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PulSAR ADC Buffer stage

Question asked by PDES on Oct 9, 2013
Latest reply on Oct 9, 2013 by maithil

Hi all,

 

I need to acquire eight 0-5V analog signals using a 14bit multichannel PulSAR ADC, like the AD7949.

The bandwith of each analog signal is some kHz and I use a 2-nd order sallen-key low pass filter to provide both filtering and buffering function. The sallen-key cell is based on the ADA4610-2, whose features seems suggest that it could be a good choice as ADC buffer for this application. The filter is followed to a passive RC filter in order to attenuate the opamp residual noise.

In order to protect the opamp from the overvoltage event I use a series resistor and a clamping diode as depicted below.

 

filter.jpg

  • Since the series resistor R1 contributes to vary the corner frequency of the sallen-key cell, could it be recommended  to use directly R2 as limiting resistor and place the clamping diode between R2 and R3 ?

 

If the opamp is powered at a voltage higher than the ADC, say +/-15V and supposing that the maximum current deliverable from the opamp (79mA in this case)  is below the maximum current that the ADC can tolerate (130mA ), could I avoid to use overvoltage protection on the ADC input ?

 

  • If I use resistor a clamping diode to protect the ADC input, what value of resistance could be reasonably used without affecting the ADC performance?
  • Is it preferrable to use a filter followed to a buffer stage ?

 

Thanks in advance

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