Cirrus Logic CRD5378 - Manual (Page 28)

CRD5378
28 DS639RD2
The +2.5 V and -2.5 V power supplies to the analog components on CRD5378 include reverse biased
Schottky diodes to ground to protect against reverse voltages that could latch-up the CMOS analog com-
ponents. Also included on +2.5 V and -2.5 V are 68 uF bulk capacitors for bypassing and to help settle
transients plus individual 0.1 uF bypass capacitors local to the power supply pins of each device.
2.5 PCB Layout
2.5.1 Layer Stack
CRD5378 layers 1 and 6 are dedicated as analog routing layers. All critical routes are on these two lay-
ers. Some microcontroller digital routes are also included on these layers away from the analog signal
routes.
CRD5378 layer 2 is a solid ground plane without splits or routing. A solid ground plane provides the best
return path for bypassed noise to leave the system. No separate analog ground is required since analog
signals on CRD5378 are differentially routed.
CRD5378 layer 3 is dedicated for power supply routing. Each power supply net includes at least 68 µF
bulk capacitance as a charge well for settling transient currents.
CRD5378 layer 4 is dedicated as a digital routing layer.
CRD5378 layer 5 is a solid ground plane without splits or routing. A solid ground plane provides the best
return path for bypassed noise to leave the system. No separate analog ground is required since analog
signals on CRD5378 are differentially routed.
2.5.2 Differential Pairs
Analog signal routes on CRD5378 are differential with dedicated + and - traces. All source and return an-
alog signal currents are constrained to the differential pair route and do not return through the ground
Specification Value
Negative Analog Supply
-2.5 V
Low Noise Micropower Regulator - Linear Tech LT1964ES5-BYP
Surface Mount Package Type SOT-23
Load Regulation, -40 C to +85 C +/- 30 mV
Quiescent Current, Current @ 100 mA Load 30
µA, 1.3 mA
Output Voltage Noise, 10 Hz - 100 kHz 20
µV
RMS
Ripple Rejection, DC - 200 Hz > 45 dB