What is the input impedance of AD8609?

Question

Planning to use AD8609 for pH application, which needs an amplifier with very high input impedance and low bias current. Couldn't find the impedance information in AD8609 datasheet.

krisf · Accepted Answer

Hi Hasan, 
 I have this plot of the AD8609 Ibias vs Vcm at 125C. 
 
 Based on the scope of these plots, we can get an idea of the worst case input impedance of the device. Taking data from one of the plots will give, 
 R(cm) = (4.5-0.5)/((370-270)E-12) = 40Gohms 
 Does that help? 
 May I also ask what the major considerations of your design are? How important is power consumption in your design? 
 Thanks, 
 Kris

krisf · Answer

Hi Hasan, 
 I cannot provide an exact value, but it should have an input impedance in the Gohm range. I will check whether this has been measured before. 
 We have a good reference circuit that you might also want to check; it uses the AD8603 which is the single option of the AD8609. Please see the link below: 
 CN0326 Circuit Note | Analog Devices 
 Thanks, 
 Kris

krisf · Answer

Hi Hasan, 
 Glad you found the information helpful! 
 I am still confirming the value. I will get back to you when it is available. 
 Thanks! 
 Kris

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What is the input impedance of AD8609?

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