MAX5441_09 Datasheet, PDF (9/12 Pages) Maxim Integrated Products – +3V/+5V, Serial-Input, Voltage-Output, 16-Bit DACs

+3V/+5V, Serial-Input,
Voltage-Output, 16-Bit DACs
additional 1µF between REF and GND provides low-fre-
quency bypassing. A low-ESR tantalum, film, or organic
semiconductor capacitor works well. Leaded capaci-
tors are acceptable because impedance is not as criti-
cal at lower frequencies. The circuit can benefit from
even larger bypassing capacitors, depending on the
stability of the external reference with capacitive loading.
Unbuffered Operation
Unbuffered operation reduces power consumption as
well as offset error contributed by the external output
buffer. The R-2R DAC output is available directly at
OUT, allowing 16-bit performance from +V_REF to GND
without degradation at zero-scale. The DAC’s output
impedance is also low enough to drive medium loads
(R_L > 60kΩ) without degradation of INL or DNL; only
the gain error is increased by externally loading the
DAC output.
External Output Buffer Amplifier
The requirements on the external output buffer amplifier
change whether the DAC is used in the unipolar or bipo-
lar mode of operation. In unipolar mode, the output
amplifier is used in a voltage-follower connection. In
bipolar mode (MAX5442/MAX5444 only), the amplifier
operates with the internal scaling resistors (Figure 2b). In
each mode, the DAC’s output resistance is constant and
is independent of input code; however, the output ampli-
fier’s input impedance should still be as high as possible
to minimize gain errors. The DAC’s output capacitance is
also independent of input code, thus simplifying stability
requirements on the external amplifier.
In bipolar mode, a precision amplifier operating with
dual power supplies (such as the MAX400) provides
the ±V_REF output range. In single-supply applications,
precision amplifiers with input common-mode ranges
including GND are available; however, their output
swings do not normally include the negative rail (GND)
without significant degradation of performance. A sin-
gle-supply op amp, such as the MAX495, is suitable if
the application does not use codes near zero.
Since the LSBs for a 16-bit DAC are extremely small
(38.15µV for V_REF = 2.5V), pay close attention to the
external amplifier’s input specification. The input offset
voltage can degrade the zero-scale error and might
require an output offset trim to maintain full accuracy if
the offset voltage is greater than 1/2LSB. Similarly, the
input bias current multiplied by the DAC output resis-
tance (typically 6.25kΩ) contributes to the zero-scale
error. Temperature effects also must be taken into con-
sideration. Over the -40°C to +85°C extended tempera-
ture range, the offset voltage temperature coefficient
(referenced to +25°C) must be less than 0.24µV/°C to
add less than 1/2LSB of zero-scale error. The external
amplifier’s input resistance forms a resistive divider with
the DAC output resistance, which results in a gain error.
To contribute less than 1/2LSB of gain error, the input
resistance typically must be greater than:
6.25kΩ × 2¹⁷ = 819MΩ
The settling time is affected by the buffer input capaci-
tance, the DAC’s output capacitance, and PC board
capacitance. The typical DAC output voltage settling
time is 1µs for a full-scale step. Settling time can be sig-
nificantly less for smaller step changes. Assuming a
single time-constant exponential settling response, a
full-scale step takes 12 time constants to settle to within
1/2LSB of the final output voltage. The time constant is
equal to the DAC output resistance multiplied by the
total output capacitance. The DAC output capacitance
is typically 10pF. Any additional output capacitance will
increase the settling time.
The external buffer amplifier’s gain-bandwidth product
is important because it increases the settling time by
adding another time constant to the output response.
The effective time constant of two cascaded systems,
each with a single time-constant response, is approxi-
mately the root square sum of the two time constants.
The DAC output’s time constant is 1µs / 12 = 83ns,
ignoring the effect of additional capacitance. If the time
constant of an external amplifier with 1MHz bandwidth
is 1 / 2π (1MHz) = 159ns, then the effective time con-
stant of the combined system is:
⎡⎣⎢_(83ns)²
+
(^159ns)²
⎤
⎦⎥
= 180ns
This suggests that the settling time to within 1/2LSB of
the final output voltage, including the external buffer
amplifier, will be approximately 12  180ns = 2.15µs.
Digital Inputs and Interface Logic
The digital interface for the 16-bit DAC is based on a
3-wire standard that is compatible with SPI, QSPI, and
MICROWIRE interfaces. The three digital inputs (CS,
DIN, and SCLK) load the digital input data serially into
the DAC.
A 20ns (min) logic-low pulse on CLR clears the data in
the DAC buffer.
All of the digital inputs include Schmitt-trigger buffers to
accept slow-transition interfaces. This means that opto-
couplers can interface directly to the MAX5441–
MAX5444 without additional external logic. The digital
inputs are compatible with TTL/CMOS-logic levels.
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