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
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
We have some preliminary testing done with a TOPAC probe that we have available here and we have seen no anomalies on the output signal. We have tested it with varying resistor values in place…
Is there a conductivity probe connected on J5? If there is, can you send me the product page for it? Also, can you send a snapshot from the signal captured on your scope? I'll try to recreate the cause of the floating effect you're seeing
It is correct that a current return path is needed, especially for AC coupled signals. The CN0359 does not use AC-coupling on the signal. The current driving the conductivity probe enters through one of the electrodes (the one connected to pin 1 of J5 and/or pin 2 if it is a 2-electrode probe) and has its return path via the trans-impedance amplifier U19.
Nikko-I am attaching pics of the scope shots.
The first one is a time lapse showing the gradual drift of the overall signal. The other shots are of pins 1,2,and 3 w.r.t ground.
Thanks Glenn-I shipped out two sensors today and I would expect them to arrive late in the week.
Keep in mind that we see this same problem with 4-pole sensors from other vendors so if you have one on hand you may get started with that.
Thank you for sending the probes. We look forward to its arrival.
Do you have a tracking number you can share for the probes? Also have you tried this setup using precision resistors instead of the conductivity probe? Its a good exercise to go through to ensure any drift that you are seeing is in fact due to the probe/solution input or related to the hardware/board configuration.
Just wanted to leave a note saying Fritz has also sent me a probe to do some testing at our Wilmington site.
We have some preliminary testing done with a TOPAC probe that we have available here and we have seen no anomalies on the output signal. We have tested it with varying resistor values in place of the probe and was also able to get correct values with the readings and the output signal looks as expected concluding that the circuit works as expected with the given test cases.
We have received the probes yesterday and have been able to start the testing of the board with your probe. We are able to see and replicate your test case where the output of the in-amp swings to the negative rail.
We are now trying to explore the possible instances why this would happen. Lia and our team are sharing our data to gain better traction in solving this issue.
Please stay tuned on the advancements of our testing.