5V, Low-Power, Voltage-Output,
Serial 10-Bit DACs
REFOUT
C
REFOUT
TEK 7A22
R
S
C
S
TOTAL
REFERENCE
NOISE
1.8
SINGLE POLE ROLLOFF
REFERENCE NOISE (µV
RMS
)
250
C
REFOUT
= 3.3µF
200
150
100
50
0
0.1
1
10
100
FREQUENCY (kHz)
C
REFOUT
= 47µF
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1000
MAX504-FIG02
300
code 0000.... REFIN’s input capacitance is also code
dependent and has a 50pF maximum value at several
codes.
If an upgrade to the internal reference is required, the 2.5V
MAX873A is suitable: ±15mV initial accuracy, TCV
OUT
=
7ppm/°C (max).
MAX504/MAX515
Logic Interface
REFERENCE NOISE (mVp-p)
The MAX504/MAX515 logic inputs are designed to be
compatible with TTL or CMOS logic levels. However, to
achieve the lowest power dissipation, drive the digital
inputs with rail-to-rail CMOS logic. With TTL logic levels,
the power requirement increases by a factor of approxi-
mately 2.
Figure 2. Reference Noise vs. Frequency
Internal Reference (MAX504 only)
The on-chip reference is laser trimmed to generate 2.048V
at REFOUT. The output stage can source and sink current
so REFOUT can settle to the correct voltage quickly in
response to code-dependent loading changes. Typically,
source current is 5mA and sink current is 100µA.
REFOUT connects the internal reference to the R-2R DAC
ladder at REFIN. The R-2R ladder draws 50µA maximum
load current. If any other connection is made to REFOUT,
ensure that the total load current is less than 100µA to
avoid gain errors.
For applications requiring very low-noise performance, con-
nect a 33µF capacitor from REFOUT to AGND. If noise is
not a concern, a lower value (3.3µF min) capacitor may be
used. To reduce noise further, insert a buffered RC filter
between REFOUT and REFIN (Figure 2). The reference
bypass capacitor C
REFOUT
is still required for reference sta-
bility. In applications not requiring the reference, connect
REFOUT to V
DD
(to save power and to eliminate the need
for C
REFOUT
) or use the MAX515 (no internal reference).
Serial Clock and Update Rate
Figure 1 shows the MAX504/MAX515 timing. The maxi-
mum serial clock rate is given by 1/(t
CH
+t
CL
), approxi-
mately 14MHz. The digital update rate is limited by the
chip-select period, which is 16 x (t
CH
+ t
CL
) + t
CSW
.
This equals a 1.14µs, or 877kHz, update rate. However,
the DAC settling time to 10 bits is 25µs, which may limit
the update rate to 40kHz for full-scale step transitions.
____________Applications Information
Refer to Figures 3a and 3b for typical operating con-
nections.
Serial Interface
The MAX504/MAX515 use a three-wire serial interface that
is compatible with SPI™, QSPI™ (CPOL = CPHA = 0), and
Microwire™ standards as shown in Figures 4 and 5. The
DAC is programmed by writing two 8-bit words (see Figure
1 and the
Functional Diagram).
16 bits of serial data are
clocked into the DAC in the following order: 4 fill (dummy)
bits, 10 data bits, and 2 sub-LSB 0s. The 4 dummy bits are
not normally needed, and are required
only
when DACs
are daisy chained. The 2 sub-LSB 0s, however, are
always
needed, and allow hardware and software compatibility
with the 12-bit MAX531/MAX538/MAX539. Transitions at
CS
should occur while SCLK is low. Data is clocked in on
SCLK’s rising edge while
CS
is low. The serial input data is
held in a 16-bit serial shift register. On
CS’s
rising edge, the
10 data-bits are transferred to the DAC register and update
the DAC. With
CS
high, data cannot be clocked into the
MAX504/MAX514.
The MAX504/MAX515 inputs data in 16-bit blocks. The SPI
and Microwire interfaces output data in 8-bit blocks, there-
by requiring two write cycles to input data to the DAC. The
QSPI interface allows variable data input from 8 to 16 bits,
and can be loaded into the DAC in one write cycle.
External Reference
An external reference in the range (V
SS
+ 2V) to (V
DD
- 2V)
may be used with the MAX504 in dual-supply operation.
With the MAX515 or the MAX504 in single-supply use, the
reference must be positive and may not exceed V
DD
- 2V.
The reference voltage determines the DAC’s full-scale out-
put. The DAC input resistance is code dependent and is
minimum (40kΩ) at code 0101... and virtually infinite at
SPI and QSPI are trademarks of Motorola, Inc. Microwire is a trademark of National Semiconductor Corp.
_______________________________________________________________________________________
9
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