ADS7865 - 470nF capacitance at REFout and REFin

Hirotaka Matsumoto

Other Parts Discussed in Thread: ADS7865

Hi all

Would you mind if we ask ADS7865?

There is some discription about 470nF capacitance at REFout and REFin on the datasheet.

<REFIN>
The converter generates reference, the internal reference buffer requires a rail-spikes on the reference input voltage because of to-rail input and output.
Such buffers typically contain internal switching.
Therefore, an external capacitor to two input stages; when the input voltage passes the the analog ground (AGND) should be used to mid-range area,
a transition occurs at the output stabilize the reference input voltage

<REFOUT>
A minimum of 470nF is required to keep the reference stable (see the previous discussion of REF IN ).

Is this 470nF used for smoothing or buffering mainly?

Kind regards,

Hirotaka Matsumoto

over 8 years ago

0 Hirotaka Matsumoto over 8 years ago

Guru 27585 points

Hi team

WE guess that your team is so busy, however could you let us know the reply?

Kind regards,

Hirotaka Matsumoto

0 Evan Sawyer over 8 years ago

TI__Expert 3175 points

Matsumoto-san,

The DAC inside the ADC will pull large amounts of current from the voltage reference during a conversion, creating voltage spikes on the reference trace. The capacitor is used to supply the necessary current, without a voltage drop that would impact the precision of the converter. We usually refer to it as a 'charge bucket,' as it is large enough to provide the charge needed by the switching capacitors inside the DAC, while the reference simply 'refills' the charge on the capacitor.

Below is a link to a new blog that with an image and explanation of these voltage spikes. Page 16 of the ADS7865 datasheet also provides a brief explanation of this process.

e2e.ti.com/.../why-is-it-so-challenging-to-design-a-voltage-reference-circuit-for-an-adc

0 Hirotaka Matsumoto over 8 years ago in reply to Evan Sawyer

Guru 27585 points

Evan san

Thank you for your very kind reply!

We got it and we have another question from our customer.

The datasheet show that this capacitor should be at least 470nF.
How much is the maximum value? 1uF?

We need your help.

Kind regards,

Hirotaka Matsumoto

0 Evan Sawyer over 8 years ago in reply to Hirotaka Matsumoto

TI__Expert 3175 points

Matsumoto-san,

Will the design use the internal reference, or an external reference?

0 Hirotaka Matsumoto over 8 years ago in reply to Evan Sawyer

Guru 27585 points

Evan san

Thank you for your reply!
Our customer will use the internal reference.

Kind regards,

Hirotaka Matsumoto

0 Hirotaka Matsumoto over 8 years ago in reply to Hirotaka Matsumoto

Guru 27585 points

Evan san

We are very sorry to rush you however could you give us the reply?
And then, if you have some precautions on using ceramic capacitance, could you let us know?
(For example, ESR, temperature characteristics, size(0402?))

We appreciate your help always.

Kind regards,

Hirotaka Matsumoto

0 Evan Sawyer over 8 years ago in reply to Hirotaka Matsumoto

TI__Expert 3175 points

Matsumoto-san,

Sorry for how long it is taking to find the answer. I am am working with the designers of this device to determine the max size of the capacitor and will let you know as soon as I have heard back.

0 Hirotaka Matsumoto over 8 years ago in reply to Evan Sawyer

Guru 27585 points

Evan san

Thank you for your reply.
We are waiting for your reply.

Kind regards,

Hirotaka Matsumoto

0 Hirotaka Matsumoto over 8 years ago in reply to Evan Sawyer

Guru 27585 points

Evan san

Would you inform us your circumstance?
It is possible, we would like to reply the answer by this Friday.

Kind regards,

Hirotaka Matsumoto

0 RahulVK over 8 years ago in reply to Hirotaka Matsumoto

TI__Expert 3785 points

Hi Matsumoto-san,

The capacitor on REFout pin has a minimum value driven from a stability and performance perspective. You can use higher values ; 1uF will work fine.

The characteristics of capacitor should be chosen to suit the application; ADS7865 is rated from -40C to 125C hence a capacitor with X7R rating would be necessary to use this temperature range. If the application is not going to be used over the full temperature range then a different grade capacitor may be used. Footprint should be selected as per PCB area constraints. Ceramic-1uF-0402-X7R should be fine.

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ADS7865 - 470nF capacitance at REFout and REFin