Cirrus Logic CDB5378, Manual

Page: 19 / 75

CDB5378
DS639DB4 19
2.2.2 Differential Amplifiers
The CS3301A/02A amplifiers act as a low-noise gain stage for internal or external differential analog sig-
nals.
2.2.2.1 MCLK Input vs. GUARD Output
By default, CDB5378 uses the CS3301A geophone amplifier which is chopper stabilized. The CS3301A
connects pin 13 to a clock source (MCLK) to run the chopper circuitry synchronous to the modulator an-
alog sampling clock. The CS3302A hydrophone amplifier is not chopper stabilized (with 1/f noise typically
buried below the low-frequency ocean noise) to achieve very high input impedance. To minimize leakage
from high-impedance sensors connected to the CS3302A amplifier, pin 13 produces a GUARD signal out-
put to actively drive a sensor cable shield with the common mode voltage of the sensor signal.
Comparing the CS3301A and CS3302A amplifiers, the functionality of pin 13 (MCLK input vs. GUARD
output) is the only external difference. CDB5378 can be converted to use either the CS3301A or CS3302A
by replacing the amplifier device and properly setting the pin 13 jumper (J42). By default this jumper is not
populated and has a shorting trace between pins on the back side of the PCB. Converting between am-
plifier types requires
carefully cutting the default short and installing a jumper. A replacement amplifier
can be requested as a sample from your local Cirrus Logic sales representative.
2.2.2.2 Rough-Fine Outputs - OUTR, OUTF
The analog outputs of the CS3301A/02A differential amplifiers are split into rough-charge and fine-charge
signals for input to the CS5373A
∆Σ
modulator.
Analog signal traces out of the CS3301A/02A amplifiers and into the CS5373A modulator are 4-wire
INR+ / INF+ / INF- / INR- quad groups, and are routed with INF+ and INF- as a traditional differential pair
and INR+ and INR- as guard traces outside the respective INF+ and INF- traces.
2.2.2.3 Anti-alias RC Filters
The CS5373A
∆Σ
modulator is 4th order and high-frequency input signals can cause instability. Simple
single-pole anti-alias RC filters are required between the CS3301A/02A amplifier outputs and the
CS5373A modulator inputs to bandwidth limit analog signals into the modulator.
Analog Signals Description
INA Sensor analog input
INB Test DAC analog input
OUTR, OUTF Analog rough / fine outputs
GUARD CS3302A guard output (jumper selection)
Digital Signals Description
MUX[0..1] Input mux selection
GAIN[0..2] Gain range selection
PWDN Power down mode enable
CLK CS3301A clock input (jumper selection)