---+ 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:
- 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:
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.
This topic: CryoElectronics
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CryoElecRev3_Issues > HeaterChannelOpAmpCareAndFeeding
Topic revision: r1 - 2010-12-03 - GraemeSmecher