OpAmp filter drive capabilities

Question

My application requires a high input impedance. Therefore I am using 2 x TLV313 OpAmps as unity-gain buffers for my ADS1296. They also provide an adequate common-mode output voltage (2.5 V) for my ADC. My signal of interest is 0 - 10 kHz. The ADCs modulator frequency (as I understand is the actual sampling frequency) is, depending on the mode, 512 kHz or 256 kHz, which puts it's Nyquist frequency to 256 kHz or 128 kHz. At and above the Nyquist frequency I would want to have significant attenuation of my input signal to not have components above that frequency folding back into my spectrum of interest.

However, I cannot build a reasonable (RC) low-pass at the input of the TLV313, because the capacitor to ground would decrease my input impedance, which I need. E.g., the TLV313 has an input capacitance of 1 pF and an RC low pass with a cutoff frequency of 25 kHz for example, in combination with a 10 kohm series input resistance of the TLV313 (recommended to decrease input current at a reasonable noise level), would already require a capacitance to GND of 630 pF (and hence strongly decreasing my input impedance). I am willing to build a strong filter (e.g., third order Chebyshev) at my TLV313 output stage. However, I only find that the TLV313 can drive up to 1 nF of a purely capacitive load (TLV313 OpAmps, page 17). How do I know if this works with a more complicated filter?

0. Am I overcomplicating things? I thought this is not something special you would want to do.
1. What is the usual/good practice to employ a strong filter after unity-gain buffers?
2. How do I know if my TLV313 will function with a complicated mix of R-L-C in case I use a higher order (Chebyshev) filter?
3. How far can I trust the LTSpice simulations? If it shows well behavior, can I take it for granted to some degree?
4. In the TLV313 datasheet the test conditions always relate to "R_L connected to Vs/2". What is meant by that? How can I imagine this connection?
5. What are the resistance limits for my TLV output/my ADC input? I read with another ADC from Analog that the series resistance towards the ADC cannot be larger than 2 kOhms, for the ADC to be able to buffer / get enough current. For this ADC I don't find this information.

Any input is highly appreciated!

Answer 1

I am willing to build a strong filter (e.g., third order Chebyshev) at my TLV313 output stage. However, I only find that the TLV313 can drive up to 1 nF of a purely capacitive load (TLV313 OpAmps, page 17). How do I know if this works with a more complicated filter?

Yes, I believe you are over-complicating things.

The anti-alias filter capacitor would be preceded by a resistor of several kΩ as you analysed. You can usually get a good feel for how low this resistance can be here in the data sheet: -

So, if you ensure you have at least 2 kΩ in series with your reactive components then it should not be a problem. However, the data sheet does suggest you can go much lower than this: -

In the TLV313 datasheet the test conditions always relate to "R_L connected to Vs/2". What is meant by that? How can I imagine this connection?

Imagine an op-amp powered by split + and - rails. RL would connect to the midpoint i.e. to 0 volts or GND. On a single rail application "10k to Vcc/2" implies 20k to Vcc and 20 k to GND.

What is the usual/good practice to employ a strong filter after unity-gain buffers?

That entirely depends on the filter. To answer this generally requires dozens pages of work so, to avoid this, indicate what filter you are considering.

How far can I trust the LTSpice simulations? If it shows well behaviour, can I take it for granted to some degree?

If a sim tells you something won't work then, it probably won't. If it tells you something does work then it might work providing you have done your homework, read all the applicable data sheets and not gone crazy in circuit ideas unless you are a proficient designer.