Question

Is it possible to eliminate the 2777A-02-10 output offset with a decoupling capacitor? If yes, I need to know the recommended value and proposed schematic for this differential charge converter.

Answer

During normal operation, the output line of Endevco® IEPE (Integrated Electronics Piezoelectric) accelerometers and remote charge amplifiers such as the model 2771C and 2777A have a DCV (direct current voltage) bias established at or near the mid point of the compliance supply voltage as shown in Figure 1. The DCV bias is common to most output stages of single ended devices allowing a dynamic AC signal to swing above and below the DCV bias operating point. Often the DCV bias is undesirable to the monitoring stage downstream especially if it is DC coupled with additional gain because it introduces a significant DCV offset to the signal line. In most cases the user would want to block the DCV bias while allowing the AC signal to pass through with minimum attenuation. This article will guide the user how to do this.

The most effective and simplest way is to use a decoupling capacitor Cd and a load resistor RL as shown in Figure 2. The load resistance RL is not critical and can be assigned a value roughly in the range from 100K to 500K ohm. One needs to make sure that RL is significantly higher (at least 1000 times higher) than the output impedance of the charge amplifier in order to reduce the error caused by excessive loading. Rm is the input resistance of the monitoring instrument and can range from 1 meg to several hundred meg ohms. The value can be obtained from the equipment manufacturer.

The low frequency cutoff f(-3dB) can be calculated using the formula below. Similarly the decoupling capacitance can be calculated if f(-3dB) is known. An example calculation of typical application is also shown. As you can see, the combination of Cd and Re forms a single pole filter. The user can take advantage of this configuration to attenuate unwanted low frequency signals by selecting the proper value of Cd and RL. This can offer attenuation of unwanted signal at 6 dB per octave roll-off.

EXAMPLE: If I use 1 μfd capacitor for Cd, 220KΩ for RL and the input resistance Rm of my data acquisition is 2 Meg ohms: What is my -3 dB low frequency corner?

The type of capacitor used is not critical. A high quality 50 volt non-polarized cap is preferred although aluminum electrolytic or tantalum will work as well. Make sure that if electrolytic or tantalum capacitor is used the positive lead of the capacitor is connected to output of the IEPE or charge amplifier.