Sensor Amplifier/Filter/Driver Board Documentation

Last Updated: 11/18/2002

Board Specifications:

  Number of Channels:      1, 2, or 3
  Gain:                    ~170 to ~6,000 - Set by Trim Pot and Jumper
  Frequency Response:      0.02Hz (50 Sec.) to 5Hz for Lehman Type Sensors
                           or 0.05Hz (20 Sec.) to 10Hz for Geophone Sensors (Note 1)
  Lowpass Filter Type:     8 Pole Butterworth Filter
  Highpass Filter Type:    1 Pole Butterworth Filter
  Input Impedance:         ~10K (Note 2)
  Input/Output Connectors: RCA Jack (Stereo equipment type) or
                           .156 Inch 2-Pin Molex Connector or
                           Terminal Screw Block
  Output Driver:           Drives ~100 feet of 75 Ohm Coax Cable into a 600 Ohm Load.
  Power Connector:         2.1mm X 5.5mm Male Jack on the Board
  Power Requirements:      12 to 18 VAC or 16 to 28 VDC
  Note 1: Other lowpass and highpass cutoff frequencies available.

  Note 2: Other input impedances available.

Using the Board

Power Supply:

The power connector on the board is a standard 2.1mm X 5.5mm male jack. The board can be powered with either AC or DC voltages. The DC input range is 16 volts to 30 volts and the AC range is 11 to 18 VAC. The amount of current needed to supply the board depends on the input voltage and number of channels: 

               Volts DC    Current    Power in Watts
Single Channel:   16        64ma           1.02
                  20        54ma           1.08
                  24        48ma           1.15
                  28        44ma           1.23
Two Channels:     16       106ma           1.70
                  20        92ma           1.84
                  24        82ma           1.97
                  28        76ma           2.13
Three Channels:   16       144ma           2.30
                  20       126ma           2.52
                  24       114ma           2.74
                  28       106ma           2.97

If you use your own power supply, you should verify that you have the correct voltage feeding the +- 12 volt regulators on the board. If you have a voltmeter, you should check the DC voltage across the large power supply capacitor C90. The minimum voltage necessary for proper operation of the regulators is ~15.5 VDC. The maximum voltage is ~28 VDC. If you exceed this voltage you will be exceeding the maximum voltage range of the power supply components.

Gain Adjustment:

Each channel has a two-pin jumper that controls the gain of the first op-amp. With the jumper in, the gain of the board will be ~170 to ~1700. This range should be used with sensors that have a high output voltage, like a geophone. With the jumper removed, the gain range is ~600 to ~6000. This input range should be used with sensors that produce less of an output voltage.

Each channel also has a variable gain stage with a range of 1x to 10x. The gain trim pot for each channel is marked "Gain" next to the part. When setting up your system, the user should first try the board with the gain jumper removed and the gain pot turned to minimum (fully counter clockwise). If there is too much signal the jumper should be inserted and the gain pot adjusted if needed. 

If there is still too much signal you will need to reduce the voltage from the sensor by using a resistor divider (schematic). Each channels as a 10k resistor to ground. This resistor can be used as one leg of the divider. The other resistor is placed in the signal path. A 10k resistor will reduce the voltage by 2 and a 100k resistor will reduce the voltage by 10. If you have a long wire between the sensor and the input to the board the resistor in the signal path should be placed near the board.

Offset Adjustment:

Each channel also has a DC Offset pot marked "Offset" next to the part. This adjustment has been set at the factory. If you do need to set the DC offset, adjust the pot for 0.0 volts at the output with no input signal. Wait for 2 to 3 minutes after you turn on the power before setting the Offset trim pot.

Input Signal Cable:

Connection between the pickup coil and the input to the channel should be kept as short as possible (less then 6 feet) if the impedance (resistance) of the coil is greater then 600 ohms. A stereo equipment patch cable will work fine. Sensors with low impedance coils can drive longer cables.

Output Signal Cable:

The board can drive up to 100 feet of shielded coax cable. This can be either 75 or 50 ohms. Adapters from RCA to BNC or type "F" connectors can be found at any Radio Shack or similar electronics store.

If you have any problems or questions, please call or e-mail me.

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Larry Cochrane -