Hard-disk drives for laptop and other portable applications need to withstand significant g-force shocks when operating. A typical 1.8-in. drive writes servo information over less than 15% of the disk's active area, which means that the read/write head spends more than 85% of the time outside the servo loop's control. A g-force shock during this time could cause the read/write arm and head to move away from the current write track and to permanently write corrupting data on an adjacent track or elsewhere.
The circuit in Figure 1 presents one solution to this problem. The circuit incorporates a piezoelectric shock sensor circuit into the drive electronic. When it senses a g-force shock, the sensor (a small accelerometer) outputs a pulse that disables the write head and prevents possible data corruption.
Figure 1's circuit represents the SMA-11 shock sensor as a voltage source in series with a capacitor. IC1 operates on a single 5V supply. This amplifier's output saturates with a large leak current, and the device's technology should be CMOS or JFET. This example uses a Texas Instruments TLC271 with high-bias mode.
The SMA-11 sensor features a typical voltage sensitivity of 4.7 mV/g; the output voltage for a 20g shock threshold would be 94 mV. You set the preamp gain for a 1V output at 20g by setting IC1's gain (G1) equal to 1/94×e-3, or 10.63. R1, R2, and R3 establish this gain according to the following equation:
Setting R1=R2 sets the preamp's output offset to 2.5V (one-half of 5V). Therefore, make R1=R2=20 k(ohm). Set R3=100 k(ohm). Then, G1=(100+10)/10=11=20.8 dB.
You can use the SMA-11's output capacitance of 150 pF to establish the low-end cutoff frequency, FC, at 20 Hz.
Unfortunately, you can't get 53-M(ohm) resistors as surface-mount devices. The maximum SMT (surface-mount-technology) resistor value generally available is only 10 M(ohm). Therefore, to get a 10-M(ohm) input resistor to appear as 53 M(ohm), this circuit uses a bootstrap circuit comprising R4 through R7 to feed back part of the in-phase output signal to the input.
If the circuit feeds back 80% of the output voltage through R5 to R4 (10 M(ohm)), then the voltage across R4 is only 20% of VOUT. If the current through R4 is 20% of IOUT, then R4 is effectively five times the input resistance, or 50 M(ohm). For an output-voltage offset of 2.5V, R6=R7=2 k(ohm).
The preamp's gain is 11, so the circuit feeds back 80% from this point, as follows:
The output of the preamp drives a window detector that consists of two open-collector comparators, IC2 and IC3, and R8 through R10. The output of the preamp at a shock level of 20g is 2.5V±1V. Therefore, high and low limits are 3.5 and 1.5V. Figure 2 shows the gain-vs-frequency curve. (DI #1884)