echem328SSmart TTransducer IInterface MModule

Data Sheet

Order Info

The echem328 Smart Transducer Interface Module (STIM) can autonomously sense atmospheric or process concentrations of a target gas (ozone, carbon monoxide, nitrogen dioxide, hydrogen, sulfer dioxide, etc.) with low noise and outstanding precision. It is compatible with most electrochemical gas sensors now on the market, provides other remote sensing and data acquisition functions via spare I/O pins, and is programmable using the standard Arduino development environment.

The echem328 introduces a new more integrated form factor that is easily added to embedded systems using a variety of standard interface protocols such as serial TTL, RS232, I2C, SPI, USB, BLE, and wired or wireless ethernet. This microcontroller-based “smart sensor” hub enables features like auto-calibration, timed network data feeds, and a local user interface and display.

The echem328’s sophisticated four-layer PCB design provides excellent noise shielding and ensures signal integrity. The board is compact (1.75" square, 2.35" diagonal) and consumes little power (<10mA max@ 3.3V). A universal pad layout on the back side of the board allows a socket for most any gas-sensing electrochemical cell to be installed. Many options are selectable by jumper installation or software modification, and thoughtful signal layouts make system integration a snap. The connectors are stackable and on a 0.1” grid for easy interfacing and prototyping.

Features● Familiar, low-cost, low-power AtMega328P microcontroller just like on the popular Arduino Uno board ,

with the standard Optiboot bootloader.

● LMP 91000 Analog Front End is a programmable interface to most electrochemical gas sensors .

● ADS 1115 16-bit / 4-channel analog to digital converter with Programmable Gain Amplifier can also can be used for thermocouple, RTDs, load cells, pH probes, etc. (12-bit option for lower-cost OEM versions)

● ADR 380 high accuracy 2.048V reference for the AFE, usable for external sensor excitation.

● LMP 2231 op-amp transconductance amplifier/unity buffer for “aux” temperature-compensation terminal or as picoamp-sensitive input buffer for pH probes, photodiodes, and other high-z sensors.

● SPX 3819 precision low-noise LDO voltage regulator provides either 3.3V or 5V (jumper-selectable) down to 0.1mA minimum load, up to 100mA peak for peripherals like Bluetooth modules (limited by sensor self-heating error). A power disable input allows low-power sleep. The on-board regulator can be bypassed via jumpers on PVR connector if a clean 3.3-5V power supply is already available.

● DS 1339 Real Time Clock with alarm functions and rechargeable lithium cell backup battery (optional)

● Standard I2C EEPROM pinout accommodates up to 128Kx8 M24M01 for on-board calibration and logging memory.

● Polyfuse overcurrent/Schottky diode reverse protection circuitry for bulletproof power.

Documentation● Schematic (pdf)● More documentation to be released after further validation

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Electrical Specifications @20 ℃

Parameter Test Conditions Min Typ Max Units

VS Supply Voltage Input to VREG, (Vcc jumper = 3.3V), Vcc jumper = 5V (3.5) 5.4 16 V

IQ Quiescent Current Venable ≤0.4V (Voltage regulator output off) 0.05 3 μA

VCC Circuit Voltage 3 5.25 V

ICC Circuit Current (depends mostly on Vcc level) 5 15 mA

ICCout Ext Circuit Current 0 500 mA

IL Current Limit Short circuit regulator limit, same as polyfuse 750 900 mA

PSRR Power Supply Rejection Ratio “Ripple Rejection” 70 dB

eNO Power Supply Output Noise @ 10mA load current 500 5000 μVRMS

ADDR Analog to Digital Data Rate auto-sample mode 8 860 Samples/sec

ADVR Converter Voltage Resolution 1 bit weight with 216 bits, per Prog Gain Amp set 62.5 1500 μV

VRERR Reference Error Over common operating range of 0-40 ℃ -0.5 0.5 % total error

BiasRW Bias Programming Range Percentage of voltage as a differential between Ref and Working sensor electrodes

±24 % referred to VREF / VDD

TIAAFE Programmable Gain Resistors 2.75, 3.5, 7, 14, 35, 120, 350 selectable 5 KΩ ±5%

TIAEXT Fixed Gain Resistor Optional on-board precision metal film resistor 200 KΩ ±0.1%

Environmental SpecificationsParameter Test Conditions Min Typ Max Units

Operating Temp sensor’s range may be less -40 20 60 ℃Humidity sensor’s range may be less, no condensing moisture 0 99 % RH

Weight w/o sensor, depends on options 10 15 g

Sensor Specifications (CO shown*, specs from vendor)

Vendor Model Sensitivity (nA/ppm)

Noise(ppb)

MSRP (USD)

