0

2

4

6

8

10

12

14

16

18

20

0

15

30

45

60

75

90

105

120

135

150

Time (sec)

C002

0

50

100

150

200

250

300

0

15

30

45

60

75

90

105

120

135

150

Time (sec)

C003

0.00

0.25

0.50

0.75

1.00

1.25

1.50

1.75

2.00

2.25

2.50

0

15

30

45

60

75

90

105

120

135

150

Time (sec)

Vc

Vw

Vtia

Vdif

C007

17

LPV811, LPV812

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SNOSD33B – NOVEMBER 2016 – REVISED NOVEMBER 2016

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

8.2.3 Application Curve

Figure 47. Monitored Voltages when exposed to 200ppm CO

Figure 47 shows the resulting circuit voltages when the sensor was exposed to 200ppm step of carbon monoxide

gas. VC is the monitored CE pin voltage and clearly shows the expected CE voltage dropping below the WE

voltage, VW, as the concentration increases.

VTIA is the output of the transimpedance amplifier U2. VDIFF is the calculated difference between VREF and VTIA,

which will be used for the ppm calculation.

Figure 48. Calculated Sensor Current

Figure 49. Calculated ppm

Figure 48 shows the calculated sensor current using the formula in Equation 7 :

ISENSOR = VDIFF / RF = 1.52V / 110 kΩ = 13.8uA

(7)

Equation 8 shows the resulting conversion of the sensor current into ppm.

ppm = ISENSOR / IPERPPM = 13.8µA / 69nA = 200

(8)

Total supply current for the amplifier section is less than 700 nA, minus sensor current. Note that the sensor

current is sourced from the amplifier output, which in turn comes from the amplifier supply voltage. Therefore,

any continuous sensor current must also be included in supply current budget calculations.