Cirrus Logic CDB5374 - Manual (Page 15)

CDB5374
DS862DB1 15
2.2.1.4 Input Protection
Sensor inputs must have circuitry to protect the analog electronics from voltage spikes as 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.
2.2.1.5 Input RC Filters
Following the diode clamps is an RC filter network that bandwidth limits the sensor inputs into the ampli-
fiers 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 marine applications that use the CS3302A amplifier, the inherent capacitance of the piezoelectric sen-
sor is combined with large resistors to create an analog high-pass RC filter to eliminate the low-frequency
components of ocean noise.
2.2.1.6 Common Mode Bias
Differential analog signals into the CS3301A/02A amplifiers are required to be biased to the center of the
power supply voltage range, which for bipolar supplies is near ground potential. This common mode bias
voltage is created by buffering the voltage reference, which is nominally +2.5 V relative to the VA- power
supply.
Resistors to create the common mode bias are selected based on the sensor impedance and may need
to be modified from the CDB5374 defaults depending on the sensor to be used. Refer to the recommend-
ed operating bias conditions for the selected sensor, which are available from the sensor manufacturer.
Specification Value
Dual Series Switching Diode - ON Semiconductor
BAV99LT1
Surface Mount Package Type SOT-23
Non-Repetitive Peak Forward Current (1
µs, 1 ms, 1 s) 2.0 A, 1.0 A, 500 mA
Reverse Bias Leakage (25 C to 85 C) 0.004
µA - 0.4 µA
Reverse Bias Capacitance (0 V to 5 V) 1.5 pF - 0.54 pF
Marine Differential Filter Specification Value
Hydrophone Group Capacitance 128 nF + 10%
Differential Resistance 412 k
Ω + 2 kΩ = 400 Ω
-3 dB Corner @ 6 dB/octave 40 kHz + 10%
Specification Value
Hydrophone Sensor Bias Resistance
18 M
Ω || 18 MΩ = 9 MΩ