9.2.1 Design Requirements

The application requires a microcontroller (MCU) such as Texas Instruments MSP430x, or TM4C129x series of MCUs connected via Synchronous Serial Interface (SPI) to the LMP93601 AFE. As shown in Figure 29, the X and Y decoder lines of the thermopile sensor (transducer) needs to be interfaced to the GPIO (General Purpose Input Output) pins of the MCU. The LMP93601 would require an external clock signal of 4 MHz (±3%).The timer subsystem of the MCU is well suited to generate the clock signal. A timer output pin shall be used to interface the output of the MCU timer to the XCLK pin of the LMP93601.

The device provides a reference voltage output (VCM) source to center the output of the sensor around AVDD/3. If the sensor does not provide a reference input pin, the INPx, INNx inputs of the LMP93601 should be connected (pulled-up) to VCM output via 2.2 MΩ resistors.

The data converter of the LMP93601 is an incremental delta-sigma type ADC, the modulator is reset after every conversion. Therefore, after each thermopile element is sampled and the ADC modulator goes through a reset, there is no possibility of any trace of signal from the previously sampled element.

To sample each thermopile element in the sensor array, the X and Y decoder signals must be provided over the GPIO, then firmware should allow settling of the data on the decoder. The SYNC pin is provided to signal the beginning of conversion to LMP93601 ADC. A GPIO of the MCU needs to be interfaced to the SYNC input pin of the LMP93601 for this purpose.

9.2.2 Detailed Design Procedure

In thermopile array systems, settling time of the signal path plays an important role when higher frame transfer rates are desired. A frame is an array of X by Y thermopile elements. While the data from each pixel is being transferred out of the sensor in a sequence, the MUX output must settle to the proper voltage from the element in the array that is being accessed. The total analog signal path settling time is determined by combined sensor’s settling time (t_ssnsr) and AFE’s settling time (t_safe). The settling time is determined by the sum of capacitances of the following: pixel in the array, sensor’s MUX, AFE’s MUX, PCB, AFE’s input stage, and sensor’s resistance.

To achieve faster settling time, total capacitance in the signal path should be kept as low as possible. Therefore, the system designers should take the resistance of the sensor and the related capacitance into consideration to achieve optimal performance of the signal path. For example, the pixel-to-pixel settling time should be keep below 70 μs to process five frame per second (1326 SPS) using a 16 x 16 array sensor. A simplified thermopile array sensor equivalent circuit is shown in Figure 30.

Figure 30. Simplified Thermopile Sensor Array

The value of biasing resistors; R1 and R2, should be much higher than the value of the sensor output resistance RSx (that is, R1 = R2 > 10 * RSx).

The bias current of the sensor and the leakage current of the sensor’s MUX should be considered as well. In Figure 31 R1 and R2 need to be matched closely to avoid introduction of differential offset error voltage in the signal path due to mismatched current flow through these resistors. Moreover, Ios through RSx needs to be calibrated out over temperature. To simplify the circuit in Figure 31 the MUX inside the AFE is not shown.

Figure 31.