Datasheet

If a transformer is not used, the outputs must have a
resistive termination to ground. Figure 17 shows the
MAX5894 output configured for differential DC-coupled
mode. The DC-coupled configuration can be used to
eliminate waveform distortion due to highpass filter
effects. Applications include communication systems
employing analog quadrature upconverters and requir-
ing a high-speed DAC for baseband I/Q synthesis.
If a single-ended DC-coupled unipolar output is desir-
able, OUTIP (OUTQP) should be selected as the out-
put, and connect OUTIN (OUTQN) to ground. Using the
MAX5894 output single-ended is not recommended
because it introduces additional noise and distortion.
The distortion performance of the DAC also depends
on the load impedance. The MAX5894 is optimized for
a 50Ω double termination. It can be used with a trans-
former output as shown in Figure 16 or just one 25Ω
resistor from each output to ground and one 50Ω resis-
tor between the outputs (Figure 17). Higher output ter-
mination resistors can be used, as long as each output
voltage does not exceed +1V with respect to GND, but
at the cost of degraded distortion performance and
increased output noise voltage.
Reference Input/Output
The MAX5894 supports operation with the on-chip 1.2V
bandgap reference or an external reference voltage
source. REFIO serves as the input for an external, low-
impedance reference source, and as the output if the
DAC is operating with the internal reference.
For stable operation with the internal reference, REFIO
should be decoupled to GND with a 1µF capacitor.
MAX5894
14-Bit, 500Msps, Interpolating and Modulating
Dual DAC with CMOS Inputs
______________________________________________________________________________________ 27
Figure 17. The DC-Coupled Differential Output Configuration
MAX5894
OUTQP
OUTQN
QDAC
14
25Ω
50Ω
25Ω
OUTIP
OUTIN
IDAC
14
25Ω
50Ω
25Ω