Time (10 ms/div)

0.3 V

2.5 V

20 PA

C012

VTIA

VCE

ISENS

17

TLV6003

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SLOS981 – OCTOBER 2019

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8.2.2 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.

Therefore, 300 mV is used as the minimum VZERO to add some headroom.

VZERO = VREF = 300 mV

where

•

VZERO is the zero concentration voltage.

•

VREF is the reference voltage (300 mV).

(2)

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

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

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.

(3)

Then, find the available output swing range greater than the reference voltage available for the measurement:

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

where

•

VSWING is the expected change in output voltage

•

VOUTMAX is the maximum amplifer output swing (usually near VCC)

(4)

Finally, calculate the transimpedance resistor (RF) value using the maximum swing and the maximum sensor

current:

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

(5)

8.2.3 Application Curve

Figure 34. Sensor Transient Response to Simulated 300-ppm CO Exposure