LMP91000: LMP91000 bias voltage setup

user5953015

Part Number: LMP91000

please, i need help,

i am using LMP91000 working with HCL gas sensor. this sensor needs a 200mV bias voltage.

i configured REFCN as following:

i measured the Vout. After powred on ,it reached a very high output, and after a certain while, about 5 minutes, it slowly goes down. So far, it's OK.

but wired that it always fell to a very low voltage, and after 2 seconds, went high, and then fell down. i captured as following:

I tested with CO sensor, and CL2 sensor. LMP91000 worked well.

Does anyone know what's wrong with my test?

Best Regards,

WangpPing

over 4 years ago

0 Josh Wyatt over 4 years ago

TI__Guru* 91086 points

Dear WangpPing -

Would you please let us know the part number of the sensor you are using and show us the schematic you are working with.

0 user5953015 over 4 years ago in reply to Josh Wyatt

Prodigy 40 points

Dear Josh,

Many thank for replying.

I am using 4HCL-LS, from SUSA. Sorry it's datasheet is in Chinese, but it's similar to City's sensor, except for Sensitivity. I attach both datasheets:

SUSA: http://www.susatech.com/UploadFile/file/20180521/20180521130414371437.pdf

CITY: https://www.citytech.com/en-gb/PDF-Datasheets/hclhbr3e30.pdf

below is my schematic.

0 Gordon Varney over 4 years ago in reply to user5953015

TI__Expert 8860 points

Dear WangpPing -

From the scope shot we would expect a large inductance either in the sensor or in the wires going to the sensor.

The ramp down is from the capacitance after the 1k resistor and the overvoltage spike is inductive.

If other sensors are working with the LMP91000 and this one doesn’t, there may be a compatibility issue. This sensor is not in our list of tested sensors.

Reach out to the sensor manufacture about the spike and see if they can explain where it is coming from.

0 user5953015 over 4 years ago in reply to Gordon Varney

Prodigy 40 points

Dear Gordon,

Thanks for your solution. I removed E1, and the system works well till now. I will follow more sensors to check the result.

Would you explain another question. I measured Vw and Vr, Vw is 817mV, and Vr is 649 mV. The bias voltage is 168mV, not desired 200mV. I removed sensor, shorted R and C. The value was unchanged.

Shoud I enlarge BIAS setting to achieve 200mV?

Thanks again!

Br,

wangping

0 Gordon Varney over 4 years ago in reply to user5953015

TI__Expert 8860 points

Wangping,

Remember the LMP91000 is measuring current for this sensor. If W and R are shorted, and you are seeing 168mV, then this is the max dynamic current range for this setting. Increasing the bias voltage to 200mv will increase the dynamic range and provide better resolution.

The sensor data sheet says Bias Potential is 200mV but does not state if this is the preferred potential or the max potential.

Operating the sensor to the max potential will shorten the life of the sensor. Operating the sensor below the preferred potential will reduce the sensitivity and accuracy of the sensor.

Maybe best to ask the manufacture if the Bias Potential is preferred or max. Then adjust as needed.

0 user5953015 over 4 years ago in reply to Gordon Varney

Prodigy 40 points

Dear Gordon,

Thank you very much!

Best Regards,

WangPing

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LMP91000: LMP91000 bias voltage setup