AD9706 Output buffer question

Hello everyone,

I'm currently working on a low power chirp generator.

Someone before me working on this, used the AD9752 (12-Bit).

I would like to replace the DAC for an AD9706 (12-bit), on the output I'd like to use an

differential to single ended buffer. The buffer has an symetrical supply.

The question I have is on this output interface. (see attached file for example)

On the AD9752 the output current is 20mA, so the voltage over R1 and R2 is

20mA x 25 Ohms is 0.5V

On the new situation the AD9706 gives 2mA output. So with the same R1 and R2 the

output voltage is ten times smaller.

How can I compensate for this voltage loss, in such a way that I preserve the signal specifications

of the AD9706?

Can I raise R1 and R2 to 240 Ohms without consequences?

Or is there another way?, and no I cannot use any transformer

  • 0
    •  Analog Employees 
    on Jul 16, 2012 5:38 PM

    Hi -

    You can raise R1 and R2 to 240 ohms without consequences. The OTCM pin on the AD9706 should grounded. With 240 ohms in use, the full scale voltage from IOUTA and IOUTB to OTCM is .48 volts. This is well with the compliance range.

    Figure 89 in the data sheet is a possible alternative for your application that uses fewer components.

    Thanks,

    Larry

  • Hi Larry

    I was confused about figure 89 in the datasheet, the caption says it is an

    single supply single ended buffer.

    Should that not be dual supply, if you look at the picture you can see -5V and +5V supply

    on the buffer amp.

    By the way can you explain how this circuit works, Why is the REFIO pin tied to the

    non inverting input op the buffer opamp. Does this has a reason?

    Figure 89 only uses IOUTA for making an symetrical signal, this gives less preformance than

    using both current outputs in the configuration I attached to my question, am I right?

    What is the value of Rs?

    The Capacitance (C) in figure 89, is that the 0,1µF capactor to decouple REFIO to ground?

    Thanks for you quick response larry

  • 0
    •  Analog Employees 
    on Jul 17, 2012 7:11 PM

    Hi Andrenaline -

    The ADA4899 op amp in figure 89 does not have a single supply it has two supplies. The caption is incorrect.You could replace the -5V supply voltage with ground and have this circuit provided Vmin is above ground.

    The REFIO sets the common mode voltage at the out put of the ADA4899. The REFIO voltage is Vref in the equation Vcm = Vref * (1 + Rfb/Rb) - Vout.

    Using both IOUTA and IOUTB will give you 6db higher amplitude and common mode rejection.

    Yes, the .1uf cap is a decoupling cap for the REFIO voltage reference.

                       

    25 ohms is a good value for Rs.

     

    - Larry