Abstract: This application note provides some simple design rules for maximizing the ESD, EMI and general performance of your sensor conditioning IC and system. Guidance on the use of ground planes and decoupling capacitors is provided, together with tips on component location, PRT wiring and PRT element protection.
Tips
Index
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if you need applications support for any of Maxim's conditioner products
or additional help with any of the tips listed. We'd also like to hear
from you if you have some tips of your own you'd like to share.
Locate the power decoupling capacitor as close to the IC as possible.
All conductors, even a straight copper PCB track, exhibit inductance
effects at high frequencies. The greater the distance between an IC's
power-supply pin and its associated decoupling capacitor, the greater
this inductance will be. The high-frequency impedance produced by this
inductance adds to other circuit-impedance elements and reduces the effectiveness
of the power-supply decoupling, rendering the circuit more susceptible
to high-frequency EMI and ESD effects. This inductance can be minimized
by placing the power-supply decoupling capacitors as close as possible
to the IC power-supply pins.
Also see tips on using low-impedance capacitors and a ground plane.
Back to tips index
Where possible,use a ground plane.
A solid-copper plane presents the lowest impedance path. Using a ground
plane to connect all ground points on a PCB is the best way to ensure
that all circuit ground points are at the same potential. A ground plane
also reduces the effect of radiated EMI on the performance of a circuit
by reducing the electrical field strength in the vicinity of the ground
plane. Electrical noise, together with EMI and ESD performance, can be
significantly improved by the use of a ground plane.
Back to tips index
Use low-inductance capacitors for power-supply decoupling.
Some capacitor types exhibit higher series inductance than others. Higher
values of series inductance limit the effectiveness of capacitors when
used at high frequencies by adding to the series impedance of these capacitors
and lowering the circuit's tolerance to EMI and ESD effects. Ceramic
surface-mount capacitors generally offer low series inductance values
and should be used wherever possible. Electrolytic capacitors typically
have high values of series inductance, and in circuits where they are
required they should be bypassed by ceramic capacitors. The 0.1_F power-supply decoupling capacitors are specified for all Maxim sensor conditioner ICs; these should be surface-mount ceramic types.
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Protective gels and coatings on the PRT element cause hysteresis.
Any material that contacts the diaphragm of a Piezo Resistive Transducer
(PRT) can be detrimental to the performance of the sensor. Gels and protective
coatings often produce hysteresis effects that cannot be corrected by
sensor-conditioning techniques. The subsequent impact on sensor accuracy
and repeatability can be quite substantial. If a protective coating or
gel is to be used for media compatibility purposes, then it is important
to understand the properties of the protective coating with respect to
temperature (Tg, Tempco, creep, etc.), mixing ratio variances, aging,
absorption, etc.
Back to tips index
Protect the IC and PRT passivation layer during assembly.
An IC's passivation layer helps to protect the delicate silicon structures
prior to packaging. This layer is in intimate contact with the surface
of the IC and has little mechanical strength. Damage done to the IC as
a result of physical contact with the passivation layer during handling
or assembly may not be immediately apparent but could lead to device malfunction
or failure at a later time. Any compromise of the IC passivation layer
should be considered reason to scrap the assembly.
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Keep the electrical wiring connection between the PRT element and the ASIC as short
as possible. Long wires act as antennas that can pick up stray signals. They are
one of the prime causes of mediocre sensor performance in the brutal automotive
2MHz-to-2GHz at-200-volts-per-meter EMI tests. The sensor should be mounted
as close as possible to the conditioning IC.
The use of a ground plane will help reduce noise. If a ground plane is
not available, then try to run grounded PCB tracks as close as possible
to the sensor connection tracks, particularly the connections to the conditioner
inputs; see connections marked INP and INM on Maxim data sheets.
Back to tips index
Buffer the power input with a low-impedance resistor and capacitor as close to
the connector as possible.
The resistance of a circuit to overvoltage transients and transmitted
noise can be improved by the addition of a resistor and a capacitor in
the power-supply line. This arrangement helps to filter out unwanted noise.
A 10-ohm resistor and a 0.1_F capacitor will greatly reduce the effects of conducted EMI and potentially damaging overvoltage transients on the power-supply lines.
Back to tips index
Don't intermingle PRT wiring traces with other circuit traces.
The sharp rise-and-fall transitions of digital lines can couple onto
the sensitive analog signals produced at the sensor outputs. To help reduce
inter-track-coupling effects, use a ground plane or fill all unused space
of the printed wiring board with a ground conductor. If a noisy signal
line, such as a clock signal, must cross a more sensitive track, then
the coupling effects can be minimized by arranging to have the tracks
cross at 90°.
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Select the correct type of PRT die attach for the application.
Use RTV for low-pressure (<100psi) rear-pressure designs and top-pressure
designs; use epoxies for rear-pressure designs >100psi. Make sure
you select fluoro-silicon RTV to prevent expansion from silicon oil absorption
in oil-filled assemblies. Check the die attach for chemical compatibility
with the sensed media.
Back to tips index
Place the ESD/EMI capacitors adjacent to the connector pins.
Exposed connector pins present the most susceptible path for ESD strikes
in a finished unit. The cable that connects with the unit can act as an
antenna for picking up EMI. The conducted emissions that result from this
are normally the largest of any EMI-induced effect. Minimize the effect
of conducted ESD and EMI on the circuit by placing capacitors as close
as possible to the connector. The capacitors should ideally be connected
between the connector pins and the case. This ensures that the case and
all electrical connections are, as far as possible, at the same electrical
potential at higher frequencies. Be sure to use high-quality ceramic monolithic
capacitors specified for use in ESD protection circuits.
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Connect the circuit ground to the case with a resistive path to drain ESD charge.
Very high voltages can be generated on the ESD protection caps when
the sensor is handled by the conductive case, with possible damage occurring
when the connector is mated. Connect the circuit ground to the case with
a resistive path to drain any accumulated charge. The value of this resistance
is not critical. A few kilohms of resistance is fine for most applications.
Back to tips index
Tie unused IC inputs to VDD or GND, as appropriate, either directly or with a resistor.
Undefined, floating inputs can cause havoc with an IC's internal
operation. Stray EMI signals can couple to floating inputs and cause them
to behave as if driven by a clock signal. Maxim ICs have integrated pullup
or pulldown devices on digital inputs to help prevent these effects, but
to be safe always tie unused inputs to a known potential such as VDD or
GND.
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Minimize the number of wiring connections to the conditioning IC.
All interconnect systems have a "per-pin" cost associated with
them. The true cost of each wire connection encompasses not only the cost
of the wire and termination but also of any additional features such as
EMI or ESD protection devices. There are also per-pin costs associated
with calibration and test fixtures. Reducing the number of wire connections
to the conditioning IC reduces the per-pin costs. The MAX1452 and MAX1455
conditioner ICs allow the serial communications pin to be wired to the
conditioner's output pin. This provides the facility to program the
IC through the output connection, thus removing the need for a separate
communications connection.
Back to tips index
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APP 637: Aug 28, 2000
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AN637,
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Appnote 637
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