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PSoC™ 4 Forum Discussions

SuSh_1535366
Level 5
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Level 5

Hello,

When using an external VREF for the SARADC of CY8C4246AZI-L445, what is the maximum current required for the reference?

To be more specific, we are thinking of using LT6656AIDC-4.096 as the external reference IC.
The output current of this IC is 1uA.
Will the SARADC work properly with this IC?

Regards,
Shimamura

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DennisS_46
Employee
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Employee

The LT6656 is specified for a load current of 5 mA. The VrefIn load current of 588 uA at 18 MHz and Vref = 4.096 should not be a problem for the LT6656. Application schematics and text references in the Linear datasheet show a 1.0 uF cap on the Vref output for stability. This works further to reduce the transient noise (at the ADC clock frequency) on the Vref input load.
---- Dennis

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Vasanth
Moderator
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Hi Shimamura,

The ADC will not load an external reference significantly. You should be able to use LT6656AIDC-4.096 without any problem.

Best Regards,
Vasanth

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SuSh_1535366
Level 5
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Level 5

Hello Vasanth, 

Thank you for your answer.

If there is a defined maximum/minimum current required for VREF, could you please tell me how much it is?

Regards,

Shimamura

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Vasanth
Moderator
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Hi Shimamura san,

I need to correct my previous statement. The current drawn from the VREF is actually proportion to the clock. The actual equation is

I = Vref *fC  , where Vref is the external reference voltage, f is the adc component clock frequency and C is 8pF.

The current value can go to 147uA ( if Vref=1.024,f=18MHz). So you need a better Vref current. Apologies for the inaccuracy in the initial statement.

Best Regards,
Vasanth

 

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DennisS_46
Employee
50 sign-ins 100 replies posted 25 solutions authored
Employee

The LT6656 is specified for a load current of 5 mA. The VrefIn load current of 588 uA at 18 MHz and Vref = 4.096 should not be a problem for the LT6656. Application schematics and text references in the Linear datasheet show a 1.0 uF cap on the Vref output for stability. This works further to reduce the transient noise (at the ADC clock frequency) on the Vref input load.
---- Dennis

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SuSh_1535366
Level 5
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Level 5

Hello Vasanth-san, Dennis-san,

Thanks for the information.

In the original design, they used LT1790BIS6-4.096 (output current 60uA) with 1MHz frequency.
According to the information you provided, the current at this time is 32.768uA.

However, (as you know) it is becoming difficult to purchase this part, so they are looking for an alternative part.

It looks like LT6656AIDC-4.096 is not usable.
I will ask them to look for other parts.

Thank you for your support.

Regards,

Shimamura

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SuSh_1535366
Level 5
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Level 5

Sorry, I misread the model number of the reference IC.
LT6656AIDC-4.096 can be used for Vref for ADC.


Thank you for your answer.

Regards,

Shimamura

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