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AD8226 low output swing

Question asked by RogerW on Jun 5, 2013
Latest reply on Jun 6, 2013 by RogerW

I have a simple circuit using an AD8226 (see attached PDF). The Rg pins are unconnected so the Gain = 1. The amplifier is acting as a buffer between a joystick and an LPC1768 uC ADC input.

 

The Vcentre voltage is a reference from the joystick and in this system is 2.44V at all times. This is correctly divided down by R24 and R54 so that -IN = 1.85V.

 

The Vin voltage is the output from the joystick and swings from 0.00V to 4.40V. Again this is correctly divided down by R37 and R25 so that +IN swings from 0.00V to 2.88V.

 

If Vout of this configuration is simply the difference in the +IN and -IN voltages plus the Vref as I think it should be, then:

 

Vout (maximum) = 2.88 - 1.85 + 1.80 = 2.83V

Vout (minimum) = 0.00 - 1.85 + 1.80 = 0.00V (can't go negative as only a single supply rail)

 

In practise Vout swings from 0.00V to 2.54V i.e. it never gets anywhere near the expected 2.83V.

 

If anyone can tell me what I'm missing here, I'd be very grateful. Thanks,

 

Roger.

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