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I am using ad8232 ECG with 3 electrode configuration,

Edited ad8232 used schematic

I need to protect circuit from any voltage more than 5V, since ecg measure only around 300mV with common mode signals. the circuit should protect from even high voltage such as 5V to 150V surge current max 50mA at 50Hz-200Hz range,

I have a doubt,

I hope to use bidirectional TVS/ESD due to all leads are on body. and same GND is on body for another application

How to choose correct TVS that will not affect ECG measuring signal range (0-300mV) at all. or with minimum affect, what parameters of TVS/ESD should I consider?

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    \$\begingroup\$ Add a schematics of the analog front end. \$\endgroup\$
    – Marko Buršič
    Commented Jul 21, 2020 at 7:56
  • \$\begingroup\$ @Marko Buršič i have added sch. \$\endgroup\$
    – kobi89
    Commented Jul 21, 2020 at 8:02
  • \$\begingroup\$ What surge current protection do you require? \$\endgroup\$
    – Andy aka
    Commented Jul 21, 2020 at 8:27
  • \$\begingroup\$ You can take a horse to water etc... \$\endgroup\$
    – Andy aka
    Commented Jul 21, 2020 at 9:19
  • \$\begingroup\$ @Andy aka I have updated 50mA max \$\endgroup\$
    – kobi89
    Commented Jul 21, 2020 at 9:35

1 Answer 1

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R3 and R4 serve for this purpose, they limit current through the in-built ESD diode which is 5mA maximum. V=180k*5m= 900V.

Using standard TVS may spoil the measurement due to leakage currents.

Read the document:

INPUT PROTECTION All terminals of the AD8232 are protected against ESD. In addition, the input structure allows for dc overload conditions that are a diode drop above the positive supply and a diode drop below the negative supply. Voltages beyond a diode drop of the supplies cause the ESD diodes to conduct and enable current to flow through the diode. Therefore, use an external resistor in series with each of the inputs to limit current for voltages beyond the supplies. In either scenario, the AD8232 safely handles a continuous 5 mA current at room temperature. For applications where the AD8232 encounters extreme overload voltages, such as in cardiac defibrillators, use external series resistors and gas discharge tubes (GDT). Neon lamps are commonly used as an inexpensive alternative to GDTs. These devices can handle the application of large voltages but do not maintain the voltage below the absolute maximum ratings for the AD8232. A complete solution includes further clamping to either supply using additional resistors and low leakage diode clamps, such as BAV199 or FJH1100. As a safety measure, place a resistor between the input pin and the electrode that is connected to the subject to ensure that the current flow never exceeds 10 µA. Calculate the value of this resistor to be equal to the supply voltage across the AD8232 divided by 10 µA.

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  • \$\begingroup\$ AD8232 should be able to protect itself from 5-150V 5mA surges at 50Hz-200Hz frequency, do i need additional protection circuits? \$\endgroup\$
    – kobi89
    Commented Jul 21, 2020 at 8:19
  • \$\begingroup\$ Not itself, rather with external series resistor which are 180kohm according to your schematics. The current will be less than +/-5mA if the voltage is less than +/-900V., so it meets your demand as it is. \$\endgroup\$
    – Marko Buršič
    Commented Jul 21, 2020 at 8:23
  • \$\begingroup\$ input voltage range is up-to Vs =3.3V , so having 150V 50mA surges wont be an issue since it handle it self with external 180KOhm resistors? \$\endgroup\$
    – kobi89
    Commented Jul 21, 2020 at 8:25
  • \$\begingroup\$ Yes, the board will sustain such surge without any modification and/or any external component. However it shall be protected with an enclosure, since you can damage the IC with ESD while touching it, only the inputs are protected. \$\endgroup\$
    – Marko Buršič
    Commented Jul 21, 2020 at 8:33

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