AD9963 DAC common mode voltage

Hello,

The text associated with Figure 60 highlights your particular case.  See below.

Hello,

So what is the value of R in this example?

Hello,

Looking at the circuit schematic,  the "R" resistor is redundant since it serves the same function as RCML generating an additional common-mode offset.  It should not have been included in the circuit since its value would be 0 ohms.

Hi,

Ok. It make sense now.

I actually need to drive 50ohm load, so I would need differential buffered output. Figure 62 looks like potential circuit. According to my understanding, output common mode voltage depends on also Rb. I'm not sure how to connect TXCML in this case? So how to calculate, Rfb, Rb and Rs and how to connect TXCML, if I need 2Vpp differential output with 1V common mode voltage?

Hello,

You are correct in stating that a differential buffered output is required should you need to drive a differential 50 ohm (?) or more commonly differential 100 ohm load.  Comments are as follows:

1) Since you need an output common-mode voltage of >0.8 V than the TXCML pin must be set to 0.5 V using a 250 ohm resistor such that the 2 mA full-scale current generates a 0.5 V signal.  Note that this value is fixed and any additional is fixed and any additional common-mode offset would need to be generated with the 2 mA full-scale current flowing through parallel combination of RL  (i.e. RL/2 ) and "R" resistor if required to generate additional offset.
*Note that my previous comment of "R" resistor was somewhat incorrect since use of "R" resistor provides additional means of generating a common-mode offset if value of RL/2 is insufficient to generate the offset.

2) RL is chosen to achieve the max differential peak-peak "signa-dependent" voltage at DAC output.....assuming that the DAC output connects to a high impedance (buffered) device.  From an AC perspective, the DAC output can be modeled as +/-1 mA current source.  By setting RL to 500 ohms, one can generate a 2 Vpp differential signal since the differential load is 2*RL (1 Kohm) hence output swings +/1 V or 2 Vpp.

3) With RL selected to be 500 ohms, the additional common-mode offset from this portion of the circuit with IOUTFS=2 mA will be  0.5 V (=2mA * 500/2).  This common-mode offset is added to the common-mode offset of 0.5 V that establishes the required TXCML voltage resulting in overall 1 V common-mode offset seen at DAC output.  ................hence the value of "R" resistor can be set to 0 V since no additional shift is required.

Hello,

Thank you for further clarification of the circuit in Figure 60. I understood it now.

I'm not understanding circuit in Figure 62.

According to my understanding, output common mode voltage depends on also Rb. I'm not sure how to connect TXCML in this case? So how to calculate, Rfb, Rb and Rs and how to connect TXCML? if I need 2Vpp differential output with 1V common mode voltage?

Hello,

I believe a better buffered output solution is shown below.  Note the following:

-The LC filter is included so as to suppress the DAC "images" and common-mode clock feed through.  A 3rd order is shown but be increased to higher order if required.

-A negative supply is required by differential amplifier.  That said.....single supply op amp based solutions would likely require a negative supply to provide similar AC performance that extends outs to 50+ MHz.  The ADA4932 would use same 3.3 V supply as the AD9963.

Hello,

One last point.  Analog Devices has a wide selection of Quadrature Modulators that are designed to support direct interfacing to DAC's like the AD9963.  If this is your target application than perhaps worth checking these products out to see if one of them better serves your needs.

I/Q Modulators | Analog Devices

Hi,

This is the circuit I was looking for. I was planning to use AD8137 instead of ADA4932.

Hello,

The differential amplifier selected for your target application should meet your linearity requirements over the target frequency range while driving the lowest RLOAD expected.  Performance of the AD8137 is shown below for an output VCM=2.5 V and could degrade further with a output VCM of 1.0 V.