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st-dt continous monitor
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st-dt continous monitor

ST/DT Continuous ESD monitors

The ST and DT series of continuous ESD monitors are new and improved designs over the CM-920xD series monitors. The new circuits allow for improved manufacturing and specifications while still providing low cost products. They also provide more trouble free operation with fewer false alarms due to operator movement and wrist strap handling.

The ST series monitors are Single Threshold units like the AEI-920x series. They detect when the input at the wrist strap connection is below the High resistance threshold or above the body capacitance spec. The DT series monitors also detect when the input to the wrist strap is below the Low threshold.

Wrist strap High Threshold Resistance 6.8 Megohms 10 Megohms 10 Megohms
Wrist strap Low Threshold Resistance n/a n/a 750 Kilohms
Bench Mat Threshold 10 Megohms 10 Megohms 10 Megohms
Body Capacitance 180 PF 100 PF 100 PF

Continuous ESD monitors like the ST/DT series operate by sensing fairly high impedance loads in the form of wriststraps and body capacitance on the operator connections and high resistance matting on the bench ports. Relatively small signals are used and sensed to detect the operators on the wriststraps and the bench mat grounding. Both of these things make them sensitive to the electrical environment that they are used in. Other equipment with similar characteristics like audio and measurement systems have the same problems.

The purpose of ESD monitors and matting is to drain off electrical charges to eliminate damaging potentials. Low impedances like a direct ground connection let the current get too high when discharge occurs and that is what can cause ESD damage. High impedances allow only small currents that don't cause ESD damage.

The worst offenders are electrical motors. Many electrical motors put electrical noise that cannot be filtered because the signal levels are so high. Motors like these probably should not be included in an "ESD Safe" environment in the first place. The signal levels can be so high as to cause an ESD event all by themselves. Note that this noise can come through the air or be conducted through the power lines themselves. Conducted noise can be reduced by filtering the power lines. Electrical noise induced through the air can only be reduced by moving the noise source away from the affected equipment.

Fluorescent lamps can be a significant source of electrical noise also. They radiate noise as the tubes turn on and off. The electrical ballasts radiate magnetic signals that are difficult to filter.

Continuous ESD monitors should be used in a relatively benign electrical environment. That also applies to the equipment being assembled. If your electrical equipment is causing alarms in your ESD monitors, then you should question whether that equipment should be there. Clean, noise-free power and good ground systems are requirements for the ESD monitors as well as the equipment that is being assembled.

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