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LPV811, LPV812

SNOSD33B – NOVEMBER 2016 – REVISED NOVEMBER 2016

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Typical Application: Three Terminal CO Gas Sensor Amplifier (continued)

8.2.2 Detailed Design Procedure

For this example, we will be using a CO sensor with a sensitivity of 69nA/ppm. The supply voltage and maximum

ADC input voltage is 2.5V, and the maximum concentration is 300ppm.

First the VREF voltage must be determined. This voltage is a compromise between maximum headroom and

resolution, as well as allowance for "footroom" for the minimum swing on the CE terminal, since the CE terminal

generally goes negative in relation to the RE potential as the concentration (sensor current) increases. Bench

measurements found the difference between CE and RE to be 180mV at 300ppm for this particular sensor.

To allow for negative CE swing "footroom" and voltage drop across the 10k resistor, 300mV was chosen for

VREF.

Therefore +300mV will be used as the minimum VZERO to add some headroom.

VZERO = VREF = +300mV

where

•

VZERO is the zero concentration voltage

•

VREF is the reference voltage (300mV)

(3)

Next we calculate the maximum sensor current at highest expected concentration:

ISENSMAX = IPERPPM * ppmMAX = 69nA * 300ppm = 20.7uA

where

•

ISENSMAX is the maximum expected sensor current

•

IPERPPM is the manufacturer specified sensor current in Amps per ppm

•

ppmMAX is the maximum required ppm reading

(4)

Now find the available output swing range above the reference voltage available for the measurement:

VSWING = VOUTMAX – VZERO = 2.5V – 0.3V = 2.2V

where

•

VSWING is the expected change in output voltage

•

VOUTMAX is the maximum amplifier output swing (usually near V+)

(5)

Now we calculate the transimpedance resistor ®F) value using the maximum swing and the maximum sensor

current:

RF = VSWING / ISENSMAX = 2.2V / 20.7µA = 106.28 kΩ (we will use 110 kΩ for a common value)

(6)