ADC buffer

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Posted by YILMAZ KIRÇİÇEK

on Apr 13 2009 14:22 PM

Intellectual765 points

Hi,

We are using TLV274 in split supply configuration for some requirements.

For ADC we need to use a buffer with TLV274.Our ADC works fine 0V to 5V (stands for -0.3Vto 5.3V)

I use precision rectifier circuit to protect ADC against negative input. But we experienced that it has some drawbacks.

Sampling is 100 ksps, not very fast but scanning 33 channels with switches. Last chance I have using a resistor and low Vf diode as BAS85 to clamp -320mV. This requires 4.7K resistor.

But it will add some mode error because of leakage and diode capacitance. Could you please give me an advise please ?

Best regards.

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Posted by Miro Oljaca

on Apr 08 2009 12:43 PM

Intellectual2930 points

Hi KIRÇİÇEK,

Which ADC are you using and what is aquisition time?

Regards

Miro

Posted by YILMAZ KIRÇİÇEK

on Apr 08 2009 14:21 PM

Intellectual765 points

Hi Mr.Oljaca,

Weare using microcontroller internal A/D and it has 1.4us aquisition time and conversion time is 8.4us

I have to scan 33 channel within 1ms, so not reached the limit of ADC yet. (33ksps/100ksps)

I have enough time to increase aquisition time a little bit more than min. specified.

Thanks for your interest.

More details : Under normal circiumtaces ADC and the buffer will not see any negative voltage.

So when I by-passed precision rectifier and use a simple voltage follower result is quite satisfactory.

Precision rectifier degrade the overal accuracy. In some extraordinary situation there can be negative

inputs to any of my 32 apamp (8 pack). Instead of protectin all of them I preffered to protect only the

ADC against to negative input.

Posted by Miro Oljaca

on Apr 08 2009 14:39 PM

Intellectual2930 points

Hi KIRÇİÇEK,

Please can you give me a part number of uC so I can look internal sructure of ADC?

Regards

Miro

Posted by YILMAZ KIRÇİÇEK

on Apr 09 2009 02:06 AM

Intellectual765 points

Good morning,

The uC we used is PIC18F4620 and here is the internal structure.

Posted by Miro Oljaca

on Apr 09 2009 18:41 PM

Verified Answer

Verified by nancy!

Intellectual2930 points

I can see that absolute maximum ratings specify input voltage at 300mV over or under the rail. So VT=300mV.

Also current into the pin must be limited to 20mA. If curent exide this value I am asuming (This is Microchip part) that input ESD structure can be damaged.

If you know what is your maximum/minimum input voltage to the ADC, for example -10V you can calculate resistor that you will need to put between output of the opamp and input of ADC. 10V/20mA=500ohm. So this resistor with two diodes, one from GND to input and another one from input to Vcc will protect input of your ADC.

If you send me the rest of your schematic I can take a look.