CCA2D2 v1

CCA2D2 v1 Description and Specification

A new analog and digital interface we call the CC A2D2 v1 was developed at CC in Feb-Mar 2001 for several different projects at CC. The Data and Models project needed an ultrafast response temperature probe, the TEEMS and Exploratorium Electronic Guidebook projects needed an interface to which a number of different analog and digital probes could be connected. The TEEMS project has a particular interest in seeing the cost of the probes be as little as possible. All three of these projects planned primary deployment on handheld computers such as the Palm and PocketPC systems as well as secondarily on full-size computer systems.

In order to handle many types of analog sensors as inexpensively as possible we built the high resolution conversion around the LTC2402, a $7 2-channel 24-bit analog-to-digital convertor from Linear Technology. With a maximum sampling rate of 7.5 Hz this convertor is only appropriate for measuring slowly changing signals. However its tremendous resolution allows us to directly connect it to un-amplified sensors and still get useful data.

The high-resolution of the LTC2402 allowed an elegant solution to the problem of creating an ultra-fast response temperature probe. We selected 0.005 diameter Type E thermocouple wire as the sensing element. A Type E thermocouple produces a signal of approximately 40 uV per degree C difference between the two junctions of the thermocouple wire. We measure the actual temperature of one junction with the TMP36, a linear IC-based temperature sensor from Analog Devices. We then calculate the temperature of the exposed junction by measure the voltage produced by the temperature difference. Taking into account the response of the sensor, the resolution of the convertor, and the electrical noise in the system we end up with a practical resolution of 0.1 degrees C

CC A2D2 v1 Specifications

Communications:

- RS-232, 0-5V signal levels, 9600 bps, 1 stop bit, no parity, no handshaking
- DB-9M connector, similar pinout to PC serial port, no handshake output

Power:

- 1.5 mA at 3.3 V, powered from the handshake input line (pin 8) on DB-9M serial connector

High resolution 24-bit analog inputs:

- 2 channels, 24-bit resolution, 0-2.5V inputs, sampling rate 6 Hz, automatically alternating from channel 0 to channel 1

Low resolution analog inputs

- 2 channels, 10-bit resolution, 0-3.3V inputs, sampling rate to 200 Hz, automatically alternating from channel 0 to channel 1

Digital I/O

- high speed digital I/O. The first collection mode supports measurement of quadtrature encoded signals for measuring angular resolution from our smart wheel.