Measured Transfer Functions for the Drive/Sense circuit combo

This page describes the expected and measured transfer function for the cryoboard resistance measurements

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Both the voltage and current measurements deviate from their expected results.

Figure 1: The voltage measurements are plotted as stars and the expected signal due to the double-pole LPF is plotted in red	Figure 2: The voltage measurements on log-log

Figure3: The current measurements are plotted as stars and the expected signal from the single-pole LPF is plotted in red	Figure 4: The current measurements on log-log

Due to the differences in frequency reponse for the current and voltage measurements, the resistance is not flat across frequencies.

Figure 5: The resistance measurements for channel 1 (using the data from figures 1 and 2)	Figure 6: The resistance measurements on log-log

The board shows a general pattern in that resistance measurements on channels 1 and 2 on an ADC result in a positive error ( The resistance is less than measured ). The resistances on channels 3 and 4 of an ADC have a negative error ( The resistance is greater than measured ).

Figure 7. An example of a channel 4 on an ADC

Some of the effect is due to differences in the single-pole LPF on the Drive circuit, and the double-pole LPF on the sense circuit. The results of a full spice simulation and the measured resistance for channel 4 is shown in figure 8. The filtering differences do not fully account for the negative errors (and especially do not account for the positive error in resistance measurements)

Figure 8.

There is a strong bimodal effect in the current measurements, which produce the bimodal effect in the resistance.

Figure 9. The current measurements for all the channels on the board. The colours match the ADC channel number (eg. blue is the first channel on each ADC)
Figure 10 shows the voltage measurements for all the channels on the board. The colours are the same as above.
Figure 11 shows the resistance measurements for all the channels on the board.

Its interesting that the grouped channels in the current measurement are not the same as the group channels in the resistance measurements.

Board10_current_data.dat: Board10_current_data.dat
Board10_resistance.dat: Board10_resistance.dat
Board10_voltage_data.dat: Board10_voltage_data.dat

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Effects of the filter boards

We've measured the resistances for all channels on a board with and without the additional filter boards. Comparing the two results show that the filter boards contribute less than 1% to the resistance error.

In the below figures the resistance measured with the filter board is plotted in red and the resistance measured without the filter board is filtered in blue.

Figure 9. Channel 1

Figure 10. Channel 6.

* channel_1_current_loglog.png:

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This topic: CryoElectronics > WebHome > DriveSenseTransferFunctions Topic revision: r3 - 2011-06-08 - JamesKennedy

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