Ion Science MiniPID - Manual (Page 14)

MINIPID 3PIN MANUAL Ion Science Ltd
Page 14 of 27
Unrivalled Detection. www.ionscience.com
Electrical Installation
– Best Practice

An electrically grounded Faraday cage is required for MiniPIDs mounted near to, or on the outside of
an instrument for the sole reason of electro-static discharge that
may falsely give a “Lamp-Out” error
state. This is because electrical currents in the order of sub-picoamps generated within the sensing
cell are being carefully monitored by the internal electronics for a “lamp-out” occurrence. Thus any
spurious capacitive coupled emc discharge on an ungrounded case/covering will be transmitted to
these circuits and cause a false “lamp-out” error message to be registered on the signal line of The
MiniPID output.
This will be seen by the signal step changing from about 52 mV to about 32 mV. The duration of this
change will be dependent upon the severity of the close-coupled emc discharge
– it is self-resetting.
This cannot be designed-out within the product because it is part of The Signal and any attempt to
stop this other than by the use of a screening case over the whole product (particularly at the pellet)
will also effect VOC generated signal.

RF interference may affect the resonance detection only in the first second of power-up. Thus the
use of a Faraday cage will give more consistent calibrations because the same resonance frequency
will be detected each time on power-up.
Similarly circuits that use multiple MiniPIDs should have the power-up sequence for each module
staggered by about 0.5 s to ensure that power supply current surges that may cause voltage dipping
will not affect the common power rail to neighbouring modules. Or select the on-board regulator and
supply with 5V or more - to provide the local isolation of 3.3 V inside.

It is advised that if the MiniPID has been off for a period of time to pulse the power ON for about 2
seconds, then off and back on again to allow the transformer to stabilise to an ideal working state.

For maximum repeatability in sensitivity then with ambient temperature excursions of greater than +/-
8°C from power up state it is recommended to turn off for 0.5 seconds and then turn power back on
to re-set oscillator to resonant frequency. Typically the MiniPID will be ready within another 0.5
seconds after application power off. Often this is implicit in the applications.
PCB layout for EMC noise reduction
To optimise the performance out of the MiniPID it is recommended that micro-strip layout techniques be used
to reduce susceptibility to EMC noise:

To minimise the externally created noise superimposing itself onto the signal the lines should be
located close to the ground plane, balanced and directly coupled to a differential input Analogue-to-
Digital Converter (ADC) or differential input amplifier.

A separate signal 0V line should be connected direct to the 0V pin of the PID and run parallel with the
signal line to the differential input ADC or amplifier. This single pair of signal lines should ideally be
located between two ground planes or at least run for its full length directly over the top of a ground
plane.

Since the PID responds in 50-100 ms, you can include an RC network on both signal lines located
directly at the input of the differential input ADC or amplifier to remove 100Hz (and higher frequency)
noise.

While the MiniPID has its own internal screening, it is possible to achieve maximal noise reduction if
the entire MiniPID sensor is mounted within a Faraday cage, which should be electrically connected to
the ground plane.