Does OPA827 have "overdrive" protection or it needs an external one. What is the recommended input over-voltage protection (if needed) for this op amp?
Mircea,
The OPA827 input and output terminals are diode-clamped to the power supply rails by reverse-biased ESD protection diodes. For that reason, input signals that can swing more than 0.5V beyond the supply rails must be current limited to 10mA or less - see below. This can be accomplished by placing an appropriately sized resistor in series with the input terminal.
Mircea,
As long as the voltage supplies used to power OPA2140 can source and sink current, all you need to protect the input stage against over-voltage condition is to place the appropriately sized input resistors to limit the current to 10mA or less.
However, if the positive supply cannot sink current (or the negative supply cannot source current) as is the case when using most LDO regulators (they can only source OR sink current), you must also add Zener diodes or Transient Voltage Suppressors (see below) in order to provide the path for the current flow. Otherwise, even a relatively small input current, in the range of 100uA, may over time result in the increase of the supply voltage beyond the Absolute Maximum Rating of +/-20V (40V total) specified in OPA140 datasheet; this would destroy the device.
Mircea,
Shunt references could work in your application as long as they are fast enough to respond to over-voltage condition before the increase in supply voltage damages the part. Of course, the size of the feeding resistor, Rf, needs to be chosen in case of using shunt references for the OPA827 maximum expected output current – see Fig 1 below.
Having said that, a safer solution is to use the Transient Voltage Suppressors (TVS’s) which have very fast turn-on time and assure current path under over-voltage condition; using TVS's also allows you to use any voltage supply or regulator you like to power OPA827 since complete protection is assured by TVS's and the input resistors - see Fig 2 below.
Transient Voltage Suppressors must sit directly on the supply pins with decoupling capacitors across them as shown below so the series input resistors, R_series_1 and R_series_2, limit the input current and TVS's control the supply voltage in case of over-voltag input transients.
