This page describes the considerations for choosing the reference resistor, or feedback gain in the cryoboard Heater Circuit.

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Regimes
This board will be used to heat both the pumps and switches using the circuit shown in figure 1. The heater resistors range from 10 kohms (switch resistors) to 400 ohms (UC pump resistor) to 200 ohms ( inter pump resistors). We typically apply maximum currents of 0.45 mA to the switches (Set 4.5V), and 60mA to the pumps (Set 12 V across interpumps)
We would like the cryoboard to be able to work within these extreme values.

Figure 1

Dalberg,2009

We have seen that the heater DAC behaves non-linearly at low DAC voltages and has an offset, as shown in figure 2. Driving a 0.45mA current across a 1.5ohm reference resistor, should put us at 267 DAC counts. In this case, it's the offset which is hurting us.
Figure 2

The manufacturer has tested the DAC linearity from 485-64714 DAC COUNTS.

Three options for us are:

1. Feedback to adaptively correct for erroneous currents.
2. Make the gain variable, so the user can adjust its values based on the load they're working with.
3. Select a combination of feedback gain and reference resistor such that both the high and low currents are in the dac linear regime.

For option 1. We've measured some step-y structure when moving from one DAC setting to the next. We think this is due to a long time constant in the system, but need to look at this more. This time constant would require us to have a long feedback time ( 10 iterations in feedback, each waiting > 2 seconds before measuring ).

For option 2. This would require some crython development, since the board would need to store, read, and use the heater gain resistors in a similar way to the sensor gain resistors. Also this could be annoying to users.

For option 3. We see that there is an offset of 8 mV at the dac (for the low dac values). If we require that an offset of 8 mV only produces a 5% error, the voltage at the dac must be 160 mV when we request a drive current of 0.45 mA (the 8 mV offset will be relatively smaller for higher drive currents). This puts some constraint on our reference resistor and feedback gain combination.

%BEGINLATEX% $ (0.45)\times 10^{-3} \times ReferenceR \times Gain = 160 mV $ %ENDLATEX% %BEGINLATEX% $ ReferenceR \times Gain = 355 $ %ENDLATEX%

However this reference resistor - feedback gain combination is way to high for currents of 60 mA

%BEGINLATEX% $ (60) \times 10^{-3} \times ReferenceR \times Gain = $ %ENDLATEX% %BEGINLATEX% $ (60) \times 10^{-3} \times 355 = 21 V$ %ENDLATEX%

There doesn't appear to be a fixed reference resistor/feeback gain value which would work for our requested currents.

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This topic: CryoElectronics > WebHome > HeaterCircuitFeedback Topic revision: r4 - 2011-04-21 - JamesKennedy

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