OPA862: Filter Design tool gain < 1 and differential signal handling.

Hello Corey,

I can help-out here.  I am looking through my notes on differential amplifier filters, whether with a fully-differential amplifier (FDA) or an ADC driver such as OPA862.

Could you share your filter characteristics or a schematic of what you liked from the tool?  I can also try and use filter tools I have access to to help construct your desired filter.

I also want to ask: do you need the high input impedance of the OPA862, or would you be interested in using an FDA from one of our FDA families?  I can support either path .

In general you have a few quick tricks and considerations for making an op-amp filter into an FDA/diff amp filter.  You mirror the filter setup and adjust the differential & common-mode parts (such as the resistor/cap between the two differential inputs).

Once I see a mock-up or help generate your filter, I can comment more.  From what I have right now, you are looking for an MFB filter as part of an attenuator circuit.

Best,

Alec

Hello Corey, I can help-out here.  I am looking through my notes on differential amplifier filters, whether with a fully-differential amplifier (FDA) or an ADC driver such as OPA862. Could you share your filter characteristics or a schematic of what you liked from the tool?  I can also try and use filter tools I have access to to help construct your desired filter.

I also want to ask: do you need the high input impedance of the OPA862, or would you be interested in using an FDA from one of our FDA families?  I can support either path .

In general you have a few quick tricks and considerations for making an op-amp filter into an FDA/diff amp filter.  You mirror the filter setup and adjust the differential & common-mode parts (such as the resistor/cap between the two differential inputs).

Hello Corey,

You can do this with either OPA862 or an FDA.  The key is to understand the noise gain, which is Rf/Rg in magnitude for FDAs, can be set while setting filter parameters.

I would recommend setting your Rf (feedback resistor) = R and your Rg (gain setting resistor) = mR.  You can adjust 'm' by increasing it from '1' to provide yoru attenuation.  When selecting or calculating the remaining R and C values, you can use this relationship to help.  Many filter tools use an R = 1k or 10k constant value for Rf, you would need to adjust this for Rg.

When I use filter software I have to show an active LPF with a gain of 1V/V and 0V/V, I get the following two representations:

G = 0.5 V/V

G = 1V/V

As you can see, the capacitor values must change to adjust the frequency roll-off and provide the low-pass filter function alongside the changed resistor values.  If you use a system of equations to represent how you want Rf and Rg to relate to each other for your attenuator, you can generate a circuit with provides attenuation and active LPF.

The discussion on whether this is beneficial in terms of performance when compared to a two-stage solution depends very specifically on the use-case, circuit purpose, and overall system needs.  I hope this helps orient you Corey, I appreciate your attitude and curiosity here.  

The tool I am using is an Ansys tool from their product suite called Nuhertz FilterSolutions.  I do not know if they offer trial use or demos, but it is a good sanity check against the old pen and paper methods.  

We also have a legacy desktop tool which some folks prefer over Webench called TI Filter Pro; I will attach it here:

8463.FilterPro.zip

The tool is limited to G (V/V) of 1 to 1000, just like the Webench tool.

Best,

Alec

Hi Corey,

As an aside; if you get stuck and want some guidance, I can create some more examples for you using the filter software to help you get un-stuck.

Trying to balance the teaching with the learning here .

Best,

Alec