Calibration with the offset and gain changer ATL10a
8 Jan 2013

The calibration of analog input channels consists in a linear transformation of the input stream so that it shows the physical values instead of the input voltages. The linear transformation is defined by two input voltage values with known calibration (physical) values.

The operation can be done in NOTOCORD-hem™ by using the Calibration window.
The two input voltages are gathered as mean values on zones and the correspondant calibration values are entered manually.

Now, suppose that instead of two calibration values, a reference signal (RS) is available for the calibration. Then, you can use the ATL10a module (offset and gain changer) which offers three calibration modes for the measured signal (MS):

  • Change offset only: the mean of MS becomes equal to the mean of RS, amplitude of MS remains unchanged
  • Change gain only: the amplitude of MS becomes equal to the amplitude of RS, the mean of MS remains unchanged
  • Both: the amplitude and mean of MS become equal to the amplitude and mean of RS

This principle is illustrated in Figure 1. A reference pressure wave is generated and acquired by a calibrated channel composed of a transducer Tcal and an amplifier Acal. The measurement channel to be calibrated is composed of the transducer Tmeas and the amplifier Ameas. Both channels are acquired in LAS30a1 and applied to ATL10a1.
An additional digital channel connects a button to the digital input of ATL10a1. When the button is pressed during the acquisition, the calibration is done online.

Figure 1: Generation of a reference wave and NOTOCORD-hem™ dynamic calibration configuration.

The measured signal, input to the ATL10a module, is shown in Figure 2 with green color. The output of ATL10a (red) is identical with this input. After the button is pressed (trigger), the amplitude and mean of the reference signal (blue) are copied to the measured signal and the two signals show a good superposition.

Figure 2: Calibration of a measured signal (green) using a reference signal (blue) and ATL10a output (red).

The Change gain only mode is useful for drift cancelation during acquisition.
The Change offset only mode is useful to adjust two transducers that should provide the same mean. An example is the verification of the mean arterial pressure, measured with two catheters throughout the arterial tree, during the aortic valve area determination1.

References

Grossman’s Cardiac Catheterization, Angiography, and Intervention, 6th edition, by Donald S. Baim & William Grossman