OPA396 datasheet(18/30 Pages) TI

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8.2.1.1 Design Requirements

For this example, an electrical model of a CO sensor is used to simulate the sensor performance, as shown in

Figure 8-2. The simulation is designed to model a CO sensor with a sensitivity of 69 nA/ppm. The supply voltage

and maximum analog-to-digital converter (ADC) input voltage is 2.5 V, and the maximum concentration is 300

ppm.

2.5 V

OPA396

VREF

260 mF

130 mF

0 µA to 20 µA

110 kŸ

10 kŸ

CO Sensor

Model

VTIA

ISENS

VCE

VREF

Figure 8-2. CO Sensor Simulation Schematic

8.2.1.2 Detailed Design Procedure

First, determine the VREF voltage. This voltage is a compromise between maximum headroom and resolution, as

well as allowance for the minimum swing on the CE terminal because 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 180 mV at 300 ppm for this particular sensor. To allow for negative CE

swing, footroom, and voltage drop across the 10-kΩ resistor, 300 mV is chosen for VREF.

VZERO = VREF = 300 mV

(2)

where

• VREF is the reference voltage (300 mV).

• VZERO is the concentration voltage (300 mV).

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

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

(3)

where

• ISENSMAX is the maximum expected sensor current.

• IPERPPM is the manufacturer specified sensor current in amperes per ppm.

• ppmMAX is the maximum required ppm reading.

OPA396

SBOSA65 – JULY 2021

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