CDB5378 Cirrus Logic Inc, CDB5378 Datasheet - Page 31

• Manufacturer Part Number: CDB5378
• Description: EVALUATION BOARD FOR CS5378
• Manufacturer: Cirrus Logic Inc

Specifications of CDB5378

• Main Purpose: Seismic Evaluation System
• Embedded: Yes, MCU, 8-Bit
• Utilized Ic / Part: CS3301A, CS5373A, CS5378
• Primary Attributes: Single Digital Filter
• Secondary Attributes: Graphical User Interface, SPI™ & USB Interfaces
• Processor To Be Evaluated: CS5378
• Interface Type: USB
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant, Contains lead / RoHS non-compliant

8. VOLTAGE REFERENCE
The CS5373A requires a 2.500 V precision
voltage reference to be supplied to the VREF
pins.
8.1 VREF Power Supply
To guarantee proper regulation headroom for
the voltage reference device, the voltage refer-
ence GND pin should be connected to VA- in-
stead of system ground, as shown in
Figure
voltage equal to VA- and a VREF+ voltage
very near ground [(VA-) + 2.500 VREF].
Power supply inputs to the voltage reference
device should be bypassed to system ground
with 0.1 μF capacitors placed as close as pos-
sible to the power and ground pins. In addition
to 0.1 μF local bypass capacitors, at least
100 μF of bulk capacitance to system ground
should be placed on each power supply near
the voltage regulator outputs. Bypass capaci-
tors should be X7R, C0G, tantalum, or other
high-quality dielectric type.
8.2 VREF RC Filter
A primary concern in selecting a precision volt-
age reference device is noise performance in
the measurement bandwidth. The
Technology LT1019AIS8-2.5
ence yields acceptable noise levels if the out-
put is filtered with a low-pass RC filter.
A separate RC filter is required for each sys-
tem device connected to a given voltage refer-
DS703F2
16. This connection results in a VREF-
From VA+
Regulator
From VA-
Regulator
100 μF
100 μF
Figure 16. Voltage Reference Circuit
voltage refer-
0.1 μF
0.1 μF
2.500 V
Linear
VREF
±
10 Ω
ence. By sharing a common RC filter, signal-
dependent sampling of the voltage reference
by one system device could cause unwanted
tones to appear in the measurement band-
width of another system device via common
impedance coupling.
8.3 VREF PCB Routing
To minimize the possibility of outside noise
coupling into the CS5373A voltage reference
input, the VREF
differential pair from the large capacitor of the
voltage reference RC filter. Careful control of
the voltage reference source and return cur-
rents by routing VREF
will improve immunity from external noise.
To further improve noise rejection of the
VREF
pacitors to system ground as close as possible
to the VREF+ and VREF- pins of the
CS5373A.
8.4 VREF Input Impedance
The switched-capacitor input architecture of
the VREF
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 kΩ.
the size of the internal capacitor is fixed, the
voltage reference input impedance will vary
0.1 μF
± routing,
±
+
Route VREF
from the 100uF RC filter capacitor
100 μF
inputs results in an input imped-
±
traces should be routed as a
include 0.1 μF
±
as a differential pair
±
0.1 μF
0.1 μF
as a differential pair
To VREF+
To VREF-
CS5373A
bypass
While
ca-
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