# R-om resistor for variable gain differential opamp

#### tronicsguy

Joined Oct 6, 2005
9
What should be the value of the offset minimizing
resistor (Rom) for a diff op amp with variable gain(0-10)?
Is it the max value of the gain?

TIA.

#### hgmjr

Joined Jan 28, 2005
9,029
Originally posted by tronicsguy@Oct 10 2005, 04:37 PM
What should be the value of the offset minimizing
resistor (Rom) for a diff op amp with variable gain(0-10)?
Is it the max value of the gain?

TIA.
[post=10912]Quoted post[/post]​
Hi tronicsguy,

Would it be possible for you to post a schematic of your diff amp?

hgmjr (400)

#### tronicsguy

Joined Oct 6, 2005
9
Originally posted by hgmjr@Oct 10 2005, 03:35 PM
Hi tronicsguy,

Would it be possible for you to post a schematic of your diff amp?

hgmjr (400)
[post=10913]Quoted post[/post]​
Its a typical diff amp config like here -

How the hell do u build a diff op amp with variable gain (0-10) whose gain is given by V0=Rf/R1(V2-V1).

#### hgmjr

Joined Jan 28, 2005
9,029
Hi tronicsguy,

I suspected that your differential amplifier was of the topology that you have indicated with your link.

Implementing a gain adjustment scheme for this amplifier will depend on the configuration of the signal source you are trying to amplify.

What is the nature of your signal source? Is it a floating signal source that produces a voltage in response to a stimulus such as would be the case with a magnetic read head or certain types of microphones or is the signal source made up of two different voltage signals that are each referenced to ground?

As for your question on what the value of the offset minimizing resistor should be, Rom is normally addressed by making sure that the two resistors that appear in series with V1 and V2 are equal to each other and the resistor in the feedback path and the resistor to ground at the positive terminal of the opamp are both equal to each other. When the resistors are selected to meet these conditions, Rom is satisfied. That is because the idea behind Rom is that opamps are generally designed to have nearly equal input bias currents. Even though these input bias currents can be very low in value, by designing the circuit so that the resistance seen by the positive and negative input terminals of the opamp are very nearly equal, the offset between the two inputs due to this input bias current can be held to a very small value and thus introduce only a very small error in the output signal.

Note that the differential amplifier in the link you provided is powered using a negative and positive supply. If you are considering the use of a single power supply you will need to make provisions for an artificial reference voltage whose value will be your input power supply divided by 2.

I will be happy to walk you through this process if you like.

hgmjr

#### tronicsguy

Joined Oct 6, 2005
9
Originally posted by hgmjr@Oct 12 2005, 04:02 AM
Hi tronicsguy,

I suspected that your differential amplifier was of the topology that you have indicated with your link.

Implementing a gain adjustment scheme for this amplifier will depend on the configuration of the signal source you are trying to amplify.

What is the nature of your signal source? Is it a floating signal source that produces a voltage in response to a stimulus such as would be the case with a magnetic read head or certain types of microphones or is the signal source made up of two different voltage signals that are each referenced to ground?

As for your question on what the value of the offset minimizing resistor should be, Rom is normally addressed by making sure that the two resistors that appear in series with V1 and V2 are equal to each other and the resistor in the feedback path and the resistor to ground at the positive terminal of the opamp are both equal to each other. When the resistors are selected to meet these conditions, Rom is satisfied. That is because the idea behind Rom is that opamps are generally designed to have nearly equal input bias currents. Even though these input bias currents can be very low in value, by designing the circuit so that the resistance seen by the positive and negative input terminals of the opamp are very nearly equal, the offset between the two inputs due to this input bias current can be held to a very small value and thus introduce only a very small error in the output signal.

Note that the differential amplifier in the link you provided is powered using a negative and positive supply. If you are considering the use of a single power supply you will need to make provisions for an artificial reference voltage whose value will be your input power supply divided by 2.

I will be happy to walk you through this process if you like.

hgmjr
[post=10943]Quoted post[/post]​

Thanks for the comments. I am almost near to completion. I just replaced the ckt with an inst amp IN114. Its great except that it needs a bit of current boosting.

can you suggest a simple current boosting ckt?
Thanks again.

#### hgmjr

Joined Jan 28, 2005
9,029
Originally posted by tronicsguy@Oct 13 2005, 04:08 AM
can you suggest a simple current boosting ckt?
[post=10964]Quoted post[/post]​
Hi tronicsguy,

One approach you might want to consider would be to use a unity gain power buffer.

An example of such a device is the LT1010 made by Linear Technology. The web site is at www.linear.com. The datasheet for this device shows several hook-up configurations. I saw that www.digikey.com has some of these devices in stock as of this writing.

hgmjr