Post Go back to editing

Role of the p-type JFET(MMBFJ177) in EC sensor channels

Category: Hardware
Product Number: EVAL-ADuCM355, ADuCM355

Regarding the EC sensor section in this evaluation board, I would like to understand the specific role of the P-type JFET (MMBFJ177) to determine if it's necessary for my system.

I would greatly appreciate it if you could elaborate on its function. Thank you for your time and help.

    • Electrochemical sensors act like high-capacitance batteries. When your hardware is entirely unpowered, leaving the W and R electrodes open and floating, allows stray charges to accumulate. This causes the internal chemical cell to polarize.
    • If the cell polarizes while the board is off, it can take hours to re-equilibrate and stabilize once power is restored. By shorting W and R together when the system is unpowered, you force a 0V bias across the cell, keeping the sensor entirely depolarized. When the system boots up, the sensor stabilizes almost immediately, yielding valid concentration readings in seconds rather than hours.
    • One final caveat; In theory drain to source and source to drain junctions on the Pch JFET should conduct equally in both directions. BUT ….there is some leakage current (Igss) across the reverse bias gate junction (1nA @ 25 C for the MMBTJ177) that cannot flow towards the W electrode. The leakage current (Igss)  will simply sum with the sensor output current. Not good! So ensure source is connected to R electrode and drain is connected to W electrode. This will eliminate any contribution of this leakage current to the EC sensors current measurement.

Before You Switch


Switching languages will make ADI Explorer unavailable. Resume your session by switching back to English and reopening ADI Explorer.