Using the DAC AD5542, I need an output swing of -5V to +5V. I'm using the configuration included in the DAC datasheet. I'm also using a Vref of 5V and a AD8643 as the output buffer. I decided on the AD8643 because it's a part that's present in the rest of my circuit, but I can change it if it's necessary. I'm also sending the data to the DAC via a PIC18F4553, but it doesn't appear in the image.
Without connecting the opamp, the output of the DAC works correctly (a 0 to 5V swing), but when I add the opamp, problems arise. As I power on the circuit, the opamp behaves unexpectedly, and I meassure -3.4V at Vin-, -0.28V at Vin+ and -0.89V at its output. If I try to set a voltage with the DAC, the opamp goes to the negative rail.
The AD8643 has 4 opamps and, as I'm only working with this area of the circuit, I left the other 3 with their pins in the air. Does it really matter or should I connect them?
Maybe the DAC changes the output faster than the opamp can follow (I don't think so, because it has a 3V/us slew rate and a 3MHz bandwidth), or maybe It has to do with the powering up sequence.
There's an application note http://www.analog.com/en/circuits-from-the-lab/CN0181/vc.htm that uses the AD8657. Should I change my opamp?
I'm not sure if the problem comes from the opamp or from the DAC, so I also posted this in the DAC support zone