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PSoC 5, 3 & 1 MCU

guzeliang
New Contributor

Hi,

I am using your CY8C5868 PSOC, use it sample lmt84(temperature sensor) P3.7 temperature as below configuration.

guzeliang_3-1632999218871.png

 

guzeliang_0-1632998903658.png

guzeliang_1-1632998909298.png

 

 

But FW can't capture stable value, use the scope capture the lmt84(temperature sensor) output signal is pull down in about 100KHZ. If I disconnet the output pin to P3.7, the lmt84 output signal is stable.

guzeliang_2-1632999138899.png

 

Best Regards,

Kent

 

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4 Replies
Vasanth
Moderator
Moderator

Hi Kent,

Is the ADC input is being sampled at the same rate ? Could you try adding an opamp buffer stage between the sensor input and ADC and see ?

Best Regards,
Vasanth

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guzeliang
New Contributor

Hi Vasanth,

yes, seems that the signal pulled down frequency is similar with ADC sample frequency. 

Yes, we tried to add OP of SOC inside, but MCU can't get any sensor siganl. Could you please show me how to add OP of SOC between our sensor input and ADC sample?

 

Best Regards,

Kent

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Vasanth
Moderator
Moderator

Hi Kent,

See the schematic below. Keep the opamp in follower mode in configuration. Make sure you call Opamp start in the firmware.

Vasanth_0-1633942788563.png

Best Regards,
Vasanth

 



Best Regards,
Vasanth

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Len_CONSULTRON
Honored Contributor II

Kent,

Based on the TI's LMT84 datasheet, this is a "medium" output impedance device (~5V/50uA = 100K ohms).

Most likely the issue you are seeing is that the SAR is a "medium" impedance input resource (~180K ohms).  When the SAR is sampling, the input impedance affects the LMT84's output drive (which can only source 50uA).

When the LMT84's output impedance is combined with the PSoC's SAR input impedance, the "effective" input voltage will drop from the expected voltage from the LMT84's table.

As Vasanth indicated, using a OPAMP in the front of the SAR will help.  (The input impedance of the OPAMP is about 10 Mohms)

Also, if you use the Delta-Sigma ADC with the input buffer enabled, you will also get about 10 Mohms input impedance.

Len
"Engineering is an Art. The Art of Compromise."
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