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- Actual acquisition time for PSoC4100S SAR ADC

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Contributor II

Jan 28, 2020
05:16 PM

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Hello,

I have a question about actual acquisition time for PSoC4100S SAR ADC.

Please see "PSoC® 4 SAR ADC Input Impedance – KBA224710".

PSoC® 4 SAR ADC Input Impedance – KBA224710

According to this,

R_{SW2}+R_{SW1 ≈1/(sample rate * 6.4 pF)}

If the clock frequency is 12MHz, the sample rate is 750000sps.

Then, R_{SW2}+R_{SW1 =1/(750000*6.4*10^-12)=208.333kohm.}

The acquisition time = 9 × (RSRC + RSW2 + RSW1) × CSHOLD.

Assuming RSRC is 0, Acquisition time = 18749nsec.

What do you think?

Best regards,

Yocchi

Solved! Go to Solution.

1 Solution

Moderator

Jan 29, 2020
08:25 PM

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Hello Yocchi san,

It is actually the input resistance seen from the source point where is signal source is connected.

Now the image you have attached is also not right. The resistor is similar to a switched capacitor resistance. Due to sampling sudden bursts of charges will be drawn into the system. If you take the average current flown over time, this can be modeled as a resistance connected.

Kindly check online references for switched cap resistances.

Best Regards,

Vasanth

11 Replies

Moderator

Jan 28, 2020
11:16 PM

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Hello Yocchi-san,

Unfortunately your inference is not right. The input impedance of the ADC includes the R_{SW2}+R_{SW1 } + 1/(sample rate * 10 pF) ,the resistance due to the switched capacitor equivalent resistance formed by sampling circuit. As the third part is dominant, it defines the total input resistance of the circuit. It is calculated based on the periodic bursts of charge going into the capacitor.

Now regarding the acquisition time, the sampling switch is closed.

So acquisition time is the time to charge the internal hold capacitor with 9 time constants, when the switch is closed.

tACQ = 9 × (RSRC + RSW2 + RSW1) × CSHOLD should be used for calculation of the acquisition time.

Best Regards,

Vasanth

Contributor II

Jan 29, 2020
01:18 AM

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Hello Vasanth-san,

I know what you mean. I agree with you.

Then, What does "An approximate value of this resistance can be found using 1/(sample rate * 6.4 pF)" in KBA224710 mean?

Best regards,

Yocchi

Moderator

Jan 29, 2020
03:04 AM

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Hi Yochi san,

Rinput = R_{SW2}+R_{SW1}+R_{SAMPLING .}

R_{SW2}+R_{SW1 }= 2.2 K Ohms

R_{SAMPLING } = 1/(sample rate * 10 pF).

Normally this will be the dominating factor in the resistance calculated. The KBA tries to tell that the acutal input impedance is not just R_{SW2}+R_{SW1 }ohms, but along with that the equivalent resistance due to sampling, R_{SAMPLING} should be added. I hope that clears your doubt.

Best Regards,

Vasanth

Contributor II

Jan 29, 2020
04:08 PM

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Hello Vasanth-san,

"Rinput = R_{SW2}+R_{SW1}+R_{SAMPLING" means the total resistances that are seen from analog input pin, right?}

Best regards,

Yocchi

Moderator

Jan 29, 2020
08:25 PM

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Hello Yocchi san,

It is actually the input resistance seen from the source point where is signal source is connected.

Now the image you have attached is also not right. The resistor is similar to a switched capacitor resistance. Due to sampling sudden bursts of charges will be drawn into the system. If you take the average current flown over time, this can be modeled as a resistance connected.

Kindly check online references for switched cap resistances.

Best Regards,

Vasanth

Contributor II

Jan 29, 2020
08:47 PM

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Hello Vasanth-san,

I am sorry. Certainly my drawing was wrong.

The other side of SWACQ appears as a switched capacitor.

Rinput = R_{SW2}+R_{SW1}+R_{SAMPLING .}

Best regards,

Yocchi

Contributor II

Jan 29, 2020
09:48 PM

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Hello Vasanth-san,

The CSHOLD is A_INCAP(10pF) in the datasheet, right?

Best regards,

Yocchi

Moderator

Jan 30, 2020
01:17 AM

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Hi Yocchi san,

A_INCAP is 10pF. Thanks for pointing out. We will do corrective actions to reflect the changes both in KBA and thread.

Best Regards,

Vasanth

Contributor II

Jan 30, 2020
01:21 AM

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Hello Vasanth-san,

May I correct the equation like below?

R_{SAMPLING }= 1/(sample rate * 10 pF)

Best regards,

Yocchi

Moderator

Jan 30, 2020
01:29 AM

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Hi Yocchi san,

I have done necessary steps to update the equation in the KBA. The responses the thread are corrected accordingly.

The resistance is, R_{SAMPLING }= 1/(sample rate * 10 pF)

Best Regards,

Vasanth

Contributor II

Jan 30, 2020
01:33 AM

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Hello Vasanth-san,

I really appreciate your help.

Thank you very much.

Best regards,

Yocchi