PSoC 1 internal capacitor needs to be higher value

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Tim
Level 2
Level 2
First solution authored 10 replies posted 10 sign-ins

I have a PSoC 1 CY8C20234.  I am using the internal 100 pF capacitor successfully in a very very simple circuit successfully.  I tie the cap to the analog bus and bring that out to a pin that has a resistor across it.  I charge up the cap and then let it discharge into the external resistor.  This all works great.   The issue is that cap is way to small.   I need about .01 uF.   Is there any way to make a cap out of the C swicthed caps or any method.   Again I need to attach the cap to the analog bus.  Very simple.  It just needs to be more like .01 uF.

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SampathS_11
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250 sign-ins 250 solutions authored 5 questions asked

Hello @Tim ,

Unfortunately, we do not have 10nF capacitor inside this PSoC. Is it possible that you can charge the 100pF capacitor through a resistor of proportionately high value (~100 times : 10nF/100pF), and achieve the same effect of having a 10nF capacitor?

Best regards,

Sampath Selvaraj 

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SampathS_11
Moderator
Moderator
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250 sign-ins 250 solutions authored 5 questions asked

Hello @Tim ,

Unfortunately, we do not have 10nF capacitor inside this PSoC. Is it possible that you can charge the 100pF capacitor through a resistor of proportionately high value (~100 times : 10nF/100pF), and achieve the same effect of having a 10nF capacitor?

Best regards,

Sampath Selvaraj 

This is a great suggestion but won't work in my situation.  I'm solving an external circuitry issue that is a very long and complicated explanation.  It all boils down to I just needed the 100 pf Cap to be around a .01 uF cap.   If there is no way to construct one out of the Capsense circuitry somehow or some other way I'm forced to do a rework that I don't want to do.  The cap would need to connect to the analog bus and ground.

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Len_CONSULTRON
Level 9
Level 9
Beta tester 500 solutions authored 1000 replies posted

Tim,

Charging and discharging is SOOOO very analog.  I'm assuming that you're trying to time something.

If so, why don't you use the timer resources (digital) inside the PSoC1?   If should be more accurate and more consistent (less prone to temperature changes, component tolerances, etc).

Len
"Engineering is an Art. The Art of Compromise."
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This is a great suggestion.  As a matter of fact I'm measuring resistance external to the  PSoC and doing a very analog physics thing external to the PSoC.  Using a timer will not work for this situation.  

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Tim,

Measuring resistance ... I can appreciate that.  

I'm now assuming the external resistor you've mentioned is the resistor to be measured.   

I'm assuming the cpa and resistor combination gives you a an RC constant (tau).   If you know C, and measure the time of one tau (63% of VDD), you can calculate R.

The common approach used in DMM to measuring R is to use a constant (known) current source.

Push that current through R.  then R = Vresistor/Iconstant_current

Another Method:

Create your own switchable bank of external resistors.    Your unknown resistor 'R' becomes a part of a resistor divider network where you turn on one external resistor to GND (using Pin_switch_Rs) and measure the center tap voltage between R and R_ref_x with an ADC.

R = R_ref_x * (VDD/V@Pin_AIN - 1)

Len_CONSULTRON_0-1640319732118.png

Here is a table of which R_ref_x resistor to use for each range to measure R

R_ref_x value Measure when R range is:
100K 10K - 10Meg
10K 1K - 1Meg
1K 100 - 100K
100 10 - 10K
10 1 - 1K

 

You could do the same with switched caps to GND.   However, it is easier to measure a static voltage and if needed average your readings.  Also you can purchase precision resistors +/- 1% but external caps tend to be +/- 10%.

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