operational amplifier - Is this clamping voltage divider for a high-impedance input a good, robust design?

I have an AC input as follows:

1. Can range from ±10V to at least ±500V continuously.


2. Runs from roughly 1 Hz to 1 kHz.


3. Needs > 100 kΩ of impedance on it, otherwise its amplitude changes.



4. May occasionally be disconnected and subject the system to ESD events.

When the input is below 20V, I need to digitize the waveform with an ADC. When it is above 20V, I can ignore it as out of range, but my system needs to not be damaged.

Since my ADC needs a relatively stiff signal, I wanted to buffer the input for further stages (in those, I will bias it, clamp it to 0V to 5V, and feed it to an ADC).

I designed the following circuit for my initial input stage to get a safe, strong output that I can feed to further stages:

^{simulate this circuit – Schematic created using CircuitLab}

My goals are:

1. Ensure > 100 kΩ of impedance on the source.



2. Change a ±20V input to roughly a ±1.66V output.


3. Provide a stiff output.


4. Safely handle continuous high-voltage inputs (at least ±500V).


5. Handle ESD events without dumping much current/voltage onto the ±7.5V rails.

Here is my rationale for my circuit design:

1. R1 and R2 form a voltage divider, reducing the voltage by 12X.


2. The TVS diode reacts quickly to protect against ESD events on the input, dumping them to my strong ground, without dumping anything onto my (weak) ±7.5V rails.


3. The TVS diode also handles extreme overvoltage (sustained ±500V) by shunting to ground. It is past R1 to limit current in these cases.



4. D1 and D2 clamp the divided voltage to ±8.5V so I don't need a high-voltage capacitor for C1; being after R1, the current through them is also limited.


5. C1 decouples the input signal. It will be a bipolar electrolytic. It needs to have a relatively large capacitance to allow the 1 Hz signals to pass unaffected: $$\frac{1}{2 \pi R_2 C_1} \ll 1 \text{ Hz}$$ $$C_1 \gg \frac{1}{2 \pi \times 1 \text{ Hz}\times220 \text{ k}\Omega} = 8 \mu\text{F}$$


6. R3 and C2, with R3=R1, compensate for input current bias and offset in the op-amp (rather than just shorting the output to the negative input); also form a low-pass filter: $$f_c= \frac{1}{2 \pi R_3 C_2} = 36 \text{ kHz}$$

Is this circuit optimal for my goals? Can I expect any problems with it? Are there any improvements that I should make, or is there a better way to accomplish my goals?

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EDIT 1

1. 
   

   I'd originally said this needed to handle ±200V continuously, but I think ±500V is a safer target.

   
   
   


2. 
   

   In order for the TVS diode to work as is, R1 needs to be split into two resistors, here R1a and R1b, as suggested by @jp314:

   
   

^{simulate this circuit}

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EDIT 2

Here is a revised circuit that incorporates the suggestions received so far:

1. Zeners across the power supply (@Autistic).


2. Resistors leading into them (@Spehro Pefhany).


3. Fast BAV199 diodes (@Master; a lower-leakage alternative to the BAV99 that @Spehro Pefhany suggested, albeit with a maximum capacitance of around 2 pF rather than 1.15 pF).


4. TVS diode out front and upgraded to 500 V (@Master), so it handles only ESD events, protecting R1.


5. Dead short from op-amp output to negative input (@Spehro Pefhany and @Master).


6. Decreased C1 to 10μF (@Spehro Pefhany); this introduces a 0.3% voltage drop at 1 Hz which isn't as good as original the 220μF cap, but will make sourcing the capacitor easier.


7. Added 1 kΩ resistor R6 to limit the current into OA1 (@Autistic and @Master).

^{simulate this circuit}