CDB5378 Datasheet, PDF (17/72 Pages) Cirrus Logic – Single-channel Seismic Evaluation System

CDB5378
2.2.2 Differential Amplifiers
The CS3301/02 amplifiers act as a low-noise gain stage for internal or external differential analog signals.
Analog Signals
INA
INB
OUTR, OUTF
GUARD
Description
Sensor analog input
Test DAC analog input
Analog rough / fine outputs
CS3302 guard output (jumper selection)
Digital Signals
MUX[0..1]
GAIN[0..2]
PWDN
CLK
Description
Input mux selection
Gain range selection
Power down mode enable
CS3301 clock input (jumper selection)
2.2.2.1 MCLK Input vs. GUARD Output
By default, CDB5378 uses the CS3301 geophone amplifier which is chopper stabilized. The CS3301 con-
nects pin 13 to a clock source (MCLK) to run the chopper circuitry synchronous to the modulator analog
sampling clock. The CS3302 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 CS3302 amplifier, pin 13 produces a GUARD signal output to
actively drive a sensor cable shield with the common mode voltage of the sensor signal.
Comparing the CS3301 and CS3302 amplifiers, the functionality of pin 13 (MCLK input vs. GUARD out-
put) is the only external difference. CDB5378 can be converted to use either the CS3301 or CS3302 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 CS3301/02 differential amplifiers are split into rough-charge and fine-charge
signals for input to the CS5373A ∆Σ modulator. The amplifier outputs include integrated series resistors
to create the anti-alias RC filters required to limit the modulator input signal bandwidth.
Analog signal traces out of the CS3301/02 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 CS3301/02 amplifier outputs and the CS5373A
modulator inputs to bandwidth limit analog signals into the modulator.
DS639DB3
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