Datasheet

MAX5894
REFIO must be buffered with an external amplifier, if heavy
loading is required, due to its 10kΩ output resistance.
Alternatively, apply a temperature-stable external refer-
ence to REFIO (Figure 18). The internal reference is over-
driven by the external reference. For improved accuracy
and drift performance, choose a fixed output voltage ref-
erence such as the MAX6520 bandgap reference.
The MAX5894’s reference circuit (Figure 19) employs a
control amplifier, designed to regulate the full-scale cur-
rent I
OUT
for the differential current outputs of the DAC.
The output current can be calculated as:
I
OUTFS
= 32 x I
REF
x 16383/16384
where I
REF
is the reference output current (I
REF
= V
REFIO
/
R
SET
). Located between FSADJ and DACREF, R
SET
is the
reference resistor, which determines the amplifier’s output
current for the DAC. Use Table 5 for a matrix of different
I
OUTFS
and R
SET
selections.
14-Bit, 500Msps, Interpolating and Modulating
Dual DAC with CMOS Inputs
28 ______________________________________________________________________________________
Figure 18. Typical External Reference Circuit
1.2V
REFERENCE
CURRENT-
SOURCE
ARRAY DAC
REFIO
EXTERNAL
1.25V
REFERENCE
R
SET
FSADJ
I
REF
10kΩ
DACREF
1µF
MAX5894
1.2V
REFERENCE
CURRENT-
SOURCE
ARRAY DAC
REFIO
FSADJ
I
REF
10kΩ
DACREF
1µF
MAX5894
R
SET
FULL-SCALE
CURRENT
REFERENCE
CURRENT
R
SET
(kΩ) OUTPUT VOLTAGE
I
OUTFS
(mA) I
REF
(µA) CALCULATED 1% EIA STD V
IOUTP/N
* (mV
P-P
)
2 62.50 19.2 19.1 100
5 156.26 7.68 7.5 250
10 312.50 3.84 3.83 500
15 468.75 2.56 2.55 750
20 625.00 1.92 1.91 1000
Figure 19. MAX5894 Internal Reference Architecture
Table 5. I
OUTFS
and R
SET
Selection Matrix Based on a Typical 1.20V Reference Voltage
* Terminated into a 50 Ω load.