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Metrology / Re: Experiments with Vref Ovens
« Last post by dietert1 on Today at 09:22:22 pm »Yesterday i tested the experimental TEC oven driver shown in the image.
It has a Nucleo STM32L432 module, an ADS1256 ADC module, a linear power supply for the output stage with 4x IRF540 and a 0.235 Ohm shunt. The SIC473 buck converter adapts the supply voltage according to TEC output current, so the lower mosfets of the bridge stay cool and work in linear mode. The design TEC current is +/- 2 A.
The output stage is controlled by a DG4053 MUX for heat/cool switching and an OPA140. Hidden below the ADC module is another MUX for voltage reversal of the thermistor and its reference resistor (blue, 10K2, 5 ppm/K). Also there is a DAC8551 for TEC current control.
The thermometer with voltage reversal gives one measurement per second at a noise level of 10 uK (standard deviation). Noise of the resistance measurement is 0.4 ppm.
So i connected a little oven and implemented configurable PID control. Gains were adjusted by hand. They are pretty high and the first diagram shows the damped oscillation (47 s period). At steady state the effectve gain is less due to thermometer noise and TEC current digitization noise. The diagrams show temperature and TEC current logs at two different time scales. Oven temperature in °C in red and TEC current in A in blue. The oven and its heatsink are protected from air movement. During the last hour the oven temperature log exhibits a 11 uK standard deviation.
The thermometer circuit has unused channels, so one can also measure ambient or TEC heatsink temperature and/or use a second thermistor mounted on the oven lid.
Last image shows a similar four channel TEC output stage that i made for a LTFLU reference array. It has an OPA4140 and a DAC8554. There is only one buck converter as all four ovens run at similar temperatures.
Regards, Dieter
It has a Nucleo STM32L432 module, an ADS1256 ADC module, a linear power supply for the output stage with 4x IRF540 and a 0.235 Ohm shunt. The SIC473 buck converter adapts the supply voltage according to TEC output current, so the lower mosfets of the bridge stay cool and work in linear mode. The design TEC current is +/- 2 A.
The output stage is controlled by a DG4053 MUX for heat/cool switching and an OPA140. Hidden below the ADC module is another MUX for voltage reversal of the thermistor and its reference resistor (blue, 10K2, 5 ppm/K). Also there is a DAC8551 for TEC current control.
The thermometer with voltage reversal gives one measurement per second at a noise level of 10 uK (standard deviation). Noise of the resistance measurement is 0.4 ppm.
So i connected a little oven and implemented configurable PID control. Gains were adjusted by hand. They are pretty high and the first diagram shows the damped oscillation (47 s period). At steady state the effectve gain is less due to thermometer noise and TEC current digitization noise. The diagrams show temperature and TEC current logs at two different time scales. Oven temperature in °C in red and TEC current in A in blue. The oven and its heatsink are protected from air movement. During the last hour the oven temperature log exhibits a 11 uK standard deviation.
The thermometer circuit has unused channels, so one can also measure ambient or TEC heatsink temperature and/or use a second thermistor mounted on the oven lid.
Last image shows a similar four channel TEC output stage that i made for a LTFLU reference array. It has an OPA4140 and a DAC8554. There is only one buck converter as all four ovens run at similar temperatures.
Regards, Dieter