AD8055 differential amplifier

Hello,

I am using the AD8055 as output stage for the DAC AD9767. As in the DAC Datasheet described, I use it as a differenital amplifier with the same resistor values (page 29 figure 74). I simulated the circuit and the following voltages should be with an open input:

Vin- = 0,711 V

Vin+ = 0,714 V

Vout = 2,13 V

Now I measured the circuit on my board. Therefore I opened the connection lines to the DAC, so the differential amplifier has an open input (just the 25 Ohm to ground and the 225 Ohm to the input). The output is open (without any resistor or capacitor).  There I measured the following voltages:

Vin- = 1,13 V

Vin+ = 0,719 V

Vout = 3,37 V

It seems so that the amplifier is completely at driven high (3,37 V is the maximum output). If I put a differential voltage to the inputs, the output voltage sinks but never goes higher (It is just one sided).

If I remove the 25 Ohm, the measured voltages are nearly the same as the simulated (new simulation with removed 25 Ohm resistors).

I also removed the AD8055 and put a new one in to make sure it is not damaged. I use a 5 V single supply (as it is mentioned in the AD9767 datasheet). The resistors have the right values.

Do I make a measurement mistake or is the circuit shown in the AD9767 datasheet wrong?

Thanks for the help.

Best regards,

Stefan

  • Hello Harry,

    that is my circuit. I would expect that the output is about 2,1 V if there is no inputsignal. I also tried to put the Input+ and Input- on 250 mV, but that also does not help bringing the Output to 2,1V. It is still about 3,3V. Putting a load (100 Ohm) at the output doesn´t help.

  • Hello Harry,

    thank you for your help. It works with +/-5V supply voltage.

    Just one question:

    In the datasheet of the AD9767, where the op amp circuit is described, is written that it is for single-supply systems. In that case, what is meant by "single-supply system"? I understood it that way, that I can use this output stage in a system with just +5V supply voltage, so that I do not have to create a negative supply voltage.

    Here is the mentioned part of the AD9767 datasheet:

    Can you explain to me how it is meant?

    I also simulated the differential op amp circuit with the spice model from the AD8055 website and it worked with a single-supply voltage of +5V (just for your information).

    Thank you!

    Best regards,

    Stefan