# Zero Drift Infrared Radiation Thermometer Using Chopper Stabilised Pre-Amplifier
## 1 Introduction
A particular type of op-amp that offers improved performance over a precision op-amp is the chopper stabilised or zero-drift, op-amp. Typically, chopper stabilised op-amps incur very low input offset voltage, achieving an order of magnitude lower offset voltage than precision op-amps. Equally, the temperature-related drift in this offset voltage is typically three orders of magnitude lower for chopper-stabilised op-amps compared to precision op-amps. Both precision and chopper-stabilised op-amps afford sufficiently low noise magnitude to enable small signals to be amplified to measurable magnitudes with signal-to-noise ratios greater than unity [9, 10, 19, 20]. Chopper-stabilised op-amps modulate the input voltage with a square wave carrier signal and demodulate the resulting signal to isolate the output voltage from low-frequency noise [20]. This modulation and demodulation occur within the op-amp's integrated circuit, using transistor switches to afford kilohertz modulation frequencies. This modulation method results, ultimately, in the very low offset voltage and offset voltage drift of the chopper stabilised op-amp output voltage [20, 21, 22]. Chopper-stabilised op-amps have been used for the measurement of signals within various applications, including signals from physiological potential sensors, micro-electro-mechanical systems (MEMS) gas measurement sensors and thermopiles in presence detectors. [PERSON] et al., [PERSON] et al., [PERSON] et al., [PERSON] et al., and [PERSON] et al., have demonstrated the application of chopper stabilised amplifiers to the acquisition of measurements from sensors having low output signal magnitudes [5, 13, 14, 15, 23]. [PERSON] et al., and [PERSON] et al., have further demonstrated the suitability of applying chopper stabilised op-amps to the amplification of signals from low signal sensors with elevated impedance to achieve measurements with low noise and offset [23, 24]. Chopper-stabilised op-amps have been demonstrated to be capable of amplifying weak sensor output voltages, of the order of microvolts, with sufficient signal to noise ratio to enable extraction of measurements [5, 12, 13, 15, 25, 26]. Given the benefit provided by chopper stabilised op-amps for such applications, it would be reasonable to expect them to also offer benefits within an IRT incorporating an uncooled MWIR photodiode. Use of a chopper stabilised op-amp within such an IRT could enable the measurement of lower minimum temperatures, associated with low magnitude photocurrents generated and voltages amplified from them, without compromising the IRT response time, sensitivity, measurement uncertainty or range of the temperature measurement. A fibreoptic coupled MWIR thermometer is presented that comprised a bootstrapped Indium Arsenide Antimony (InAsSb) photodiode connected to a novel TIA circuit that utilised a chopper stabilised op-amp. The usage of a chopper-stabilised op-amp within the TIA should afford lower input voltage offset, noise and offset voltage drift than an equivalent IRT using a precision op-amp, thereby affording lower minimum temperatures to be measured. The noise and offset voltage of the resulting 'zero-drift' IRT was compared to a similar MWIR thermometer comprising an identical circuit, except that the TIA used a precision op-amp. The comparison between the IRTs was used to infer whether the advantages expected from the chopper stabilised op-amp IRT was demonstrable in laboratory-based measurements. Performing analyses without any lens optics, both with and without the fibreoptic in place, enabled a direct comparison of the IRTs to be undertaken, without the uncertainties associated with optical alignment. In this work, we measure the temperature of calibrated blackbody sources in units of degree Celsius, \({}^{\circ}\)C. However, for subsequent analysis of instrument noise performance and resolution, we differentiate performance from a position within the temperature scale by using the different unit of kelvin, K, where 1 K = 1 \({}^{\circ}\)C.