We have set up EVAL-CN0359 and have the following problem. We are measuring the voltage from J5 pins 2 or 3. These feed AD8253. We see the expected distorted square wave on the scope but it is floating up and down between the rails. Page 18 of the datasheet for AD8253 possibly implies that the inputs are not stabilized by a bias current path to ground in the CN0359 layout. See also fig. 6 of your article by Kitchin concerning "Common Problems When Designing Amplifier Circuits". Any advice here? Thanks--Fritz
Just an update on this in case someone encounters this problem: further discussion on this topic has been done directly through email. If you are using the IST LFS1505 sensor and are encountering this drift issue, please connect a 10 µF capacitor in series with pin 1 of J5 and 1 µF capacitors in series with pins 2 and 3. In addition, place 1 MΩ resistors on pins 2 and 3 of J5 to ground. The below diagram shows the connections for this:
This should stabilize the output signal of the U15 in-amp as shown below:
Please note that we are still working with Fritz on this; an update may be posted at a later date. Thank you.
Hello EZ! Below is a more detailed explanation of the fix above:
CN0359 setting: Excitation voltage: 0.3V, Excitation frequency: 100Hz~10kHz, Cell constant: 0.68/cm
300mV~400mV for all LFS1505 electrode
150nA~300nA for LFS1505 V1, V2 electrode
400nA~800nA for LFS1505 I1, I2 electrode
500nA~1.5uA for LFS1505 V1, V2 electrode
2uA~3uA for LFS1505 I1, I2 electrode
Thank you for your extensive comments. Here are some points that apply here:
(2) The vendor is not forthcoming here but I believe the electrodes are regular platinum with a physical "frosting" to increase surface area. They are shipped this way--it is not due to corrosion. I do not think there is an oxide layer with a new electrode. Operation under 0.3Vpp insures we are well below the electromotive potentials for any ions that could be involved in Redox (OH, H, K , Cl, Pt). Examination after long term operation does not show physical differences so I expect oxidation is not happening at least to a notable degree.(3)Your experiment is interesting. My intuition is that if we are dealing with very small offsets, the typical noise and precision issues for a reference electrode would not reveal this due to inherit inaccuracy. More important I don't think a DC voltmeter will work here. What I see in the lab is a very quick development of a polarization layer/drift that would upset an accurate measurement.