What happens when the constant current source and compliance voltage of an IEPE accelerometer varies outside the specification limits?
Most IEPE accelerometers available today are powered by a constant current source, with the power supply specified in minimum and maximum currents and voltages. It is best to stay within this range if the performance specification is to be expected. This article will describe the performance of a typical IEPE accelerometer operating outside the power supply specification limits.
Figure 1 shows the typical output behavior of an IEPE device operating at a maximum output swing at four different conditions. For simplicity, the output swing is assumed to be rail to rail (no output nonlinearity at or near saturation).
Compliance voltage variation
During normal operation at room temperature as shown in Figure 1 (a), the bias is established at V2 and the output signal is normally an undistorted full swing sine wave. Now let's moderately reduce the compliance voltage below the minimum supply limit from V5 down to V4. This reduction will bring the compliance voltage near the saturation point of the positive peak of the output signal.
As you can see, further reduction of the compliance voltage will eventually distort the output even at room temperature. Now, if we lower the temperature, the bias will move up to V3 as shown in (b) of Figure 1. The upper half cycle will now be saturated (clipped). This is the condition that we must be concerned about when the compliance voltage is lowered below the specified limit. On the other hand, going the other way by increasing the compliance voltage way beyond the upper limit of V6 should be avoided. Some components will break down and permanently damage the circuit.
Constant current variation
Within the specified current range, varying the constant current source at room temperature has no or negligible effect on the bias voltage. Operating near the low limit within the specification allows lower circuit dissipation but reduces the output line driving capability. It is important to note that, if a long line is to be driven, the constant current source should be increased but stay within the specified limit. Operating near the upper current and upper temperature limit is shown in (c) of Figure 1. The bias will drop to V1, and this moves the negative peak closer to signal ground.
Current setting below the specification should be avoided, because the charge amplifier will simply cease to function. Increasing the current beyond the specified limit makes the charge amplifier run warmer and at high temperature, which will further reduce the bias to V0 as shown in (d) of Figure 1. This is due to the added heat generated by the I2R dissipation of the circuit. This is the condition of concern, because it will cause the negative swing to distort as it drifts closer to signal ground. Increasing the current way beyond the upper limit will eventually burn the circuit.
Things to remember
I have a model 7251A-10 accelerometer operating at room temperature. The minimum compliance voltage required is 2Vdc, but I only have 20Vdc available. What specification will be sacrificed?
From model 7251A data sheet:
Typical charge amplifier characteristics:
Amplifier output non-linearity
3V (for simplicity, this term was ignored in previous discussion)
Voltage required for full output swing:
Full scale output voltage + amplifier non-linearity
5V + 3V = 8V
Minimum compliance voltage needed:
Maximum bias + voltage required for full output swing
13.5V + 8V = 21.5V
Maximum output voltage attainable without distortion:
Full scale output voltage minus difference between minimum compliance voltage needed and voltage available
5V - (21.5V - 20Vdc) = 3.5Vdc
Convert 3.5Vdc to (g) based on 10mV/g:
3500mV/10 mV/g = 350g
At room temperature, the model 7251A-10 which has 500g range can have only 350g maximum range if the compliance voltage is reduced from the required 23Vdc minimum down to the available 20Vdc. Please be aware that the maximum range will continue to drop as the temperature is lowered.