AD9706 Output buffer question

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

ADIApproved · Accepted Answer

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

Andrenaline · Answer

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