Analog Devices: CN0234 Reference Design for Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor

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Kit Overview

The EVAL-CN0234-SDPZEVAL-CN0234-SDPZ circuit is a single-supply, low power battery operated, portable gas detector using an electrochemical sensor. Electrochemical sensors offer several advantages for instruments that detect or measure the concentration of many toxic gases. Most sensors are gas specific and have usable resolutions under one part per million (ppm) of gas concentration. They operate with very small amounts of current, making them well-suited for portable, battery powered instruments.

The circuit uses the ADA4505-2, dual micro-power amplifier, which has a maximum input bias current of 2 pA at room temperature and consumes only 10 μA per amplifier. In addition, the ADR291ADR291 precision, low noise, micropower reference consumes only 12 μA and establishes the 2.5 V common-mode pseudo-ground reference voltage.

The ADP2503ADP2503 high efficiency, buck-boost regulator allows single-supply operation from two AAA batteries and consumes only 38 μA when operating in power-save mode.

Total power consumption for the circuit (excluding the AD7798AD7798 ADC) is approximately 110 μA under normal conditions (no gas detected) and 460 μA under worst-case conditions (2000 ppm CO detected). The AD7798 consumes approximately 180 μA when operational (G = 1, buffered mode) and only 1 μA in the power-save mode.

This circuit uses the EVAL-CN0234-SDPZEVAL-CN0234-SDPZ circuit evaluation board and the EVAL-SDP-CB1ZEVAL-SDP-CB1Z system demonstration platform (SDP-B) evaluation board. In addition, a small adapter board included with the EVAL-CN0234-SDPZ is required to connect the two circuit boards together. The EVAL-CN0234-SDPZ includes an AD7798 16-bit Σ-Δ ADC to digitize the output voltage of the circuit.

Key Applications: Chemical Analysis, Environmental Monitoring

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Development Tools

PC with USB port and Windows XP or Windows Vista (32-bit), or Windows 7 (32-bit)
EVAL-CN0234-SDPZEVAL-CN0234-SDPZ circuit evaluation board and adapter board.
EVAL-SDP-CB1ZEVAL-SDP-CB1Z SDP evaluation board
- Blackfin Processor ADSP-BF527
- USB 2.0 Interface to PC
- JTAG header footprint
- 32MB SDRAM
- 2 x Small Footprint 120 Pin Connectors
- Configurable Peripheral Interfaces - SPI - SPORT - I2C - GPIOs - Parallel - Timers
CN0234 evaluation software
Two AAA batteries
Calibration gas (less than 250 ppm CO recommended)

Technical Documents

Learning Center

Type	Description
User Manual	ADI: User Manual for CN-0234 Single Supply, Micropower Toxic Gas Detector Using an Electrochemical Sensor
User Guide	ADI: User Guide for EVAL-SDP-CB1Z
Datasheet	ADI: Datasheet for ADA4505-1/ADA4505-2/ADA4505-4: 10 μA, Rail-to-Rail I/O, Zero Input Crossover Distortion Amplifiers (Rev D)
Datasheet	ADI: Datasheet for ADR291/ADR292: Low Noise Micropower 2.5 V and 4.096 V Precision Voltage References (Rev F)
Datasheet	ADI: Datasheet for ADP2503/ADP2504: 600 mA/1000 mA, 2.5 MHz Buck-Boost DC-to-DC Converters (Rev B)
Datasheet	ADI: Datasheet for AD7798/AD7799: 3-Channel, Low Noise, Low Power, 16-/24-Bit, Sigma Delta ADC with On-Chip In-Amp (Rev A)

Design Elements

Type	Description
Schematics	ADI: Schematics for EVAL-CN0234-SDPZ Evaluation Board
Layout	ADI: Layout for EVAL-CN0234-SDPZ
BOM	ADI: Bill Of Material (BOM) for EVAL-CN0234-SDPZ Evaluation Board

