We're using the LTC2271. The analog inputs, could occasionally have voltages as high as 10 V. Under absolute maximum ratings on page 2 the datasheet says the range is -0.3V to VDD+0.2V (2.0V). But then note 3 on page 6 says:
Note 3: When these pin voltages are taken below GND or above VDD, theywill be clamped by internal diodes. This product can handle input currentsof greater than 100mA below GND or above VDD without latchup.
This seems to imply we can let the voltage go to 10 V so long as we use say a 100 ohm resistor to limit the current. But then the purpose of the absolute maximum rating on the voltage is unclear.
Or do we need to add clamps on Ain to limit the voltage to VDD + 0.2 V?
Using 100ohms to limit the current on the analog input is advisable if you are taking the analog inputs up to 10V. The clamps will protect the inputs and keep them within the abs max range of Vdd+0.2V.
Are you saying the internal diodes of the ADC will keep the voltage within the max voltage range (so long as we limit the current) or ones we add external to the chip?
Thanks for the help.
Yes, the internal diodes are sufficient as long as the current ls less than 100mA.
That's the answer I was hoping for since we have a severely space constrained board. And it's hard to find diodes that don't distort the signal.
I should note, I got a somewhat different answer from Analog support, but it made me wonder what the point of the internal diodes was:
You should clamp the voltages externally using schottky diodes. Note 3 could be clearer, but what they're tryingto convey is the internal diodes will conduct if theinputs go beyond the rails, and that they can withstand100mA without the device latching up. It's best to clamp those signals externally, as repeated overvoltagecan degrade the internal diodes.
We will in fact be going over Vdd+0.2 fairly often.
Can you forward me that email: firstname.lastname@example.org
I'll have a talk with them.
I notice that the LTC2271 demo board does not seem to use clamps on AIN1 and AIN2 (see page 8).
There is no need for clamp diodes on the demo board, there are internal diodes that will clamp the voltage and can withstand 100mA. The current is limited with resistors so the voltage should never get high enough to cause damage.