Electronic – Protect ADC – Input HF

Yes, the AD6644 has a differential input stage. Citing its datasheet, the maximum damage free input voltage is your analog supply voltage. Which doesn't mean that's the maximum usable voltage, i.e. a point that isn't already clipping. That would be, as per the datasheet, p.3 a 2.2V betweeen AIN and /AIN.

To stress this further (emphasis by me):

The AD6644 input voltage range is offset from ground by 2.4 V. Each analog input connects through a 500 Ω resistor to a 2.4 V bias voltage and to the input of a differential buffer (Figure 21). The resistor network on the input properly biases the followers for maximum linearity and range. Therefore, the analog source driving the AD6644 should be ac-coupled to the input pins.

In other words: you don't use a 65MS/s ADC to measure a DC voltage. You use it with capacitors in series to its inputs, and the ADC itself establishes the 2.4V bias. It doesn't really matter what absolute voltage offset your analog signal has – that offset doesn't pass through the capacitors – it's only important that the peak-to-peak voltage doesn't exceed 2.2V.

The best way is offset and less attenuation.

Spec

• yes convert 20~30 to 0 to 5V with 0.1% accuracy if poss.

Rev A

• 22~30 converted to 0~4 becomes Vin * 0.5000 -11.00V = Vout
  • 11.000V = k* 4.380V using k= 2.5114
  • with a non-inv OpAmp of gain 1+Rf/Rin
• one way: V+/2 - 11.00 V using precision Vref or programmable zener.
• using R array with matched R to <0.1%
• This will be 3x better resolution using 1/2 instead of 1/6 attenuation
• ensure your connections are twisted pair and filtered with low ESR RF cap

What supplies are avail?

p.s. I grew up in awe of Aurora , chasing smoke trails that appeared to be at ground level, watching sky waves like the ocean for hours at night and the fiery colours in Churchill Mb at the NRC rocket research range for Plasma Physics.