General Design Overview

K-type thermocouples will convert skin temperatures into small voltages which are amplified by AD595 thermocouple amplifiers. Payload ambient temperature and battery temperature will be measured using AD590 temperature sensors. A Motorola MPX5100AP pressure sensor will be used to measures the ambient air pressure. Figure 3.2.2.12 shows a block diagram of the flight instruments board.

 

Circuit Level Design

Skin Temperature Sensor- The skin temperature will be measured using a K-type thermocouple. Temperature measurement using thermocouples typically requires two thermocouples : one thermocouple for the actual measurement and a second for a reference junction, usually a 0°C reference. The AD595 provides this reference internally to the chip. This eliminates the need for a
second thermocouple. The chip also eliminates the need for an instrumentation amplifier for the thermocouple output voltage. The AD595 is designed specifically for K-type thermocouples (chromel-alumel). As shown in Figure 4, a single +5V supply powers the skin temperature thermocouple amplifier (U5). This allows temperatures from 0° Celsius to +300° Celsius to be measured. When using single supplies, pin 7 is connected to pin 4. Pin 8 is connected to pin 9 in order to provide a 10 mv/°C temperature transfer characteristic using an internal, pre-calibrated feedback network. The AD595 contains an alarm which indicates an open circuit at the thermocouple inputs. The voltage at pin 12 will be pulled low if the thermocouple opens. The thermocouple will interface to the PCB board through a SIP2 Molex connector. It is important that the connector junction be placed in a location where there is no temperature gradient. A temperature difference at the junction will cause measurement errors. The connector should be placed as close to the AD595 as possible to minimize temperature differences between the junction. Figure 3.2.2.13 shows the thermocouple amplifier circuit.