Difference between a operational amplifier and an instrument

[markdem]

Hi all, I am building a PH sensing device, and am stuck with deciding what amp to use. I don’t quite understand the difference between an operational amplifier and an instrumental amplifier. Would I have any benefit in using an instrumental amplifier over an op amp? From what I can gather, a instrumental amplifier is just three op amps connected, but I am not too sure what it does. Can any one please help me out??

Thanks, Mark

 

[electronics_kumar]

high cmrr ,input impedance,gain...than opamp

 

[antedeluvian]

Re: Difference between a operational amplifier and an instru

Mark,

An IA is a combination of 2 or 3 op-amps. It is differential in that there are 2 inputs to the IA, (call them V1 and V2). The output of the IA is equal to some amplification of the difference between the input signals, that is
Vout=A*(V1-V2). The advantage of the IA is that it gives you a high input impedance, good common mode noise rejection, and the ability to measure the voltage difference, sometimes with a high common mode voltage.

The output is referenced to the ground of the system, but on some IAs it is possible to add a DC offset by providing a DC voltage on one of the pins. that is the output would be
A*(V1-V2) +Vdc

Some IAs also allow a common mode voltage of the inputs to be greater than the supply voltage of the IA. The AD626 (Analog Devices) will allow a common mode voltage of +50V. There is a variation of an IA called a difference amplifier which normally has a skew on the input impedance to the two input voltages, and worse common mode noise performance. There are some of these that allow for common mode voltages of +/-200Vdc (TI: INA148). Note there are some circuit conditions where this range may not be attainable.

You should check out the Analog Devices, TI (BurrBrown)and Linear Tech websites for more information on IAs.


In particular
AN202 from Analog Devices seems to deal with this in detail.

Also check out
**broken link removed**
"A designers guide to IAs" which will give you a far better explanation than I could ever do.

-Aubrey

 

[lambtron]

Re: Difference between a operational amplifier and an instru

An instrumentation amp outputs the difference between its two inputs, times its gain. For example, assume an i-amp gain is set to 1, its positive input is +2V and its negative input is -3V. In this case its output will go to +5V, which is the difference of its input voltages. Note that an i-amp requires no external feedback to "close the loop."

If you apply the same input voltages to an opamp, its output will swing to the positive rail because it is running open loop.

 

[markdem]

Re: Difference between a operational amplifier and an instru

Thanks guys, that cleared al lot up for me.
I can now see that a in amp has a internal feedback network, internal gain resistors and better common mode rejection.
Am i right in saying that a in amp will be a far better device in a ph meter than a op amp?

Thanks, Mark

 

[antedeluvian]

Re: Difference between a operational amplifier and an instru

Am i right in saying that a in amp will be a far better device in a ph meter than a op amp?

Yes

-Aubrey