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Part Number: TIDA-00854
Hello, I'm using electrochemical sensors to measure different gases using this design. Some sensors need a bias voltage to operate correctly. I think the sheet says the difference between the Vreference and the Vworking is the bias but it's not clear to me. Can someone confirm this? And if it's true, how can I give positive or negative bias? Thanks.
The bias voltage is defined as the difference between the Reference electrode and the Working Electrode.
Vbias = ( Vworking_electrode - Vreference_electrode )
If you look at the simplified Potentiostat circuit:
The potential desired on the Reference Electrode (RE) is applied to the input on servo loop amp, labeled "Vcounter" above. The loop will maintain the set voltage at RE by adjusting the voltage on the Counter Electrode (CE) as needed.
Note in the circuit example above, "reference" is somewhat misleading...since the Vreference *input* is actually named for the transimpedance amplifier reference point - NOT the Reference Electrode - even though Vreference does set the potential on the *Working Electrode*. Do not confuse them...
The potential on the Working Electrode (WE) is set by the Vreference voltage input on the Transimpedance amplifier.
If the cell is a "zero bias" cell, then Vcounter and Vreference would be set the same.
This is where it can be confusing. The "reference" of the cell itself is actually the Working Electrode. The "Reference" pin just sets an internal operating point.
A sensor requiring a +300mV bias would require that the Reference Electrode potential be 300mV BELOW the Working Electrode potential.
A sensor requiring a -600mV bias would require the Reference Electrode to be 600mV ABOVE the Working Electrode potential.
So for a +300mV cell, assuming Vreference is +1V, Vcounter would need to be set to +700mV.And for a -600mV cell, Vreference would be set to +1V and Vcounter set to +1.6V.
SGX Sensortech has a nice application note, AN2, that shows how to bias "biased' cells.
Regards,Paul GroheTI Comparators (LAMPS-CMPS) Applications Group
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In reply to Paul Grohe:
Thank you very much Paul!!
In reply to Fons van Heese:
Yes. Send a friend request.
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