VCA2618 Internal HPF

toshi-e2e

Hi all,

According to the overview of VCA2618 datasheet of page 9,
" By using the internal coupling into the PGA,
the result is a high-pass filter characteristic with cutoff at
approximately 75kHz. The output PGA naturally rolls off at
around 30MHz, making the usable bandwidth of the VCA2618
between 75kHz and 30MHz."

However the section of PGA post filter of page 11 described that
"The lowfrequency bandwidth is set by the following equation:
1/(2 •π • 500kΩ• (220pF + CEXTERNAL))"

And when CEXTERNAL is 0(pF), the result is about 1.447kHz.
The result is not 75kHz.

Why is the calculation result not 75kHz?

Regards,
Toshi

over 11 years ago

0 Chuck Smyth over 11 years ago

TI__Expert 6320 points

Hi Toshi,

Yes, you are right. This shows a maximum cutoff of 1.5kHz. So any Cext would only lower the cutoff.

I will check to see if this is a typo.

Thanks,

Chuck Smyth

0 toshi-e2e over 10 years ago in reply to Chuck Smyth

Genius 4830 points

Hi Chuck-san,

My Customer has evaluated VCA2618 and reported the result as follows.
It shows that maximum cutoff is not 1.5kHz. even if adding external cap.

1. Is the formula of " 1/(2 •π • 500kΩ• (220pF + CEXTERNAL))" correct?
2. Can VAC2618 perform 2kHz signal?
3. In this case, what must it be careful of?
4. Is there the data of Gain vs Frequency which is below 100kHz?

Regards,
Toshi

0 Noriyuki Takahashi over 10 years ago in reply to toshi-e2e

TI__Genius 10980 points

Hi, Toshi-san

I will take a measurement in lab and respond with the result.

Regards,

Noriyuki Takahashi

0 toshi-e2e over 10 years ago in reply to Noriyuki Takahashi

Genius 4830 points

Hi Takahashi-san,

Thanks for your support.

My customer hopes to receive the result as soon as possible..

Regards,
Toshi

0 toshi-e2e over 10 years ago in reply to toshi-e2e

Genius 4830 points

Hi Takahashi-san,

Would you tell when the result can be got?
It is necessary to change a design depending on a result,
and my customer is wanting to get to know a result urgently.

Thanks and regards,
Toshi

0 Amy Schnoor over 10 years ago in reply to toshi-e2e

TI__Mastermind 18285 points

Toshi-san,

I will check on the shipment of the EVM boards. We were waiting for them before we could test in the lab. I will provide an update soon.

0 Amy Schnoor over 10 years ago in reply to Amy Schnoor

TI__Mastermind 18285 points

Toshi-san,

We received the boards late today. We will do our testing in the lab and send you the results shortly after.

0 toshi-e2e over 10 years ago in reply to Amy Schnoor

Genius 4830 points

Amy-san,

Thanks a lot for your support.

I hope that VCA2618 can perform 2kHz signal.

Regards,
Toshi

0 Noriyuki Takahashi over 10 years ago in reply to toshi-e2e

TI__Genius 10980 points

Hi Toshi-san

Sorry for late reply.

We measured the filter response of the EVM like below.

VCA_CNTL=0.2V

There is no C_EXTERNAL

Regarding the fomula, we are still confirming.

Regards,

Noriyuki Takahashi

0 toshi-e2e over 10 years ago in reply to Noriyuki Takahashi

Genius 4830 points

Hi Takahashi-san,
Thanks for your support.

How was the result about the formula?

Regards,
Toshi

0 Noriyuki Takahashi over 10 years ago in reply to toshi-e2e

TI__Genius 10980 points

Hi Toshi-san,

Sorry for late reply.

We are still checking whether the equation is correct by asking the designer.

We made some tests using VCA2618EVM.

The C_EXTERNAL helps to lower the fc of HPF, however fc isn’t lowered to 1.5kHz.

fc changed to about 2 or 3 kHz by adding 10uF capacitor, but fc at 20uF capacitor was almost same with fc at 10uF in lab test.

Please see following pictures.

3618.MGS=001, VCA_CNTL=0.2V, C_EXTERNAL=0uF.pdf

1651.MGS=001, VCA_CNTL=0.2V, C_EXTERNAL=10uF.pdf

6281.MGS=001, VCA_CNTL=0.2V, C_EXTERNAL=20uF.pdf

Regards,

Noriyuki Takahashi

0 toshi-e2e over 10 years ago in reply to Noriyuki Takahashi

Genius 4830 points

Hi Takahashi-san,

Thanks for providing the test results.
I am waiting for a reply about the formula.

Regards,
Toshi

0 Lijoy Philipose over 10 years ago in reply to toshi-e2e

TI__Intellectual 1315 points

Hi Toshi,

I wanted to expound a bit more. We'll have to trust our measurements results and advise the customer accordingly. We'll update once we have more data. It may take sometime.

Regards,

Lijoy

0 toshi-e2e over 10 years ago in reply to Lijoy Philipose

Genius 4830 points

Hi Lijoy-san,

I understood.
I will try to explain the situation to my customer.

Thanks and regards,
Toshi

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VCA2618 Internal HPF