Are there any suggested protection schemes against ESD that should be considered with these products?
These converters are manufactured on a standard CMOS process and, therefore, all standard practices and protection schemes apply to these devices as with all other CMOS devices. There are ESD protection diodes on all the inputs that protect the device from possible ESD hits due to handling and production. These ESD protection diodes will clamp the voltage at any pin to within 0.5 V of the supplies. They can carry quite high currents but only for a short periof of time so, they can protect the IC from large pulses of short duration (the total energy is quite low). The latchup current is typically 100mA on all pins.
The maximum DC current that these protection diodes can withstand is 10 mA. Therefore, the maximum current that can be applied to any input is 10 mA. If it is possible for a current in excess of 10 mA to be applied to a pin due to an over-voltage, external protection is required. Protection schemes that can be applied include transzorbs on the power supply lines, series resistors on digital input lines, and resistors and diodes on analog inputs. For example, the external protection could be a resistor in series with the input pin to limit the current into the pin to less than 10 mA. For example, if the maximum overvoltage applied to a pin is 5V, a 1 Kohm series resistor in each line will limit the current to 5 mA.
There are a number of application notes and seminar material available on this topic. These are available on www.analog.com:
AN-202 Application Note, An IC Amplifier User’s Guide to Decoupling, Grounding and Making Things Go Right for a Change. www.analog.com/static/imported-files/application_notes/135208865AN-202.pdf
AN-311 Application Note, How to Reliably Protect CMOS Circuits Against Power Supply Overvoltage. www.analog.com/static/imported-files/application_notes/2915025627482692953221605948252614692AN311.pdf
AN-397 Application Note, Electrically Induced Damage to Standard Linear Integrated Circuits. www.analog.com/static/imported-files/application_notes/262799190AN-397.pdf
Overvoltage Effects on Analog ICs. www.analog.com/static/imported-files/seminars_webcasts/36900313320057Section7.pdf
In one of our application, we are connecting 4 wire load cell to the input of AD7797 through a connector, and the connector is accessible by everyone. What are the possible ways to protect the input up to system ESD?
The AD7124 has lower noise and lower current consumption than the AD7797. Would you like to consider using our newer product as it has all the building blocks of AD7797 with more advanced feature. It operates from a 3.6V max supply but it is worth considering if using a 3V supply is acceptable for your design. The AD779x family has an ESD rating of 1.5kV (HBM) and 250V(FICDM). The AD7124 has an ESD rating of 4kV (HBM) and 1250V (FICM). So, the AD7124 is a lot more robust than the AD7797.
The link below shows information on how to achieved the best radiated immunity performance from AD7192, taking into account the effects of PCB layout and parts placement. This can also be applicable for AD7124 and AD7797. You can use this as your reference and can be useful in designing your weigh scale if you are worrying about overvoltage due to some spikes in the presence of radiated immunity https://www.analog.com/media/en/technical-documentation/application-notes/AN-1186.pdf
For ESD, Protection schemes that can be applied include transzorbs on the power supply lines, series resistors on digital input lines, and resistors and diodes on analog pins. Just like stated on the previous replies, you can have a tranzorbs on the power supply to limit the overvoltage let say for example to 10V and then have a series resistor with the input pin for overvoltage of 10V to limit the current into the pin to less than 10 mA.
Thanks for the reply, We are operating AD7797 at 3.1V and according to datasheet maximum rating at analog input is AVDD + 0.3V i.e 3.4V in our case. Adding 500 ohm resistor in series will limit the current. But how can I limit the voltage up to maximum rating at analog pins? A suggestion of transzorb will be appreciated.
If an overvoltage is seen on the analog input, the internal ESD protection diodes will clamp the voltage at any pin to within 0.5 V of the supplies. These diodes can only tolerate a current less than 10mA so using external limiting resistors prevents the ADC from getting damaged. However, the external overvoltage may cause the on-chip registers to be corrupted.
To prevent the internal registers from being corrupted, the overvoltage needs to be directed away from the ADC. Connecting schottky diodes from each analog input to AVDD and GND or tranzorbs is one method that can be used. We do not have a standard recommendation for non ADI products but the schottky diode/tranzorbs must have a turn-on voltage of 300mV. This ensures that the external diode turns on before the ADC’s internal diodes so that the overvoltage is directed away from the ADC.
Thanks for the response. Placing tanzorbs near the connector (which is away from ADC) will direct overvoltage away from ADC.