Variable Gain OP-Amps

[awan]

variable gain op amp

Hi;

I want to implement a gain that can be varied from 10 upto 50 for frequencies upto 250Hz.

I have been searching some OPAMPs and think there is a method to vary gain by changing resistance between two nodes of the OPAMP. I will need to use an electronic potentiometer so that my microcontroller can change the gain.

Does someone have experience on designing such circuit with a Variable Gain OP-Amp and an electronic potentiometer.

Thanks

 

[Audioguru]

variable gain op amp circuit

It is easy to change the value of the feedback resistor with a digital pot to make a variable gain opamp circuit.

 

[awan]

variable gain op amp, application note

Could you please recommend a commercial digital pot that can be used for feedback resistance of the amplifier. I can only find digital pots with I2C and SPI interface. I am not sure if they can be used for my application.

 

[FvM]

digital gain op amp

I can only find digital pots with I2C and SPI interface. I am not sure if they can be used for my application.

Yes a digital pot is supposed to have some kind of digital interface. You should know about the available control options in your application.

Furthermore, properties as usable voltage range, resistance, resolution, bandwith and capacitive load, volatile or nonvolatile operation have to be considered. Available devices differ in all of these parameters. It's simply analog circuit design. You may want to follow a manufacturers application note, if you don't have a clue.

For an overview of available devices, the product search offered by a catalog distributor, e.g. Farnell or Digikey can help you. Or browse product catalogs of e.g. Analog, MaximIC, Microchip.

 

[GetDeviceInfo]

digital pot variable gain opamp

I employ digital pots in a multistage amplifier circuit for variable gain, as well as in a window comparator ciruit to control window spread.

These circuits are used in an IR triggering system where the controller tunes the sensitivity of the circuit (gain / window) via the digital pots.

You may find that higher values are uncommon, making thier use in feedback for high gain restricted. This is where I found that the more common values fit between multistage nicely. I prefer I2C for it's reduced line count.

 

[LordAli]

variable gain op amplifier with pot

I would rather use circuit without digital pot. For example oamp with three ranges : 10x, 30x, 50x switched through electronic switch, like ADG412.

 

[kender]

variable gain op amp pot

I would rather use circuit without digital pot. For example oamp with three ranges : 10x, 30x, 50x switched through electronic switch, like ADG412.

Electronic switches can give you a bigger bandwidth than digipots. If your signal bandwidth is on the order of 250Hz, either would work. Here's an example schematic for ADG412 in an OpAmp feedback loop http://www.prolifictec.com/filehost/programmable_gain_peak_detector_rev01.pdf. Nothing magic, although that circuit was built and tested.

 

[hanifmoallem]

hi.
i have question. i want to control the amplitude of a sinusoidal voltage signal in 800khz with a dc voltage( gigital pulses).
can anybody help me?
recomend me an IC that do thi?

 

[enchanter]

If I just want to use some registers with transaction gates as switch to control the gain. What about the typically resisters value?

According to the TSMC 0.18um, and the formula Ron= 2LVdd/(uCoxW(Vdd-Vth)2). I think the Ron for the transaction gate should be around 2K. So the resisters should be greater than 200K to ignore the Ron's affect. Am I right?