Electronics > Projects, Designs, and Technical Stuff
High speed variable gain amplifier
(1/3) > >>
ricko_uk:
Hi,
I have to make a 250 KHz variable gain amplifier controlled by a micro.

The input signal can (in extreme cases) swing between -4V and +4V i.e. below GND. Initially I thought of the attached schematic using a digital pot. But that way the wiper (connected to the op-amp negative input) would go negative when the signal goes below ground. So it would go out of specs. And the rare digital pots that can go below ground are expensive.

Then I thought of using a DAC. But similar issue.

The variable high speed gain op-amps tend to be expensive... So what is the best low cost solution?

Many thanks :)



SiliconWizard:
A possible approach to lower cost is NOT to go with a dual supply at all.
To deal with a signal that can go below ground, you can shift it up (and attenuate it) so that it fits within 0/+5V, and then use a single +5V supply for both the digital pot and the opamp. Of course the opamp output will be shifted as well. What is it going to be used for?
ricko_uk:
Thank you Silicon Wizard,
it is used for detecting the amplitude of a sensor.

The sensor generates an AC signal proportional to the quantity measured. That amplitude, depending on the application, can vary between few tens of mV to 4.5V peak to peak - hence the reason for having a variable gain amplifier.

The output is then fed into a high speed peak detector to detect the amplitude and convert it into DC for a ADC.

I did think about shifting it but that would then introduce DC which could then reduce the ADC level to half (unless I add another stage - but because we have up to 42 channels it would impact the small board space available).

Any other suggestion? Or perhaps a refinement on your initial one?

Many thanks!! :)
SiliconWizard:

--- Quote from: ricko_uk on February 26, 2020, 05:28:59 pm ---I did think about shifting it but that would then introduce DC which could then reduce the ADC level to half (unless I add another stage - but because we have up to 42 channels it would impact the small board space available).

--- End quote ---

If you're going to feed the signal to an ADC, shifting the signal makes even more sense IMO (and is what we usually do)? As most ADCs out there can only deal with positive inputs anyway?
Marco:

--- Quote from: ricko_uk on February 26, 2020, 12:41:40 pm ---The variable high speed gain op-amps tend to be expensive

--- End quote ---
A MCP6S91 is too expensive?
Navigation
Message Index
Next page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod