Rainbow Electronics MAX545 - Manual (Page 9)

MAX544/MAX545
+5V, Serial-Input, Voltage-Output, 14-Bit DACs
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Applications Information
Reference and Analog Ground Inputs
The MAX544/MAX545 operate with external voltage ref-
erences from 2V to 3V, and maintain 14-bit performance
if certain guidelines are followed when selecting and
applying the reference. Ideally, the reference’s
temperature coefficient should be less than 1.5ppm/°C to
maintain 14-bit accuracy to within 1LSB over the 0°C to
+70°C commercial temperature range. Since this convert-
er is designed as an inverted R-2R voltage-mode DAC,
the input resistance seen by the voltage reference is code
dependent. The worst-case input-resistance variation is
from 11.5k
Ω (at code 8554 hex) to 200kΩ (at code 0000
hex). The maximum change in load current for a +2.5V
reference is +2.5V / 11.5k
Ω = 217µA; therefore, the
required load regulation is 28ppm/mA for a maximum
error of 0.1LSB. This implies a reference output imped-
ance of less than 71m
Ω. In addition, the signal-path
impedance from the voltage reference to the reference
input must be kept low because it contributes directly to
the load-regulation error.
The requirement for a low-impedance voltage reference
is met with capacitor bypassing at the reference inputs
and ground. A 0.1µF ceramic capacitor with short leads
between REFF and AGNDF (MAX545), or REF and
AGND (MAX544), provides high-frequency bypassing.
A surface-mount ceramic chip capacitor is preferred
because it has the lowest inductance. An additional
10µF between REFF and AGNDF (MAX545), or REF
and AGND (MAX544), provides low-frequency bypass-
ing. A low-ESR tantalum, film, or organic semiconductor
capacitor works well. Leaded capacitors are accept-
able because impedance is not as critical at lower fre-
quencies. The circuit can benefit from even larger
bypassing capacitors, depending on the stability of the
external reference with capacitive loading. If separate
force and sense lines are not used, tie the appropriate
force and sense pins together close to the package.
AGND must also be low impedance, as load-regulation
errors will be introduced by excessive AGND resis-
tance. As in all high-resolution, high-accuracy applica-
tions, separate analog and digital ground planes yield
the best results. Tie DGND to AGND at the AGND pin to
form the “star” ground for the DAC system. Always refer
remote DAC loads to this system ground for the best
possible performance.
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 14-bit performance from +V
REF to AGND
without degradation at zero scale. The DAC’s output
impedance is also low enough to drive medium loads
(R
L > 60k Ω) 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 unipolar or bipolar
operational mode. In unipolar mode, the output amplifi-
er is used in a voltage-follower connection. In bipolar
mode (MAX545 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 amplifier’s input
impedance should still be as high as possible to mini-
mize 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 AGND are available; however, their output
swings do not normally include the negative rail
(AGND) without significant degradation of performance.
A single-supply op amp, such as the MAX495, is suit-
able if the application does not use codes near zero.
Since the LSBs for a 14-bit DAC are extremely small
(152.6µV 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 6.25k
Ω) contributes to zero-scale error.
Temperature effects also must be taken into considera-
tion. Over the 0°C to +70°C commercial temperature
range, the offset voltage temperature coefficient (refer-
enced to +25°C) must be less than 1.7µV/°C to add
less than 1/2LSB of zero-scale error. The external