MAX520/MAX521
Quad/Octal, 2-Wire Serial 8-Bit DACs
with Rail-to-Rail Outputs
______________________________________________________________________________________ 15
(R
IN
) is code dependent, it must be driven by a circuit
with low output impedance (no more than R
IN
÷ 2000) to
maintain output linearity. The REF input capacitance is
also code dependent, with the maximum value occur-
ring at code FF hex (typically 30pF for the MAX520/
MAX521’s REF0–REF3, and 120pF for the MAX521’s
REF4). The output voltage for any DAC can be repre-
sented by a digitally programmable voltage source as:
V
OUT
= (N x V
REF
) / 256, where N is the numerical value
of the DAC’s binary input code. Table 1 shows the
unipolar code.
MAX520 Unbuffered DAC Outputs
The unbuffered DAC outputs (OUT0–OUT3) connect
directly to the internal 16kΩ R-2R network. The outputs
swing from 0V to V
DD
.
The MAX520 has no output buffer amplifiers, giving it
very low supply current. The output-offset voltage is
lower without the output buffer, and the output can also
slew and settle faster if capacitive loading is minimized.
Resistive loading should be very light for highest accu-
racy. Any output loading generates some gain error,
increasing full-scale error. The R-2R ladder’s output
resistance is 16kΩ, so a 1µA output current creates a
16mV error. Linearity is not affected because the ladder
output resistance does not change with DAC code.
Ladder-resistance changes with temperature are also
very small.
DACs are often used in trimming applications to
replace hardware potentiometers. Figure 15a shows a
typical application, which requires a buffered output so
that a precise current can be injected into the summing
node through precision resistor R
T
. For this application,
the MAX520A features a precise ±1% (T
A
= +25°C,
±2.5% over temperature) factory-trimmed output resis-
tance. Because the MAX520A’s output resistance is
precisely trimmed, there is no need for an internal
buffer or external precision resistor (Figure 15b). For
applications where the output resistance value is not
critical, use the MAX520B.
All DACs exhibit output glitches during code transitions.
An output filter is sometimes used to reduce these
glitches in sensitive applications. The MAX520 simpli-
fies output filtering because its internal resistive ladder
network serves as the “R” in an RC filter. Simply con-
nect a small capacitor from the DAC output to ground.
See the
Typical Operating Characteristics
for oscillo-
scope photos of the worst-case 1LSB step change both
without and with 25pF of capacitance on the MAX520’s
output.
MAX521 Output Buffer Amplifiers
The MAX521 voltage outputs (OUT0–OUT7) are inter-
nally buffered precision unity-gain followers that slew
up to 1V/µs. The outputs can swing from 0V to V
DD
.
With a 0V to 4V (or 4V to 0V) output transition, the
amplifier outputs typically settle to 1/2LSB in 6µs when
loaded with 10kΩ in parallel with 100pF. The buffer
amplifiers are stable with any combination of resistive
loads ≥2kΩ and capacitive loads ≤300pF.
2R
R
RR
2R
2R 2R 2R 2R
D0 D5 D6 D7
REF_
AGND
SHOWN FOR ALL 1s ON DAC
OUT_
(MAX521)
OUT_
(MAX520)
Figure 14. DAC Simplified Circuit Diagram
Table 1. Unipolar Code Table
0V00000000
1
+ V
REF
(———)
256
00000001
127
+ V
REF
(———)
256
01111111
128 V
REF
+ V
REF
(———)= ——
256 2
10000000
129
+ V
REF
(———)
256
10000001
255
+ V
REF
(———)
256
11111111
ANALOG OUTPUTDAC CONTENTS
