The DAC outputs are derived as a percentage of Vreg, and then amplified by 1.5, so any
changes in Vreg will have a proportionate affect on the DAC outputs. Vreg is more stable than
Vs (5 volt supply voltage), as it is the output from a 3.3 volt regulator.
The DACs are derived from PWM signals that are affected by the timer clock frequency (Section
l functions). For a desired
utput voltage, the binary value can be approximated as:
5)*256
d to the analog outputs, as they are voltage sources themselves.
the event that a voltage is accidentally applied to either analog output, they do have
rvoltage) of a few volts.
he DACs on the U3 can output quite a bit of current, but have 50 Ω of source impedance that
op-amp can be used to buffer the output,
re 2-2. A simple RC filter can be added
etween the DAC output and the amp input for further noise reduction. Note that the ability of
ar the power rails must still be considered. A possible op-amp
from 4.95 volts to 10.0 volts, a gain of 2.02 is
quired. If R2 (in Figure 2-3) is chosen as 100 kΩ, then an R1 of 97.6 kΩ is the closest 1%
an 2.02. The +V supply for the op-amp would have to be
2.x). The default timer clock frequency of the U3 is set to 24 MHz, as this results in the
minimum DAC output noise. If the frequency is lowered, the DACs will have more noise, where
the frequency of the noise is the timer clock frequency divided by 2
8
.
The analog outputs have filters with a 3 dB cutoff around 16 Hz, limiting the frequency of output
waveforms to less than that.
The analog output commands are sent as raw binary values (low leve
o
Bits(uncalibrated) = (Volts/4.9
For a proper calculation, though, use the calibration values (Slope and Offset) stored in the
internal flash on the processor (Table 2-7):
Bits = (Slope * Volts) + Offset
The analog outputs can withstand a continuous short-circuit to ground, even when set at
maximum output.
Voltage should never be applie
In
protection against transient events such as ESD (electrostatic discharge) and continuous
overvoltage (or unde
2.7.1 Typical Analog Output Connections
.7.1.1 High Current Output 2
T
will cause voltage drop. To avoid this voltage drop, an
such as the non-inverting configuration shown in Figu
b
the amp to source/sink current ne
choice would be the TLV246x family (ti.com).
2.7.1.2 Different Output Ranges
The typical output range of the DACs is about 0.04 to 4.95 volts. For other unipolar ranges, an
op-amp in the non-inverting configuration (Figure 2-3) can be used to provide the desired gain.
For example, to increase the maximum output
re
resistor that provides a gain greater th
greater than 10 volts.
For bipolar output ranges, such as
±10 volts, a similar op-amp circuit can be used to provide
gain and offset, but of course the op-amp must be powered with supplies greater than the
desired output range (depending on the ability of the op-amp to drive it’s outputs close to the
24
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