Data Sheet AD5512A/AD5542A
Rev. C | Page 17 of 21
OUTPUT AMPLIFIER SELECTION
For bipolar mode, a precision amplifier should be used and
supplied from a dual power supply. This provides the ±V
REF
output. In a single-supply application, selection of a suitable op
amp may be more difficult because the output swing of the ampli-
fier does not usually include the negative rail, in this case,
AGND. This can result in some degradation of the specified
performance unless the application does not use codes near zero.
The selected op amp must have a very low-offset voltage (the
DAC LSB is 38 μV for the AD5542A with a 2.5 V reference)
to eliminate the need for output offset trims. Input bias current
should also be very low because the bias current, multiplied by
the DAC output impedance (approximately 6 kΩ), adds to the
zero-code error. Rail-to-rail input and output performance is
required. For fast settling, the slew rate of the op amp should
not impede the settling time of the DAC. Output impedance
of the DAC is constant and code-independent, but to minimize
gain errors, the input impedance of the output amplifier should
be as high as possible. The amplifier should also have a 3 dB
bandwidth of 1 MHz or greater. The amplifier adds another
time constant to the system, thus increasing the settling time
of the output. A higher 3 dB amplifier bandwidth results in a
shorter effective settling time of the combined DAC and amplifier.
FORCE SENSE AMPLIFIER SELECTION
Use single-supply, low-noise amplifiers. A low-output impedance
at high frequencies is preferred because the amplifiers must be
able to handle dynamic currents of up to ±20 mA.
REFERENCE AND GROUND
Because the input impedance is code-dependent, the refer-
ence pin should be driven from a low impedance source. The
AD5512A/AD5542A operate with a voltage reference ranging
from 2 V to V
DD
. References below 2 V result in reduced accuracy.
The full-scale output voltage of the DAC is determined by the
reference. Table 9 and Table 10 outline the analog output voltage
or particular digital codes. For optimum performance, Kelvin
sense connections are provided on the AD5512A/AD5542A.
If the application doesn’t require separate force and sense lines,
tie the lines close to the package to minimize voltage drops
between the package leads and the internal die.
POWER-ON RESET
The AD5512A/AD5542A have a power-on reset function to
ensure that the output is at a known state on power-up. On
power-up, the DAC register MSB is 1 and all other bits are 0
until the data is loaded from the serial register. However, the
serial register is not cleared on power-up; therefore, its contents
are undefined. When loading data initially to the DAC, 16 bits or
more should be loaded to prevent erroneous data appearing on
the output. If more than 16 bits are loaded, the last 16 are kept,
and if less than 16 bits are loaded, bits remain from the previous
word. If the AD5512A/AD5542A must be interfaced with data
shorter than 16 bits, the data should be padded with 0s at the
LSBs.
POWER SUPPLY AND REFERENCE BYPASSING
For accurate high-resolution performance, it is recommended
that the reference and supply pins be bypassed with a 10 μF
tantalum capacitor in parallel with a 0.1 μF ceramic capacitor.
