Clamp output of op amp

dazza000 said:

HI
I have an op amp feeding the input of an ADC. this ADC must only have 0v to +2.5v on its input. So My Question is How do I clamp the output of the op amp to strictly and only 0 to +2.5 vlolts in order to protect the input of the ADC. The supply of the op amp is -12v and +12v and so far in my practical circuit the output of the Op amp seems to drift outside of these limits. (but that is another question)
can anyone help
thanks
david

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Nigel Goodwin said:

What is your ADC?, if it's a PIC they have protection diodes, and a simple series resistor feeding the pin is all that's required (check my PIC tutorial hardware). If it's not a PIC, and it doesn't have internal protection, simply add external protection diodes, and use a similar series resistor.

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speakerguy79 said:

Do you need the full resolution of the ADC? If not, design your ckt so your signal will only fluctuate between 0.5V and 2V. Use a 2.5V zener diode in parallel with a schottky just to protect the input pin. Your ckt will then operate normally from .5V to 2V and in case anything goes wrong, the voltage at the input pin won't exceed -0.3V/2.5V (or thereabouts).

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dazza000 said:

could I add an lm358 with a 0 to 2.5+v supply to the output stage?

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audioguru said:

The minimum supply voltage of an LM358 is 3.0V.
Its max output voltage is about 1.2V less than its positive supply voltage.
Its max input voltage is about 1.5V less than its positive supply voltage.

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speakerguy79 said:

TLV2781 from TI

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dazza000 said:

looking at the data sheet for the TLV2781
the common mode input voltage range for this chip is −0.2 VDD+0.2

So if I am interperating this right, I cannot expect to send the output of my original op amp which swings (possibly) between +12 and -12 volts.

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given that i will be supplying the tlv2781 with 0 to 2.5 volts on its power supply.
Is this right?

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ronsimpson said:

Here is a method of controlling the output voltage swing of an op-amp. If the output is too high the top transistor turns on. If the output is too low the bottom transistor turns on.
I am working on an audio amplifier with controlled clipping.
Set the “top clip” and “bottom clip” voltage to correct values.
If the clipped output has less than 5 volts swing do not use the diodes.
If the clipped output is more than 5 volts swing add the diodes.

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speakerguy79 said:

This is a ridiculously long topic for such a simple problem

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