---+ Care and Feeding of the Heater Channel Op-Amps (OPA547)

Summary

• We can currently compensate for negative offsets at the heater by digitally compensating the value sent to the DAC.
• Most OPAs, unfortunately, are positively offset due to the input offset voltage. (See Pg. 6 of http://www.ti.com/lit/gpn/opa547)
• Input bias currents approximately cancel in the current circuit since R12=R15.
• Instead, skew the bias in our favour by imbalancing R12 and R15 as follows:
  • Set R12=0
  • Set R15=100k
• This introduces a negative average bias depending on the offset voltage and bias currents as shown in the table below.
• The remaining bias can be compensated digitally by bumping the DAC value

Description

OPA547 has the following specifications:

Spec	Typical	Max	Notes
Input offset voltage	+/- 1mV	+/- 5mV
Input bias current	-100nA	-500nA

These effects aren't negligible, and introduce three offsets in the circuit:

• a voltage develops across R12, when it shouldn't,
• a voltage develops across R15, when it shouldn't, and
• a voltage develops across the inputs (V+ and V-) of U5, when it shouldn't

In all cases, an offset at the op-amp input (or output) causes a corresponding offset at the heater resistance. We can compensate for negative offsets (i.e. v_heater < 0 when v_dac = 0) by adjusting the DAC output to match. The DAC is not capable of generating negative voltages, so a positive offset (v_heater > 0 when v_dac = 0) can't be compensated in this way.

Current Configuration

Input bias currents are always negative (i.e. current drawn into the device) and are approximately matched between the inverting and non-inverting input. Since R12==R15, the voltages developed across these two resistors should be approximately equal, and the effects of bias current are minimized.

Input offset voltage, however, introduces an output bias (mean 1 mV, sigma ~1.5mV) that is random and mostly positive. This biases most heaters positively, meaning we can't correctly generate 0 current flow with the heater switch closed.

Proposed Fix

If we intentionally mismatch R12 and R15, we can bias the offset negatively. Consider R12=0 and R15=100k. Then,

	Offset Voltage
Bias Current	-5mV	1mV	5mV
-100nA	-500uA	-300uA	-170uA
-500nA	-1.8mA	-1.6mA	-1.5mA
Current through Heater; DAC = 0

At nominal input offset (-100nA) and mean offset voltage (1mV), this gives us a bias of -300 uA. At worst case (offset = -5mV, bias=-500nA), we lose <2% of the DAC's dynamic range.

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This topic: CryoElectronics > DigitalFMux > CryoElectronicsDesignFiles > CryoElecRev3_Issues > HeaterChannelOpAmpCareAndFeeding Topic revision: r1 - 2010-12-03 - GraemeSmecher

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