Rainbow Electronics MAX5302 - Manual (Page 6)

MAX5302
Low-Power, 12-Bit Voltage-Output DAC
with Serial Interface
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Detailed Description
The MAX5302 contains a voltage-output digital-to-analog
converter (DAC) that is easily addressed using a simple
3-wire serial interface. The IC includes a 16-bit shift
register, and has a double-buffered input composed of
an input register and a DAC register (see
Functional
Diagram). In addition to the voltage output, the amplifier’s
negative input is available to the user.
The DAC is an inverted R-2R ladder network that converts
a digital input (12 data bits plus 1 sub-bit) into an
equivalent analog output voltage in proportion to the
applied reference voltage. Figure 1 shows a simplified
circuit diagram of the DAC.
Reference Inputs
The reference input accepts positive DC and AC signals.
The voltage at the reference input sets the full-scale out-
put voltage for the DAC. The reference input voltage
range is 0 to (V
DD - 1.4V). The output voltage (V OUT ) is
represented by a digitally programmable voltage
source, as expressed in the following equation:
V OUT = (V REF · NB / 4096) Gain
where NB is the numeric value of the DAC’s binary
input code (0 to 4095), V
REF is the reference voltage,
and Gain is the externally set voltage gain.
The impedance at the reference input is code dependent,
ranging from a low value of 14k
Ω when the DAC has an
input code of 1554 hex, to a high value exceeding several
gigaohms (leakage currents) with an input code of
0000 hex. Because the input impedance at the reference
pin is code dependent, load regulation of the reference
source is important.
In shutdown mode, the MAX5302’s REF input enters a
high-impedance state with a typical input leakage current
of 0.001µA.
The reference input capacitance is also code dependent
and typically ranges from 15pF (with an input code of
all 0s) to 50pF (at full scale).
The MAX873 +2.5V reference is recommended for the
MAX5302.
Output Amplifier
The MAX5302’s DAC output is internally buffered by a
precision amplifier with a typical slew rate of 0.6V/µs.
Access to the output amplifier’s inverting input provides
the user greater flexibility in output gain setting/signal
conditioning (see the Applications Information section).
With a full-scale transition at the MAX5302 output, the
typical settling time to ±1/2LSB is 14µs when loaded
with 5k
Ω in parallel with 100pF (loads less than 2kΩ
degrade performance).
The amplifier’s output dynamic responses and settling
performances are shown in the Typical Operating Char-
acteristics.
Shutdown Mode
The MAX5302 features a software-programmable shut-
down that reduces supply current to a typical value of
4µA. Writing 111X XXXX XXXX XXXX as the input control
word puts the device in shutdown mode (Table 1).
In shutdown mode, the amplifier’s output and the refer-
ence input enter a high-impedance state. The serial
interface remains active. Data in the input registers is
retained in shutdown, allowing the MAX5302 to recall
the output state prior to entering shutdown. Exit shutdown
mode by either recalling the previous configuration or
OUT
FB
SHOWN FOR ALL 1s ON DAC
MSB
2R 2R 2R 2R 2R
R R R
REF
AGND
Figure 1. Simplified DAC Circuit Diagram
_____________________Pin Description
NAME FUNCTION
1 OUT DAC Output Voltage
2 CS Chip-Select Input. Active low.
PIN
3 DIN Serial-Data Input
4 SCLK Serial-Clock Input
8 V
DD
Positive Power Supply
7 GND Ground
6 REF Reference Voltage Input
5 FB DAC Output Amplifier Feedback