dazza000

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

ssylee

Did you try having a rail to rail opamp with supply voltages of 2.5 and 0 V? I can't think of any cheaper solutions that might solve your problem

speakerguy79

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).

Leftyretro

Some A/D convertor can stand inputs above their measurement range. What A/D device are you using?

__________________
Measurement changes behavior

Nigel Goodwin

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.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

dazza000

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

dazza000

Er yes, Least significant bits are hard to come by: i need full scale resolution
dazza000

dazza000

Er yes, Least significant bits are hard to come by: i need full scale resolution
dazza000

dazza000

It is a LTC1297 12bit serial
dazza

dazza000

Would I need to limit the current from the op amp with a high value resistor before I feed the diodes?
dazza.

audioguru

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.

__________________
Uncle $crooge

dazza000

Hmmm, can you suggest a more versatile opamp?
dazza.

speakerguy79

TLV2781 from TI

dazza000

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.

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

audioguru

Rectify the 24V signal so it is only positive then attenuate the +12V down to +2.5V.

Yes, but the output load resistor must be very high or the output voltage will not reach +2.5V.

__________________
Uncle $crooge