TLV272 Rail to Rail problem DIODES

mikel Guest #7231323 
Hello,

I need a expert tip for my problem by using a Operational Amplifiers 
TLV272IS-13 from DIODES. Rail to Rail, 3,3VCC to GND as power supply. In 
my circuit I use the TLV272IS-13 as a “Voltage Follower” to measure the 
net supply of 230V/50Hz over a rectifier bridge and a voltage divider. 
By reaching the max limit of net supply (300VAC) the Voltage Follower is 
by 3,3V. But unfortunately, the output signal of TLV272IS-13 is cut by a 
voltage of 2,92V.

I testes bigger capacities by power supply (22uF) between 3,3V and GND, 
but without positive result. I unsoldered all capacities in the circuit, 
unsoldered the pulldown, tested an pull up of 12k to 3,3V also I copied 
the circuit to an external board, but nothing helps the signal is cut by 
2,29V.

By using a simple 3,3V over an adjustable voltage divider I can go up to 
3,3V (3,25V) without problems, but if there is a frequency in the 
signal, the Rail to Rail does not work.
I changed to an operational amplifier type TLV272CDR from Texas 
instruments and it works perfectly. The 300VAC will be displayed as 3,3V 
after TLV272CDR.

Does I overlook something in the datasheet ?
Is TLV272IS-13 from Diodes not able do drive RtR ?

Mikel
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Achim S. Guest #7231343 
mikel wrote:
> Does I overlook something in the datasheet ?
> Is TLV272IS-13 from Diodes not able do drive RtR ?

The datasheet says "Rail to Rail output". But it does not claim a rail 
to rail input. If you use this OpAmp as a voltage follower, the input 
common mode range limits the usable voltage range.

The recommended Common Mode Input Voltage according to datasheet ends at 
VDD-1.35V

You may divide the input voltage by a factor of 2 and configure the 
OpAmp as x2-amplifier - then the output should look better.
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