He says that V1 + V2 / 2 = Vco Where Vco is common mode voltage.

So gain is 1.

But quistion is then, what is common mode voltage? I guees its the averrage? What can we use it for?

He also says that the differential mode gain is 1.

I read alot, but it seems very confusing to me, also why i cant explain better what i dont understand. I hope someone can gief a better explanation of what this common mode voltage means and what difference it does?

 

[ericgibbs]

hi Simon,
A Inst Amp [IA] is configured to amplify the 'difference' of the voltages applied to to the two input pins and not the common voltage that the two input signals have in common.

ie: say the common mode input voltage is 2V and the two inputs are exactly at the same 2.0v voltage, the IA would not amplify the 2V,
If one input was at 2.1V and the other was at say 2v, the IA would amplify the 0.1V difference

 

[SimonTHK]

Okay. But where is the different in an IA and a differential amplifier? Dossent the DA does the same thing as you just mentioned?

 

[crutschow]

Okay. But where is the different in an IA and a differential amplifier? Dossent the DA does the same thing as you just mentioned?

Not quite. For example op amps are differential amps but op amps are not IA's. It's a little like saying all collies are dogs but not all dogs are collies.

The difference is that IA's are designed to have a high input impedance and high common-mode rejection with a fixed differential gain using minimal added components. You can make an op amp into an IA but it typically requires two or three op amps and significant added precision parts.

 

[SimonTHK]

When you say high input impedance, what does this mean? I know what impedance is, but is high input impedance a big resistance on the input? And why?

Why is the CMRR better in a IA than a DA?

 

[KeepItSimpleStupid]

Large input Z's don't disturb the circuit/system/transducer your measuring. There is a big difference between a EKG electrode and the voltage across a battery or a PH meter. OP amps have two basic problems because they are not ideal:
1) input bias current
2) input impeadance
3) input voltage offset
These effects also change with temperature.

The CMRR is better because the resistors in an IA are laser trimmed. After the IA is made, a laser can remove material of the resistor changing it's value. The CMRR is very dependent on matching of components such as resistors.

 

[crutschow]

OP amps have two basic problems because they are not ideal:
1) input bias current
2) input impeadance
3) input voltage offset

Perhaps some remedial math would help.

 

[KeepItSimpleStupid]

Good catch. It's bad when the number s less than fingers+toes. <G>

 

[crutschow]

Why is the CMRR better in a IA than a DA?

It's not necessarily better, but you are still comparing apples and oranges. An IA is a specific type of DA. DA is a generic term for any amp with differential inputs. It can have a high or low CMRR depending upon it's design.

An op amp is a differential amp with a very high CMRR. But when you add the resistors to make it an IA the resistors can degrade the CMRR. It's basically determined by the resistor match.

See this for a discussion of instrumentation amps and how they are made using op amps.

 

[SimonTHK]

hi Simon,
A Inst Amp [IA] is configured to amplify the 'difference' of the voltages applied to to the two input pins and not the common voltage that the two input signals have in common.

ie: say the common mode input voltage is 2V and the two inputs are exactly at the same 2.0v voltage, the IA would not amplify the 2V,
If one input was at 2.1V and the other was at say 2v, the IA would amplify the 0.1V difference

When you say "and not the common voltage that the two input signals have in common" does it mean that this is what the differential opamp does instead? If i have 5v and 5,1v into a DA what will it then gain?

 

[crutschow]

When you say "and not the common voltage that the two input signals have in common" does it mean that this is what the differential opamp does instead? If i have 5v and 5,1v into a DA what will it then gain?

What differential op amp?

You still seem to be confusing an IA with an op amp.

An IA is a DA that normally has a fixed, relatively low value of gain. An op amp (without added resistors) as a DA has a very high gain (the open loop gain), thus it typically only takes less than a mV difference at its inputs to generate a full scale output signal.

You can convert an op amp into a controlled gain differential amp using four resistors but it will have a lower input impedance and likely poorer common-mode rejection than an IA.

The IA will amplify the 0.1V difference between the two signals by the gain setting of the IA (as typically determined by an external resistor value).

 

[SimonTHK]

Ohhh k. I see its me talking wrong here.

Im talking about the controlled gain DA with 4 external resistors as you mention. If i put 5v and 5,1v into this controlled gain DA, will it then only gain the difference as the IA?

 

[crutschow]

Ohhh k. I see its me talking wrong here.

Im talking about the controlled gain DA with 4 external resistors as you mention. If i put 5v and 5,1v into this controlled gain DA, will it then only gain the difference as the IA?

For exact resistor values it will. But any tolerance change in the resistor ratios will also cause a small amplification of the CM signal. Thus a 1% resistor ratio error for a DA with a gain of one will amplify the 5V common-mode signal by 1/100 and give an output offset error of 50mV, which is half of the example 0.1V signal of interest.

 

[KeepItSimpleStupid]

Here is a datasheet to an IA that I have used. Note the wierdness of the output and the reference. https://www.electro-tech-online.com/custompdfs/2011/01/AD625.pdf Here is another IA https://www.electro-tech-online.com/custompdfs/2011/01/AMP02.pdf

Here is a differrential amplifier IC. https://www.electro-tech-online.com/custompdfs/2011/01/dn454fa.pdf

Note the big difference. The output of a differential amplifier is differential, the output of an IA is single ended.

This is similar to a couple of devices:
The BALUN transformer used for Coax to twinlead applications.
And balanced (XLR cnnectors) to unbalanced audio signals RCA. You can transmit audio signals a greater distance without noise using the balanced method. Any signal picked up from the environment will be cancelled.

For reference:
Twisted pair cable tends to resist inductive coupling.

and

Shielded cable tends to reduce RFI. For process signals the shield is grounded at the signal source only to avoid ground loops.

I added some extra stuff here and I hope I didn't confuse you.

I think we are confusing "difference amplifier" and "Differential amplifier" and IA's. I too am now seeing somewhat of a blur. Because of "differential probes for oscilloscopes", "Differential operation of a 2 channel scope (A-B mode)", and Instrumentation amplifiers. There is even somthing called pseudo differential amplifiers. The A-B operation of a scope is indeed pseudo differential because it contains a ground.
True differential amplifiers wound not require a ground. This can get complicated too because some require a bias return path.

I'm getting really confused.

If i have 5v and 5,1v into a DA what will it then gain?

would be either (5-5,1)*A or (5,1-5)*A where A is the gain of the amplifier.

Where common mode comes into play is say when a low level signal is transmitted where 50 or 60 Hz power is found. If the signal is transmitted differentially and received differentially, the 50 Hz noise nearly goes away. If it's received in a differential fashion, the noise of the 50 or 60 Hz is typically attenuated.

My head hurts.

 

[crutschow]

Note the big difference. The output of a differential amplifier is differential, the output of an IA is single ended.

Commonly the output of a differential amplifier is single-ended (for example most op amps). But there are some differential amps that have a differential output (such as those used to buffer differential input A/D converters).