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Electronics - Minimum design for Electromagnetic Compatability and Electrostatic Discharge

Created; 19-01-2016, Changed; 05/07/2018, 10-07-2018
The circuit below is an RS232 or V24 to TTL digital levels buffer with specified ratings for withstanding Electrostatic Discharge and bandwidth limiting features for interference control. There are lower baud rate variant of this buffer that should be selected if a baud-rate less than 1Mb/s is to be used. I have added 100p COG/NP capacitors but you will note that there is no common mode choke fitted. My experience with designs similar to those on the previous page that needs to be good for a low power bench instrument that does not have earthed power supply and may have just one I/O port. In any case this has been found to be consistently adequate.

The circuit could be used as two RS232 ports but with no modem control lines Or one RS232 port with the minimal handshake control - The pin out configuration as a DTE with the two modem control lines DSR and DTR but in a less than standard way of hold up data if the buffer is getting full and allow data if the buffer is empty or nearly empty. Note RS232 is Identical to V24 spec but V24 specification is more specific (less open ended) and some of the signal names are slightly different but the connector pin number and function is identical.

This particular IC has a number of features that probably do not add much to the performance but the IC is good value for money anyway;
  • READY - signals when the internal power supply generator is generating the correct voltage. 
  • !FORCE OFF - use this for power saving modes.
  • FORCE ON - probably best to connect this to the !FORCE OFF pin and drive both pins together. 
  • !INVALID - this could provide extra useful information not found in a basic buffer.
  • There is no certainty whether the 100pF capacitors help significantly but they do not compromise and it is mostly likely that they do make the circuit more robust in both ESD and EMC regards.
  • Other capacitor values are the values show in the data-sheet but these can be increased in value as described in the data-sheet.
The other thing that could be useful is that the power supplies generated RS232_5V5- and RS232_5V5+ could be used to power a low power circuit such as an operational amplifier if the supply's were further filtered with 100R in series with 10uF filtering say? but also read the data sheets for the op-amp to see that power supply rejection ratio is met. If you were to do that the READY signal may also be useful.

In most cases MAX3224E which has a lower bandwidth variant would be a better choice regardless of cost. It is a good strategy to not design in any greater bandwidth than is required. Otherwise the industry standard similar to; DS1488 and DS1489 type buffers do the job well.

Electrostatic sensitivity 

CMOS Logic Integrated Circuits;
In about 1975 the 4000 "A" series CMOS logic were in use. The MOSFET gate were not protect but unlike a Bipolar Junction Transistor there is no inherent avalanche diode to prove any protection. These parts were very easily destroyed by touching a pin unless very good handling practices were used.

I found that to use these parts it was best to hold all the pins or place the IC with the pins in contact with the palm of my hand. Then to take a PCB that already has all other parts fitted and soldered and hold it in a way that my hand touched metal tracks on that PCB. Then transfer the 4000A CMOS IC to the PCB socket or solder it in. This was reliable but the parts were broken very easily and when the "B" series were launched the best thing to do was to not use the "A" series any more.

Metal Oxide Silicon Field Effect Transistor;
It may still be necessary to use unprotected MOSFET transistors for special applications and the SD210 series and BSV81 transistors had a little metal or conductive plastic tie joining all the pins together for example. In some cases you took the shorting component off after soldering the transistor to the PCB and having solder every thing else prior.  In some cases these can be substituted with analogue switches that both provide protection and have had very good leakage and charge injection specifications compared to what they were before 1990.

Other Integrated circuits;
The ICL7117 digital volt meter IC on the second page was very typical of many of its time in that if current were to flow into any pin from a voltage source greater than the power supply a parasitic component would cause the IC's functions or accuracy to be compromised. In this particular case a thyristor could be triggered that would short out the power supply until the circuit was turned off then on again. Many of these parasitic issues have been overcome in modern IC's but it is always worth avoiding the issues in order to achieve the best performance from an IC with analogue functions.

Extending the internal screen beyond the instrument;

The Inter IC bus circuit below works very reliably although these IC's do not have specifically good ESD protection they do have diodes to the supply's so that supply capacitor would absorb much ESD. There are many I2C ICs such as EEPROMS that work well holding for example calibration constants for a sensor of some type. In this case there could be four connectors each with a different address so the temperature of the probe connected to a specific connector can be interrogated.

Connectors tend not to be subjected to ESD testing so much and they are a vulnerability. In any case Keyboards and display are exposed to ESD a lot. Some connectors have space inside for a very small circuit so in any case the option of providing filtering and protection is not possible for a small company that would have to use off the shelf parts. The continuation of the screen shown would not could safely have an aperture such as would occur with a plastic housing this may in any case be no other option.

When there is no convenient component but something else may be usable; 
It is sometimes necessary to characterise components for operation beyond what is specified on the data-sheet. This occurs less often now because data-sheets may have characteristics for different temperatures and ageing. The limit in a semiconductor is the storage temperature so the performance will degrade but may still function above the operating temperature. Do check this with the manufacturer of the part they will help you. But do not operate or store any component at above the storage temperature. This is because internal glues may soften causing the chip to move damaging the part. This was the advise I received from Hammatsu on some of their scientific linear image sensors for example.

Blog Page discussion - based on a working example the Power Input filter is star point between the case and the 0V plane which is a necessary compromise for safety;