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Kit Features

Devices Used for the EVAL-CN0234-SDPZEVAL-CN0234-SDPZ Evaluation Board
Key Features for AD7798AD7798: RMS noise: 27 nV at 4.17 Hz (AD7799) 65 nV at 16.7 Hz (AD7799) 40 nV at 4.17 Hz (AD7798) 85 nV at 16.7 Hz (AD7798) Current: 380 μA typical Power-down: 1 μA maximum Low noise, programmable gain, instrumentation amp Update rate: 4.17 Hz to 470 Hz 3 differential inputs Internal clock oscillator Simultaneous 50 Hz/60 Hz rejection Reference detect Low-side power switch Programmable digital outputs Burnout currents Power supply: 2.7 V to 5.25 V –40°C to +105°C temperature range Independent interface power supply	Key Features for ADP2503ADP2503: 1 mm height profile Compact PCB footprint Seamless transition between modes 38 μA typical quiescent current 2.5 MHz operation enables 1.5 μH inductor Input voltage: 2.3 V to 5.5 V Fixed output voltage: 2.8 V to 5.0 V Adjustable model output voltage range: 2.8 V to 5.5 V 600 mA (ADP2503) and 1000 mA (ADP2504) output options Boost converter configuration with load disconnect SYNC pin with three different modes Power save mode (PSM) for improved light load efficiency Forced fixed frequency operation mode Synchronization with external clock Internal compensation Soft start Enable/shutdown logic input Overtemperature protection Short-circuit protection Undervoltage lockout protection

Key features for ADA4505-2: PSRR: 100 dB minimum CMRR: 105 dB typical Very low supply current: 10 μA per amplifier maximum 1.8 V to 5 V single-supply or ±0.9 V to ±2.5 V dual-supply operation Rail-to-rail input and output 3 mV offset voltage maximum Very low input bias current: 0.5 pA typical	Key Features for ADR291ADR291: Supply range 2.8 V to 15 V, ADR291 4.4 V to 15 V, ADR292 Supply current: 15 μA maximum Low noise: 8 μV and 12 μV p-p (0.1 Hz to 10 Hz) High output current: 5 mA Temperature range: −40°C to +125°C Pin-compatible with REF02/REF19x

Devices Used for the EVAL-CN0234-SDPZEVAL-CN0234-SDPZ Evaluation Board

Key Features for AD7798AD7798:

RMS noise:
- 27 nV at 4.17 Hz (AD7799)
- 65 nV at 16.7 Hz (AD7799)
- 40 nV at 4.17 Hz (AD7798)
- 85 nV at 16.7 Hz (AD7798)
Current: 380 μA typical
Power-down: 1 μA maximum
Low noise, programmable gain, instrumentation amp
Update rate: 4.17 Hz to 470 Hz 3 differential inputs
Internal clock oscillator
Simultaneous 50 Hz/60 Hz rejection
Reference detect
Low-side power switch
Programmable digital outputs
Burnout currents
Power supply: 2.7 V to 5.25 V
–40°C to +105°C temperature range
Independent interface power supply

Key Features for ADP2503ADP2503:

1 mm height profile
Compact PCB footprint
Seamless transition between modes
38 μA typical quiescent current
2.5 MHz operation enables 1.5 μH inductor
Input voltage: 2.3 V to 5.5 V
Fixed output voltage: 2.8 V to 5.0 V
Adjustable model output voltage range: 2.8 V to 5.5 V
600 mA (ADP2503) and 1000 mA (ADP2504) output options
Boost converter configuration with load disconnect
SYNC pin with three different modes
Power save mode (PSM) for improved light load efficiency
Forced fixed frequency operation mode
Synchronization with external clock
Internal compensation
Soft start
Enable/shutdown logic input
Overtemperature protection
Short-circuit protection
Undervoltage lockout protection

Key features for ADA4505-2:

PSRR: 100 dB minimum
CMRR: 105 dB typical
Very low supply current: 10 μA per amplifier maximum
1.8 V to 5 V single-supply or ±0.9 V to ±2.5 V dual-supply operation
Rail-to-rail input and output
3 mV offset voltage maximum
Very low input bias current: 0.5 pA typical

Key Features for ADR291ADR291:

Supply range
2.8 V to 15 V, ADR291
4.4 V to 15 V, ADR292
Supply current: 15 μA maximum
Low noise: 8 μV and 12 μV p-p (0.1 Hz to 10 Hz)
High output current: 5 mA
Temperature range: −40°C to +125°C
Pin-compatible with REF02/REF19x

Kit Contents

The Analog Devices EVAL-CN0234-SDPZEVAL-CN0234-SDPZ kit is supplied with below contents:

Evaluation Board