THS4551: Sudo Differential input attenuation to differential output

Hi John,

The only modification that needs to happen for the above MFB filter interface is replace the input 1.2kohms with 4.8kohms. Rest of the filter components could stay the same. I have attached a TINA-TI circuit simulation which shows a flat band response across frequency with ~300kHz cut-off if you want to look over.

Best Regards,

Rohit

THS4551_MFB.TSC

Rohit,

Thanks for the response. This all looks good by I will be running at 3.3V not 5V also a vocm of 1.65V since that is what the ADC called for. For some reason there is a odd clipping that occurs when I change these voltages . I see not voltages approaching the rails either on the inputs or outputs. can you explain and or fix this?

Thanks,
John

Hello TI,

Would it be possible to provide the design equations for above circuit, or a spreadsheet incorporating these?

I am also looking to modify this circuit, but for a cut-off of 43kHz and Gain options of 1 and 2 (using the ADS127L01's Low Latency filter, 128kSps and OSR=128).

sloa054e does not include a derivative which includes the 5Ω output resistor.

John,

If you are planning to run at 3.3V and not 5V supply, then you will be violating the THS4551 input common-mode voltage spec for the current circuit with 0 to 10V input voltage swing. Looking at the ADS127L01 datasheet, it has Vref nominally set to 2.5V which will require a 1.25V Vocm out of the THS4551. 

I have attached the modified circuit below that takes care of the output clipping on a +3.3V supply. The THS4551 Vocm can be set to 1.25V on a single +3.3V supply because it has rail-to-rail output.

Best Regards,

Rohit

THS4551_MFB_3.3V.TSC

Rohit,

Thanks this looks good. The ADS127L01 datasheet says the common mode input voltage should be VCM = (VAINP + VAINN) / 2. With a 3.3V supply doesnt that mean my Vocm should be 1.65?

Thanks,
John

John,

I think you are right that a 3.3V supply on the ADS127L01 will require a Vocm of 1.65V. If that's the case, then you will start seeing the THS4551 outputs to clip with 3.3V supply due to the limited input voltage headroom towards the positive supply while level shifting a 0 to 10V input. I think the only way to resolve this is by operating the THS4551 on +3.6V or higher supply voltage, while keeping its Vocm centered around 1.65V.

Best Regards,
Rohit

Rohit,

Thanks for all your help on this. I think I'm am fine with this design but can you point me to a good white paper on doing a simple nodal/stability analysis on a FDA. For example, I have no Idea how you can up with the modifications to the attenuation and I would like to be able to explain them.

Thanks,

John

Hi John,

You could look at the below link for understanding the stability of an FDA.

training.ti.com/ti-precision-labs-–-op-amps-fully-differential-amplifiers-–-fda-stability-and-simulating-phase

Best Regards,
Rohit

Hi Bernhard,

The MFB filter designed above is very similar to the details mentioned in the sloa054e document with a slight modification as you have noted that includes the 5-ohm output resistor. As such the design equations stay the same as the sloa054e document.

You should be able to use the Webench Filter Designer for your application to design a single-ended filter and then port it over to the FDA for the differential filter topology. It is important to note that the parallel or shunt capacitors will become half in value for the differential filter topology. The link to the Webench Filter Designer is provided here: www.ti.com/.../webench-filters.html

Let me know if you are having difficulties designing the 43kHz cut-off MFB filter.

Best Regards,
Rohit