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Is the risk of the opamp overdriving the ADC too great for no input protection to be used? I would certainly say yes but, this can usually be easily solved by using a current limit resistor in...
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#2: Post edited
by
Andy aka
·
2020-12-17T10:28:52Z (almost 4 years ago)
- > _Is the risk of the opamp overdriving the ADC too great for no input protection to be used?_
- I would certainly say yes but, this can usually be easily solved by using a current limit resistor in the feed line to the ADC. Most ADCs specify a maximum current that their inputs can take. This is an overdrive level and doesn't normally apply when input signals are correctly bounded.
- $$$$
- But, on power-up situations the op-amp output may be able to deliver more than 25 mA at a voltage much larger that 3 volts so, a resistor is a simple and effective counter-measure.
- $$$$
The the maximum ADC input current spec is 5 mA then what resistor value will prevent more than 5 mA when 15 volts is applied to the ADC input? You can assume that the ADC input diode protection will try and clamp to a little above 3.3 volts so assume 3.5 volts. Then it boils down to: -- $$\dfrac{15\text{ volts} - 3.5\text{ volts}}{R} = 5 \text{ mA}$$
- So, R = 2.3 kΩ.
- > _Is the risk of the opamp overdriving the ADC too great for no input protection to be used?_
- I would certainly say yes but, this can usually be easily solved by using a current limit resistor in the feed line to the ADC. Most ADCs specify a maximum current that their inputs can take. This is an overdrive level and doesn't normally apply when input signals are correctly bounded.
- $$$$
- But, on power-up situations the op-amp output may be able to deliver more than 25 mA at a voltage much larger that 3 volts so, a resistor is a simple and effective counter-measure.
- $$$$
- If the maximum ADC input current spec is 5 mA, then what resistor value will prevent more than 5 mA when 15 volts is applied to the ADC input? You can assume that the ADC input diode protection will try and clamp to a little above 3.3 volts so assume 3.5 volts. It then boils down to: -
- $$\dfrac{15\text{ volts} - 3.5\text{ volts}}{R} = 5 \text{ mA}$$
- So, R = 2.3 kΩ.
#1: Initial revision
by
Andy aka
·
2020-12-17T10:17:41Z (almost 4 years ago)
> _Is the risk of the opamp overdriving the ADC too great for no input protection to be used?_
I would certainly say yes but, this can usually be easily solved by using a current limit resistor in the feed line to the ADC. Most ADCs specify a maximum current that their inputs can take. This is an overdrive level and doesn't normally apply when input signals are correctly bounded.
$$$$
But, on power-up situations the op-amp output may be able to deliver more than 25 mA at a voltage much larger that 3 volts so, a resistor is a simple and effective counter-measure.
$$$$
The the maximum ADC input current spec is 5 mA then what resistor value will prevent more than 5 mA when 15 volts is applied to the ADC input? You can assume that the ADC input diode protection will try and clamp to a little above 3.3 volts so assume 3.5 volts. Then it boils down to: -
$$\dfrac{15\text{ volts} - 3.5\text{ volts}}{R} = 5 \text{ mA}$$
So, R = 2.3 kΩ.