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I am designing an AA LPF for my 8-bit ADC running at ~13.1 Msps, to sample a buffered differential signal. My goal is to down-sample this stream to 200 Hz, effectively increasing my ADC resolution to 16 bits.

I am planning to use a simple RC AA filter with cutoff at 16.6 kHz to give me a wide "unattenuated" passband and minimal phase lag on the analog domain. The output of this filter is then buffered before an ADC. The circuit is as follows:

enter image description here

Since my specs require low noise, low offset voltage, zero-drift, high accuracy signals, I thought the LTC2066/7/8 would be a good choice. However, simulations show that the buffered output is highly distorted:

enter image description here

What specs cause this? The GBP of the op-amp should be enough to drive this. Analog.com's Filter Wizard does not recommend this op-amp for my design due to its low output impedance which may be a clue, but I'm not 100% sure how this would affect my design.

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    \$\begingroup\$ DC voltage at the input (0V in the sim) is below the (common-mode) input voltage range. This is not a rail-to-rail input opamp. \$\endgroup\$
    – G36
    Commented Nov 21, 2022 at 14:52
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    \$\begingroup\$ Even a rail-to-rail opamp wouldn't be able to output below GND signals... You need a negative supply, or level-shift your Signal \$\endgroup\$
    – tobalt
    Commented Nov 21, 2022 at 14:53
  • \$\begingroup\$ I don't need the full 0-5V range so I'm fine with a smaller headroom. Rohat's answer below highlights the fact that my input signal was actually -0.5V to 0.5V that caused the error. \$\endgroup\$ Commented Nov 21, 2022 at 15:18

1 Answer 1

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Because your signal has no offset and cause the op amp's output to clip. Since the op amp is running with single supply it won't be able to handle negative inputs.

If you put a DC offset to your AC source and re-run the simulation everything will be fine:

enter image description here

NOTE: I put a 500k load because the op amp's gain and phase graphs are given for RL = 499k.

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