CN0359: Float of signal from Instrumentation Amplifier

Question

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?  T
hanks--Fritz

Ramparo · Accepted Answer

Hi Fritz, 
 
 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. 
 Thank you, 
 Glenn)

LBrodeur · Answer

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 
 
 I found the drift and DC bias only show on voltage channel of CN0359, current channel work well 
 LFS1505 electrode have a coating layer look like a kind of heterogeneous catalysis layer similar to platinum black, I don&rsquo;t know this forested surface is bring by manufacture or corroded by the solution. This coating more like come from manufacture because the electrode buried in resin have same color with outside. I believe there have a primary cell between oxide layer and metal body of electrode 
 I measured electromotive force in different solution by add another platinum reference electrode. You could repeat this experiment without platinum electrode:

Use a disposable paper cup to take 100ml tap water to make sure the solution isolated with any potential 
 Immerse LFS1505 conductivity sensor into the water 
 Use a multimeter with input impedance > 10MΩ at DC voltage function, set multimeter to DC V function 
 Immerse multimeter black lead pin into the water beside LFS1505, before immersion make sure lead pin is clean and no scratch 
 Connect multimeter red lead to each LFS1505 pin to check the voltage output 
 Use the same way to measure the current generated by LFS1505 
 Repeat above test with 1g salt added solution

I got the following measure result:

Tap water: 
 
 300mV~400mV for all LFS1505 electrode 
 150nA~300nA for LFS1505 V1, V2 electrode 
 400nA~800nA for LFS1505 I1, I2 electrode 
 
 Salt solution: 
 
 300mV~400mV for all LFS1505 electrode 
 500nA~1.5uA for LFS1505 V1, V2 electrode 
 2uA~3uA for LFS1505 I1, I2 electrode 
 
 Use AC coupling could eliminate the primary cell DC bias, only V1 and V2 electrode required, the DC bias remove filter introduced load to electrode, so the cell constant need to recalibrate. 
 Fortunately the excitation current path could use DC coupling, no huge capacitor add to system. 
 Only add a resistor load to V1 and V2 electrode will give help to reduce DC bias voltage, but DC bias voltage will change relate to concentration of solution, and sensor will aging fast. 
 There have obviously drift in the V1 and V2 output in 5ms half cycle (100Hz excitation frequency) no flat top on the square wave, the primary cell charge and discharge jam excitation signal, you never can get right result in theory. 
 From search:

four- electrode sensor typically used in concentrated solution measurement to remove stray resistance, they have larger cell constant 
 platinum black two-electrode sensor typically used in middle range conductivity solution or high chemical activity measurement, they have middle cell constant, platinum black could resistance to polarization 
 the electrode with coating must to be immerse into purified water to store, it will be damaged in dry state 
 bare platinum two-electrode sensor typically used in dilute solution measurement, they have smaller cell constant 
 
 Suggestions: 
 
 Add AC coupling circuit into V1 and V2 channel, careful select R and C to minimize introduced error. Because the primary cell&rsquo;s character relate to ion concentration, the excitation frequency and setup time should be set carefully. 
 Use exposed metal surface as V1, V2 electrode body, because there is no current through those electrodes and less corrosion in dilute solution measurement, use two-electrode configuration for concentrated solution.

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CN0359: Float of signal from Instrumentation Amplifier