Rainbow Electronics MAX5351 - Manual (Page 9)

MAX535/MAX5351
Low-Power, 13-Bit Voltage-Output DACs
with Serial Interface
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_______________Detailed Description
The MAX535/MAX5351 contain a 13-bit, voltage-output
digital-to-analog converter (DAC) that is easily
addressed using a simple 3-wire serial interface. It
includes a 16-bit shift register, and has a doubled-
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 con-
verts a 13-bit digital input into an equivalent analog out-
put voltage in proportion to the applied reference
voltage input. Figure 1 shows a simplified circuit dia-
gram of the DAC.
Reference Inputs
The reference input accepts positive DC and AC sig-
nals. The voltage at the reference input sets the full-
scale output voltage for the DAC. The reference input
voltage range is 0V to (V
DD - 1.4V). The output voltage
(V OUT) is represented by a digitally programmable volt-
age source as:
V OUT = (V REF x NB / 8192 ) x Gain
where NB is the numeric value of the DAC’s binary
input code (0 to 8191), V
REF is the reference voltage,
and Gain is the externally set voltage gain.
The impedance at the reference input is code depen-
dent, ranging from a low value of 14k
Ω
when the DAC
has an input code of 1555 hex, to a high value exceed-
ing several giga ohms (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.
The REF reference input has a 14k
Ω
guaranteed mini-
mum input impedance. A voltage reference with a load
regulation of 6ppm/mA, such as the MAX873, would
typically deviate by 0.0062LSB (0.009LSB worst case)
when driving the MAX535 reference input at 2.5V.
In shutdown mode, the MAX535/MAX5351’s REF input
enters a high-impedance state with a typical input leak-
age current of 0.001µA.
The reference input capacitance is also code depen-
dent and typically ranges from 15pF (with an input
code of all 0s) to 50pF (with an input code of all 1s).
Output Amplifier
The MAX535/MAX5351’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 MAX535/MAX5351 out-
put, the typical settling time to ±1/2LSB is 16µs when
loaded with 5k
Ω
in parallel with 100pF (loads less than
2k
Ω
degrade performance).
The MAX535 output amplifier’s output dynamic re-
sponses and settling performances are shown in the
Typical Operating Characteristics .
OUT
FB
SHOWN FOR ALL 1s ON DAC
D0 D10 D11 D12
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