AnsweredAssumed Answered

AD8055 differential amplifier

Question asked by S.Artz on Apr 4, 2017
Latest reply on Apr 5, 2017 by harryh

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

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