Alphasense CO-A4 220 - 375 20 $100

Alphasense CO-B4 420 - 620 4 $125

DD Scientific S+7CO 80-120 500 $38

DD Scientific GS+4CO 50-90 1000 $32

SGX Sensortech SGX-7CO 80 - 120 500 $45*Sensors also available for: AsH3, B2H6, Cl2, CH4S, (CH2)4S, ClO2, COCl2, ETO, F2, H2, HCl, HCN, HF, H2S, NH3, N2H4, NO, NO2, O2, O3, PH3, SO2, SeH2, SiH4

Mechanical Dimensions

Calibration

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Typical sensor used with the echem328. Image by Alphasense,

Enter the calibration numbers shipped with your Alphasense sensor using our simple terminal-mode interface and they are poked into on-board non-volatile memory, referenced whenever the board is powered up or reset. If you're using another brand of sensor that's not calibrated at the factory you'll need to do your own calibration, consult CircuitSci for assistance, we're developing software and apparatus to make this easy and accurate.

Typical resolution and noise at ppb-level CO concentrations with an Alphasense CO-B4 sensor. Test conditions: Rload = 50Ω, chamber volume ~100mL, flow rate = 500mL/min, Sensirion SFC4100 mass flow controllers,

balance gas = N2, @25°C. (malformed first and last steps due to calibration rig error)

Connectors● Ten-pin expansion port PX allows for debug, user interface, or other custom functions, and includes two

spare digital pins, four analog input pins that can also be regular digital I/O pins, and two uncommitted LEDs for connecting to whatever signals you might want to monitor. PX also includes an optional jumper from board VREF to the MCU’s AREF input for much-improved A/D performance.

● 20-pin I/O port PBUS includes UART, SPI, and I2C signals and two interrupts which can be jumpered to A/D and real-time clock alert signals. Several smaller signal groupings and standard pinouts are included within this bigger connector:

○ The left 2x3 pins (1-6) form the standard Arduino ISP (In-System Programming) connection including main SPI signals. This is used to upload the Arduino bootloader at our factory but can also be used for SPI peripherals or to upload MCU code directly using another development environment.

○ Even-numbered pins 6-16 form a standard “FTDI Friend” serial connection for USB, Bluetooth, and other UART-based protocol adapters.

○ MCU interrupts INT0 and INT1 are at PBUS pins 20 and 18 respectively. These pins can alternately be used as spare digital I/O pins (D2/3).

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Sensor App Notes● Electrochemical Sensors Application Note 2 - Design of Electronics for Electrochemical Gas Sensors - SGX● Designing a Potentiostatic Circuit - Alphasense App Note AAN105-03● Environmental Changes : Temperature , Pressure , and Humidity - Alphasense App Note AAN110● CN 0234: Single Supply , Micropower Toxic Gas Detector Using an Electrochemical Sensor -

Analog Devices app note● How Electrochemical Gas Sensors Work -- Alphasense app note AAN104● Humidity Extremes : Drying Out and Water Absorption -- Alphasense app note AAN106● Modeling Amperometric Electrochemical Gas Sensors -- Alphasense app note AAN111-02

Circuit Function Table

Function Desig Description Device Notes

ADC U6 4-channel 12/16-bit Analog-Digital Converter

ADS1015 / ADS1115

PAI connector with only ADC and related passives installed on the board allows use as I2C-based 4-channel data acquisition board. SE & diff input options,programmable gain x 0.67/1/2/4/8/16.

AFE U5 echem sensor Analog Front End

LMP91000 Required for any echem sensor. AFE’s potentiostat may also be useful for other electrochemical research.

VREF VR2 Precision Voltage Reference 2.048V

ADR380 Required for precision AFE echem readings, AFE can use VDD/2 without VREF for low-precision/higher noise.

OPA U4 Precision Low input current Op-amp / Buffer

LMP2231 Required for Alphasense echem sensors with AUX temp compensation pin. Can also be used for other buffer/amp applications (pH, optodiode, etc.) via PAI.

EEP U3 Electrically Erasable Memory (EEPROM)

24M01 Standard pinout can accept other memory sizes and technologies. RE7 jumper will connect signal MNB to EEPROM address signal A0 so memory and sensor can be associated via I2C with no local MCU (see AFE datasheet).

RTC U2 Real Time Clock w/Alarms DS1339 Also requires rechargeable battery BT1 and crystal Y2 and related passives, can be used without local MCU but has only one I2C address.

MCU U1 MicroController Unit ATMega328P Mostly optional for all circuit functions although some additional control pins like MENB may be required beyond simple I2C control (MCU can also control local I2C via soft I2C or soft serial on spare pins).

RST U7 Reset Monitor TPS3808 MCU RST can be used from a remote MCU for local wired-OR reset operation of a larger system.

