Cirrus Logic CS4373A - Manual (Page 27)

CS4373A
DS699F2 27
8.4 VREF Input Impedance
The switched-capacitor input architecture of
the VREF
± inputs results in an input imped-
ance that depends on the internal capacitor
size and the clock frequency. With a 15 pF in-
ternal capacitor and a 2.048 MHz MCLK the
VREF input impedance is approximately
[1 / [(2.048 MHz) * (15 pF)]] = 32 k
Ω. While
the size of the internal capacitor is fixed, the
voltage reference input impedance will vary
with MCLK.
The voltage reference external RC filter series
resistor creates a voltage divider with the
VREF input impedance to reduce the effective
applied input voltage. To minimize gain error
resulting from this voltage divider effect, the
RC filter series resistor should be the minimum
size recommended in the voltage reference
device data sheet.
8.5 VREF Accuracy
The nominal voltage reference input is speci-
fied as 2.500 V across the VREF
± pins, and all
CS4373A gain accuracy specifications are
measured with a nominal voltage reference in-
put. Any variation from a nominal VREF input
will proportionally vary the analog full-scale
gain accuracy.
Since temperature drift of the voltage refer-
ence results in gain drift of the analog full-scale
amplitude, care should be taken to minimize
temperature drift effects through careful selec-
tion of passive components and the voltage
reference device itself. Gain drift specifications
of the CS4373A do not include the tempera-
ture drift effects of external passive compo-
nents or of the voltage reference device itself.
8.6 VREF Independence
If the test signal source is required to be fully
independent of the measurement channel, a
separate voltage reference device for the
CS4373A is required. Using a separate volt-
age reference minimizes the possibility of un-
detected ratiometric errors when the same
voltage reference is used by both the test sig-
nal source and the measurement channel.
Because modern precision voltage references
are highly reliable, requirements for separate
modulator and test DAC voltage references
should be considered carefully. In the unlikely
event of voltage reference failure independent
of other system components, the CS4373A
volts-to-codes ratio will be out of spec and per-
formance will be poor during system self-tests.