# Understanding SPEC CO Sensors

Hello
We have lost our Analog Engineer and I need some help in the fundemental understanding of these SPEC Sensors

We have followed the application guide for the CO Sensor

According to the CO Sensor it required a 3mV Bias.
Question 1.
Can I just clarify that U3 Pin 3 will be 3mV higher than U2 Pin 3 ?

When using a 3V Battery on the SPEC Analog Board Vref will be approx 1.5V
Question 2.
What is the working voltage output when CO = 1000ppm and how is this calculated?

We would like to use our ADC with a Vref of 1.5V - ie When the CO = 1000pm the working voltage is 1.5V
Question 3.
What will be the resistor values be for R1, R2, R3 on the Analog Sensor Dev Board

Thank You for you valued assistance
KR
Paul

For clarification, the image above seems to be from the schematic for the ULPSM v1.1, found here.

The companion settings table document is useful in speaking to these questions.

Yes: this document defines the “bias” as the potential of the sensor’s working electrode with respect to the reference electrode. When R2 in your schematic is chosen as 2KΩ and a 3.0V source is used, configuring the bias jumpers in the indicated positions should result in the potential at U3:A pin3 being approximately 3mV more positive than that at U2:A pin 3.

The sensor generates an output current which is imposed across the R6/R7/R11 network and added to the reference voltage to generate an output. Assuming R6=100KΩ and nominal sensitivity per the table, for concentration of 1000PPM and Vreg=3.0V:

It’s unclear what precisely you’re asking here; an ADC requires some “reference” voltage to establish the scaling between bits and volts, but this is not necessarily the same as the Vref provided by the sensor board. Many converters would be quite unhappy with that arrangement, actually.

One sets the desired sensor bias level by the ratio of R2:(R1+R2+R3), and desired Vref using the ratio of R1:(R1+R2+R3). Suggestions for achieving optimal performance would vary depending on the architecture of your chosen ADC, and what “performance” means in your specific context.