5
dc809af
DEMO MANUAL DC809A
experiments
The following experiments are intended to demonstrate
some of the outstanding features of the LTC2604. All can
be performed using the onboard LTC2428 to monitor
the DAC output voltage. The indicated output voltage will
typically agree with an HP3458A voltmeter to 5 digits. If
a DAC will be sinking or sourcing a significant current,
then the output voltage should be measured as close to
the DAC as possible.
Most of the data sheet specifications use a 4.096 volt
reference, so this is the preferred reference to use for
these experiments. Using the 5V reference has the limita-
tion that V
CC
may be slightly lower than V
REF
, which may
affect the full-scale error.
Using an external power supply is highly recommended for
these experiments, especially those that draw significant
current. Refer to the DC590 quick start guide for details.
Resolution
The onboard LTC2428 ADC has an input resolution of
6μV. This will easily resolve a 1LSB (76μV for V
REF
=
5V, 62.5μV for V
REF
= 4.096V) change in the LTC2604
output. Set one of the DAC channels to a voltage close to
mid-scale. Select the FINE slider on the control panel with
the
mouse and use the right and left arrow keys to step
the output by single LSBs. The change should be clearly
visible in the output graph. (It may be necessary to wait
for the graph to clear if a large step has just occurred.
This can be sped up by disabling all other DAC channels
in the software by unchecking them.)
Integral Nonlinearity
A rough measurement of INL can be taken using the onboard
ADC. Measure one of the LTC2604 outputs at code 256
and 65,535 and calculate the slope and intercept using a
spreadsheet. Next, take several readings at intermediate
points. The readings should not deviate from the calcu-
lated line by more than 64LSBs, and they will typically be
within 12LSBs.
Load Regulation/DC Output Impedance
Set one of the outputs to mid-scale (code 32768) source or
sink 15mA from this output by pulling it to power ground or
V
CC
with an appropriate value resistor. The voltage change
should be less than 2.25mV, corresponding to an output
impedance of 0.15Ω. Output impedance is typically less
than 0.030Ω. (Measure DAC voltage at the output pin if
using a voltmeter.)
Zero Scale Error
Set one of the DACs to code 0. The measured output should
be less than 9mV and will typically be less than 1mV.
Offset Error
Set
one of the DACs to code 256. The output voltage should
be within 1mV of the correct value, or V
REF
• 256/65535.
Gain Error
Set one of the DACs to code 65,535. The output voltage
should be within 0.7% of V
REF
, and will typically be within
0.2%.
DC Crosstalk
Set all DACs to mid-scale. Connect a 250Ω resistor from
one output to V
CC
or Power Ground (to sink or source
10mA, respectively, when the 5V reference is being used.)
A given output should not change by more than 3.5μV per
milliamp of output current in all other DACs combined.
