Differential input to single ended unipolar output

Hello Rohit,

I show in an issue of TI's Analog Applications Journal (AAJ) how to convert a conventional single-ended input/output Multiple-Feedback active filter, to one having a differential input. You can find the information on Pg. 37, of the 3Q-2009 issue:

 The active filter can be synthesized using TI's FilterPro active filter program, or TI's Filter Designer program:

The op amp you select to drive your converter requires a lot of forethought. It's not clear what supply voltage(s) you intend to use in the circuit.

Regards, Thomas

PA - Linear Applications Engineering

I intend to use a single 5V supply for the op-amp, if possible. If it is a necessity, +/- 5V supplies can be used. I have looked at that document actually. The filter shown in the document (Figure 6 on page 37) converts differential input to bipolar output, i.e. something that is both positive and negative with respect to Ground, if I have interpreted it correctly.

So, I need a bias to change the bipolar output to unipolar. Should I simply add an adder circuit (which means another op-amp is required) or is there a filter circuit that can do both jobs: differential to single and to unipolar? Please advise.

In addition, I was unable to find the differential input filter configuration in TIFilter Pro. Am I missing something? Is there an instructional video or document that explains how to use the FilterPro to design differential input to single output filters?

Hi Rohit,

FilterPro allows for the selection of the fully differential (FDA) configuration in Step 4 of the synthesis procedure. When you click on that selection the schematic is shown for that topology. A problem with that simple schematic and the one that follows for the actual filter is it has a wire connecting the two output of the FDA together. That is an error. However, you will be using a single-ended output and that shouldn't matter. The AAJ article shows you how the FDA circuit is then configured for the differential-input, single-ended output. You won't need to use an FDA for the filter in that case. 

You might be able to set up the differential in, single-ended out, filter circuit such that mid-scale would be 2.5 V, or some other dc level. Viewing Fig. 6 in the AAJ article, it appears that the ground point for R5 and C3 could be lifted and driven by a low-impedance voltage source. The impedance would have to be low to keep the point at a constant voltage and to keep from altering the filter's response characteristics.

Realize that no op amp can fully swing to 0 V when powered by a single, positive supply. That can cut out some of the intended input range for the ADC. You may want to consider adding a low-voltage negative supply for the op amp to use. The LM7705, -0.23 V low-noise charge may be an option. Then, you could power a low-voltage CMOS op amp with +5 V, and -0.23 V and still be within the specified operating supply range.

Take a look at the OPA350 for the filter/ADC driver. It is one of the easiest to use, highest performance op amps in the ADC driver role:

Regards, Thomas

PA - Linear Applications Engineering

I have one more question. In FilterPro, gain selection of less than 1.0 is not allowed. However, I need to convert a +/-5V signal (see differential signal definitions in my first post) to a 0 to 3.3V signal. That means that I need following gain and offset:
Vo = 0.33*Vin+1.65
This means that a gain of 0.33 is needed. FilterPro does not allow me to enter 0.33 as gain. How do I get around this?

Hello Rohit,

When I run into an active application where the gain required from the circuit is less than 1 V/V, I use a different tool other than FilterPro, or Filter Designer. Okawa Electric Design, of Japan, has a very good active filter design tool that I have employed. Here's the link:

sim.okawa-denshi.jp/.../OPtazyuLowkeisan.htm

Use the MFB calculator on the lower part of the page. If you use a quality factor of 0.707, a maximally flat Butterworth response will be obtained.

Another active filter resource that I use often is a filter program called Filter Wiz Pro. It will allow you to set the MFB up with a gain less than 1 V/V as well. This is not a freeware program, but in my opinion is well worth its low cost if you have to do a lot of filter design:

You would then apply the methods I explain in the cited AAJ article to convert the filter to the differential-in configuration.

Regards, Thomas

PA - Linear Applications Engineering