Serial ADC implemented as a two channel thermometer.

I used the Serial ADC and added a dual channel Platinum (PRC) sensor interface.

Serial ADC with two probes.

Inside the prototype.

The circuit uses a constant current drive and a four wire connection to the probes. This allows almost any length of probe connecting cable without re-calibration.

The current is set to 1.000mA. In the circuit I make use of the new MAXIM® MAX4194 series of precision differential amplifiers.

Circuit Diagram.

A few notes about the construction:-

Surface mount chips are wonderful, but for prototype work these can be difficult to use. To make it easier I constructed a SO to DIP adapter,

Picture of SO to DIP

I needed two simple probes for air and water temperature measurement, and constructed them using fiberglass circuit board material, the Platinum element is embedded in heatsink compound. I used a mini-DIN plug as there are plenty of cheap keyboard extension very flexible cables available with mould plugs. The entire probe is covered in heat shrink tubing and the end is sealed.

The Probe

The sensor element.

Sensor elements (PC100) are of the type CRZ-2005 class 1/3DIN by http://www.micro-ohm.com/temperature/templatm.html

Calibration was done using the Serial ADC software. Calibration references were crushed ice in water for 0°C and boiling water for 100°C. Of course all measurements were checked against a Mercury thermometer ! http://www.brooklynthermometer.com/

Probe Response.

As can be seen there is a difference in thermal response ( not accuracy), this is due to a slight difference in the amount of heatsink compound applied during construction of the each probe.

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