pH Probe Input and Amplifier
This is the only critical part of the entire
circuit — the wiring from the pH electrode BNC connector to pin 5 of U300B must
be as short and direct as possible. The reason for this is that this is an
extremely high impedance point, and any extra wire length here will act as a very
effective antenna, picking up any electrical noise in the area. If the
wiring in this area is kept short, there will be no problems. In my controller,
the IC butts up against the front panel BNC jack so that there is only about 1/4
inch of lead length between the BNC connector and the IC. Note that the pH
probe's shield connection is not connected to the minus supply, but to the reference
ground. Since the BNC jack will be electrically common with the case, this
means that the case (if metal) will be at the reference ground potential as well.
(A metal case is strongly recommended to help shield out interference.)
C352, combined with the probe input resistance,
forms a low-pass filter at the amplifier input. It will prevent even a sizable
amount of electrical noise at the input from getting into the circuit and causing
trouble. (Additional components yet to be discussed also help in this regard.)
The 3 dB point for this capacitor and the probe resistance is 1/(6.28*R*C), which
is less than 1 Hz. The tradeoff is that the RC time constant (the product
of the the capacitance and the gigaohm source resistance) is about 1 second.
This means it will take a few seconds for the capacitor to charge when there's a
change in pH, which will cause a noticeable settling time during calibration.
If you feel you don't need C352, please feel free not to use it. If you're
not sure, please include it, as it is capable of converting a very noisy input signal
into a quiet one. Since this is a very high impedance part of the circuit,
capacitor leakage here could be a problem. Usually, one uses low leakage (for
example, polystyrene) capacitors at points like this. In practice, the garden-variety
ceramic Radio Shack capacitor listed in the parts list works well, and is what is
used in my controller. This ends the discussion of the circuit's only critical
area. Keep the lead lengths here short and direct to prevent noise pickup
and no problems should be encountered.
C351 is a simple bypass capacitor. Ideally,
it should run directly between pins 4 and 8 to bypass electrical supply noise to
ground. C350 is an nonpolarized electrolytic capacitor. It, like C352,
is not strictly necessary, but exists to cut down the AC gain. If any 60 Hz
noise gets past C352, C350 will help to keep it from being amplified to any significant
extent. Again, unless you are sure you don't need it, please include it.
R352 is set to about 8500 ohms, giving a circuit
gain of about 17. This is what is sometimes referred to as the "slope"
adjustment in a pH circuit.
R354, R351, R355, and R353 are the offset
nulling resistors. This offset array compensates for the input offset voltage
of the op amp, and for the error voltage caused by the approximately 20 picoamp
input current. (20 pA * 1gigaohm = 20 millivolts, which would be amplified
to about 0.3 volts (0.3 pH error) if it wasn't canceled here. During calibration,
R354 will be adjusted to give a reading of 7.00 when the probe is in the pH 7 calibration
buffer. (Potentiometer R354 is sometimes referred to as the "Zero"
adjustment in a pH circuit.) R352 and R354 are the two potentiometers
that will have to be adjusted at calibration time, so make them easily accessible.
(On my controller, all the 15-turn potentiometers can be reached through holes in
the front panel.) Further discussion of this sort of nulling arrangement
can be found in The IC Op Amp Cookbook.