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.
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?
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
If you can check whether it's been measured before, that would be great. Thanks for suggesting CN0326, it's really helpful. In my case, both the pH and reference inputs are high impedance, so have to use a 4 channel amplifier.
Glad you found the information helpful!
I am still confirming the value. I will get back to you when it is available.
Thanks Chris, that actually helps. I am doing a pH interface design, so the input impedance is very critical as I will have to interface pH glasses with impedance up to 2Gohms. Major design decisions of selecting an Opamp are low bias current, low DC offset and it has to be a duel power opamp. Power consumption is not critical for this application.
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