VREG VR1 Power Voltage Regulator, LDO Adjustable

SPX3819 Optional if clean, stable 3.3V-5V system power is available, can be easily bypassed via PVR jumpers. 3.3V/5V jumper with precision resistors.

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Typical AFE/ADC Connections at PAIThe standard circuit configuration puts the AFE_HALF signal on AIN3, which the ADC allows to be used in pseudo-differential fashion with inputs 0 and 1 as the positive input, depending on ADC multiplexer selections (see ADC datasheet and circuit description below for more details). The following table shows the standard echem sensor jumpers for typical operation. PAI pin numbers shown in (parenthesis).

PAI pin numbers shown in (parenthesis).AFE Outputs ADC Inputs Description Notes

AIN0 (11) Sensor temp monitor input External TO-92 analog temp sensor mounted against side of echem sensor

AUX_OUT (10) AIN1 (9) Output of opamp U4 AUX pin amplifier

AFE_TEMP (8) AIN2 (7) Output of AFE transconductance amplifier Accurate to a few deg but good enough for echem temp compensation, see datasheet for cal table.

AFE_HALF (6) AIN3 (5) Halfway operating point of AFE potentiostat, also the reference for GAS and AUX signals

Useful as a diagnostic, should be 1024 mV ±2mV except right after power-up when it might start a few mV lower and drift up.

AFE_HALF (3) AFE_POS (4) Halfway point of AFE also goes to AUX opamp + input as AUX signal reference

Aux Amplifier ConnectionThe fourth “AUX” terminal on Alphasense echem sensors provides temperature compensation from sensor temperature errors. Currently Alphasense has patented this technology and is the only vendor supplying it. Basically, the Auxiliary electrode is a “dummy” working electrode that is physically in parallel but does not have the same potentiostat current from the Counter electrode. Thus the voltage impinged on the AUX pin is almost entirely due to the temperature offset of the sensor chemistry and can be subtracted from the main sensor signal.

The transistor (T1 in the circuit below, Q1 on the echem328 circuit board) performs a similar function to an internal shorting FET inside the AFE. Both are used to short sensor electrodes during power down, otherwise the electrolytic charge held in the device will typically cause large errors on the next power up that can take hours to equilibrate.

AUX amplifier connection to LMP 91000 AFE per TI app engineering note showing the Aux pin TIA (U4) and other sensor connections including an electrical model of the echem gas sensor.

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InterruptsTwo interrupts are available in this system via the Arduino software environment. INT0 and INT1 are located at the end of PBUS (pins 18 and 20) and are typically jumpered across to Real-Time Clock alert for wake on time, and to A/D Alert output for wake on signal threshold respectively. Parameters for both of these alarm settings can be set via I2C bus. However, these pins can be used for other system interrupts, or can be alternately used as standard digital I/O pins (D2/3).

Sensor SocketsAndon Gas Sensor Socket Catalog shows sockets for most available sensors. Page 28 shows adapters between A & B sizes that might be useful. Sockets available from cdiweb.com.Alphasense A-size = Andon R530-0403-02N-440P55-R27-L14 Alphasense B-size = Andon R670-0804-02N-285E-R27-L14

echem328 Ordering Information – Introductory pricing!!!Small quantities available Q1/2015, ships ~2 weeks ARO, calibration adds ~2 weeks.

echem328 Model

Cofiguration Notes Price (USD)

Datalogger No echem AFE but 16-bit 4ch A/D, RTC, EEPROM

Built and Tested

Standard echem AFE & 16-bit 4ch A/D, no RTC Built and Tested, sensor extra **

Deluxe “The Works” – with RTC and max memory

Built and Tested, sensor extra**

Pro “Ready to Go” – Plug it in, set up parameters via serial port and start collecting data

Built and Working with sensor, checked with NIST-traceable cal gas and your sensor (CO-B4 only for now), you get cal constants, graphs, certs

OEM Custom per your requirements Cheaper 12-bit version for indoor AQ available, Minimum order TBD

“contact factory”

** Sensor socket of your choice (Alphasense -A4, -B4 or SGX SensorTech EC4-style installed for additional $10 (parts+labor).

bakPAKsbakPAKs are Peripheral and Accessory Kits that provide expanded connectivity, memory, and control options to the base echem328 circuit board. A range of these are envisioned for a myriad of communication protocols and customized configurations, but the most universal one to start with is the Grove. Let us know what kind of bakPAK you'd like to use and we'll add it to our list!

A bakPAK mates with the STIM board underneath primarily via the PBUS and secondarily the PX connector using female plugs mounted on the bottom side on the bakPAK. A plug with long thru-pins may be used to extend the signal to yet another connector or bakPAK or prototype board in the stack. Also, bakPAKs can easily be prototyped using standard 0.1” grid perfboard as the corner mounting holes and all of the connectors of the echem328 are laid out in a standard 0.1” grid.

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