CDB5378 Datasheet, PDF (15/72 Pages) Cirrus Logic – Single-channel Seismic Evaluation System

CDB5378
By default, CDB5378 is populated with passive RC filter components on the INA inputs, and no filter com-
ponents on the INB inputs (though the component footprints are present on the INB inputs). Because the
CS5373A precision output will not tolerate significant loading, on CDB5378 the DAC_OUT signal should
only jumper to the INB inputs. The CS5373A buffered outputs are less sensitive to the RC filter load and
DAC_BUF can be jumpered to either the INA or INB inputs.
2.2.1.4 Input Protection
Sensor inputs must have circuitry to protect the analog electronics from voltage spikes. Geophone coils
are susceptible to magnetic fields (especially from lightning) and hydrophones can produce large voltage
spikes if located near an air gun source.
Discrete switching diodes quickly clamp the analog inputs to the power supply rails when the input voltage
spikes. These diodes are reverse biased in normal operation and have low reverse bias leakage and ca-
pacitance characteristics to maintain high linearity on the analog inputs.
Specification
Dual Series Switching Diode - ON Semiconductor
Surface Mount Package Type
Non-Repetitive Peak Forward Current (1 µs, 1 ms, 1 s)
Reverse Bias Leakage (25 C to 85 C)
Reverse Bias Capacitance (0 V to 5 V)
Value
BAV99LT1
SOT-23
2.0 A, 1.0 A, 500 mA
0.004 µA - 0.4 µA
1.5 pF - 0.54 pF
2.2.1.5 Input RC Filters
Following the diode clamps is an RC filter network that bandwidth limits the sensor inputs into the amplifier
to “chop the tops off” residual voltage spikes not clamped by the discrete diodes. In addition, all Cirrus
Logic component ICs have built in ESD protection diodes guaranteed to 2000 V HBM / 200 V MM (JEDEC
standard). The small physical size of these ESD diodes restricts their current capacity to 10 mA.
For land applications using the CS3301 amplifier, the INA input has a common mode and differential RC
filter. The common mode filter sets a low-pass corner to shunt very-high-frequency components to ground
with minimal noise contribution. The differential filter sets a low-pass corner high enough not to affect the
magnitude response of the measurement bandwidth.
For marine applications that use the CS3302 amplifier, the inherent capacitance of the piezoelectric sen-
sor is combined with large resistors connected to the input signal common mode to create an analog high-
pass RC filter to eliminate the low-frequency components of ocean noise. Following the high-pass com-
mon mode filter is a differential low-pass filter to reject high-frequency signals into the amplifier. The cutoff
frequency for the low-pass filter is high enough not to affect the magnitude response of the measurement
bandwidth.
By default, CDB5378 uses the CS3301 differential geophone amplifier and so the input RC filter on the
INA inputs are set for land applications. Marine applications using the CS3302 amplifier will need to mod-
ify the input RC filter components.
DS639DB3
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