15
DAC8532
SBAS246A
www.ti.com
DRIVING RESISTIVE AND CAPACITIVE LOADS
The DAC8532 output stage is capable of driving loads of up
to 1000pF while remaining stable. Within the offset and gain
error margins, the DAC8532 can operate rail-to-rail when
driving a capacitive load. Resistive loads of 2k
can be
driven by the DAC8532 while achieving a typical load regu-
lation of 1%. As the load resistance drops below 2k
, the
load regulation error increases. When the outputs of the DAC
are driven to the positive rail under resistive loading, the
PMOS transistor of each Class-AB output stage can enter
into the linear region. When this occurs, the added IR voltage
drop deteriorates the linearity performance of the DAC. This
only occurs within approximately the top 20mV of the DAC
’
s
digital input-to-voltage output transfer characteristic. The
reference voltage applied to the DAC8532 may be reduced
below the supply voltage applied to V
DD
in order to eliminate
this condition if good linearity is a requirement at full scale
(under resistive loading conditions).
CROSSTALK AND AC PERFORMANCE
The DAC8532 architecture uses separate resistor strings for
each DAC channel in order to achieve ultra-low crosstalk
performance. DC crosstalk seen at one channel during a full-
scale change on the neighboring channel is typically less than
0.5LSBs. The AC crosstalk measured (for a full-scale, 1kHz
sine wave output generated at one channel, and measured at
the remaining output channel) is typically under
–
100dB.
In addition, the DAC8532 can achieve typical AC perfor-
mance of 96dB SNR (Signal-to-Noise Ratio) and 65db THD
(Total Harmonic Distortion), making the DAC8532 a solid
choice for applications requiring low SNR at output frequen-
cies at or below 4kHz.
OUTPUT VOLTAGE STABILITY
The DAC8532 exhibits excellent temperature stability of
5ppm/
°
C typical output voltage drift over the specified tem-
perature range of the device. This enables the output voltage
of each channel to stay within a
±
25
μ
V window for a
±
1
°
C
ambient temperature change.
Good Power-Supply Rejection Ratio (PSRR) performance
reduces supply noise present on V
DD
from appearing at the
outputs to well below 10
μ
V-s. Combined with good DC noise
performance and true 16-bit differential linearity, the DAC8532
becomes a perfect choice for closed-loop control applica-
tions.
SETTLING TIME AND OUTPUT
GLITCH PERFORMANCE
Settling time to within the 16-bit accurate range of the
DAC8532 is achievable within 10
μ
s for a full-scale code
change at the input. Worst case settling times between
consecutive code changes is typically less than 2
μ
s, en-
abling update rates up to 500ksps for digital input signals
changing code-to-code. The high-speed serial interface of
the DAC8532 is designed in order to support these high
update rates.
For full-scale output swings, the output stage of each
DAC8532 channel typically exhibits less than 100mV of
overshoot and undershoot when driving a 200pF capacitive
load. Code-to-code change glitches are extremely low
(~10uV) given that the code-to-code transition does not
cross an Nx4096 code boundary. Due to internal segmen-
tation of the DAC8532, code-to-code glitches occur at each
crossing of an Nx4096 code boundary. These glitches can
approach 100mVs for N = 15, but settle out within ~2
μ
s.
USING REF02 AS A POWER SUPPLY FOR DAC8532
Due to the extremely low supply current required by the
DAC8532, a possible configuration is to use a REF02 +5V
precision voltage reference to supply the required voltage to
the DAC8532's supply input as well as the reference input, as
shown in Figure 10. This is especially useful if the power
supply is quite noisy or if the system supply voltages are at
some value other than 5V. The REF02 will output a steady
supply voltage for the DAC8532. If the REF02 is used, the
current it needs to supply to the DAC8532 is 567
μ
A typical
and 890
μ
A max for V
DD
= 5V. When a DAC output is loaded,
the REF02 also needs to supply the current to the load. The
total typical current required (with a 5k
load on a given DAC
output) is:
567
μ
A + (5V/5k
) = 1.567mA
FIGURE 10. REF02 as a Power Supply to the DAC8532.
REF02
DAC8532
3-Wire
Serial
Interface
+5V
1.567mA
V
DD
, V
REF
V
OUT
= 0V to 5V
SYNC
SCLK
D
IN
+15
The load regulation of the REF02 is typically 0.005%/mA,
which results in an error of 392
μ
V for the 1.5mA current
drawn from it. This corresponds to a 5.13LSB error for a 0V
to 5V output range.
BIPOLAR OPERATION USING THE DAC8532
The DAC8532 has been designed for single-supply opera-
tion but a bipolar output range is also possible using the
circuit in Figure 11. The circuit shown will give an output
voltage range of
±
V
REF
. Rail-to-rail operation at the amplifier
output is achievable using an amplifier such as the OPA703,
see Figure 11.