Add Lowpass Filtering with SCB-68 Terminal Block

Updated Jan 16, 2018

Reported In

Hardware

  • SCB-68
  • DAQ Signal Accessory

Issue Details

How do I connect an RC lowpass filter to the analog input channels of my DAQ board using an SCB-68 terminal block?

Solution

Soldering pads are provided on the SCB-68 to build RC lowpass filter circuits for the analog input channels. Refer to the SCB-68 User Guide for the location of the SCB-68 soldering pads used for analog input signal conditioning. Complete the following steps to build an RC lowpass filter circuit:

Single-Ended Lowpass Filter:
  1. Connect the resistor to position Rx(F) for channel CH(i) or to position Rx(G) for channel CH(i+8).
  2. Connect the capacitor to position RCx(B) for channel CH(i) or to position RCx(D) for channel CH(i+8).
  3. Connect the input signal (+) to the appropriate channel on the terminal block and signal (–) to AI GND.
Differential Lowpass Filter:
  1. Connect the resistor to position Rx(F).
  2. Connect the capacitor to position RCx(E).
  3. Connect a short across Rx(G).
  4. Connect the input signal (+) and signal (–) to CH(i) and CH(i+8) locations on the terminal block.
Figure 1 shows the connections for a single-ended and a differential lowpass filter.

Figure 1. RC filter connections
 
Refer to the SCB-68 User Manual for Advanced Functions or your hardware documentation for your device pinout.

Note: Filtering increases the settling time of the instrumentation amplifier to the time constant of the filter used. Adding RC filters when scanning channels greatly reduces the practical scanning rate since the instrumentation amplifier settling time can be increased to 10T or longer, where T = (R)(C). You can use RC filters with single-ended or differential inputs.

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