AD7739 AIN Protection

Question

The AD7739 datasheet states that the device can operate with +/- 2.5 volt inputs. 
 The Absolute Maximum ratings for the AIN pins are listed as -0.3V to AVDD + 0.3V which would appear to contradict the input voltage range. 
 I wish to use the device with an external instrumentation amplifier operating with supplies of ca +/- 15V and a ouput voltage range of +/- 2V in normal operation. 
 How should I protect the input of the AD7739 from overvoltage ouput of the instrumentation amplifier.

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ADIApproved · Answer

Hi Lyn, 
 Adding diodes such as schottky diodes from the analog input pin to AVDD and AGND will protect the AD7739 against overvoltages. 
 Regarding the analog input range of the AD7739, the device can accept an analog input of +/-2.5V. The device operates from a single power supply so the analog input must be biased up. For example, if the voltage on AIN+ swings from 1.25V to 3.75V and the voltage on AIN- is 180 degrees out of phase, then the voltage AIN+ - AIN- = 3.75V-1.25V = 2.5V when AIN+=3.75V and AIN-=+1.25V. When AIN+=1.25V and AIN-=3.75V, the voltage AIN+ - AIN- = 1.25V – 3.75V = -2.5V. So, the ADC sees an analog input of +/-2.5V. However,please ensure the voltage on AIN+ and AIN- is between 0.2V and AVDD – 0.3V as per the spec for the absolute voltage on AIN+/AIN- on table 1 in page 3 of the datasheet. 
 About the input to the instrumentation amplifier, what magnitude and frequency are you using? 
 We have parts like the AD7190 that features an on-chip PGA with gains of up to 128 and an output data rate of up to 4.8kHz. You might find this helpful in your application. 
 Hope this helps. 
 Thanks and Best Regards, 
 Chris

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AD7739 AIN Protection