Datasheet
MAX5895
Reference Input/Output
The MAX5895 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.
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 MAX5895’s reference circuit (Figure 19) employs a
control amplifier, designed to regulate the full-scale
current I
OUT
for the differential current outputs of the
DAC. The output current can be calculated as:
I
OUTFS
= 32 x I
REFIO
- 1LSB
I
OUTFS
= 32 x I
REFIO
- (I
OUT
/2
16
)
where I
REFIO
is the reference output current (I
REFIO
=
V
REFIO
/R
SET
) and I
OUT
is the full-scale output current
of the DAC. 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.
16-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
MAX5895
1.2V
REFERENCE
CURRENT-
SOURCE
ARRAY DAC
REFIO
FSADJ
I
REF
10kΩ
DACREF
1µF
MAX5895
R
SET
FULL-SCALE
CURRENT
REFERENCE
CURRENT
R
SET
(Ω) OUTPUT VOLTAGE
I
OUTFS
(mA) I
REF
(µA) CALCULATED 1% EIA STD V
IOUTP/N
* (mV
P-P
)
2 62.50 19.2k 19.1k 100
5 156.26 7.68k 7.5k 250
10 312.50 3.84k 3.83k 500
15 468.75 2.56k 2.55k 750
20 625.00 1.92k 1.91k 1000
Figure 19. MAX5895 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.