ADS8557: Load Capacitance when bus connected

Shinichi Inoue

Guru 20926 points

Part Number: ADS8557
Other Parts Discussed in Thread: OMAPL138, OMAP-L138

Dear Specialists,

My customer is considering ADS8557 and has a question.

I would be grateful if you could advise.

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I would like to confirm the Input Capacitance and Load Capacitance of the ADS8557.

EMIF of OMAPL138
ADS8557IPM: 4
DAC7728SPAG: 1
Intel/Altera 10M04DCF256I7G
Wiznet W5300
are BUS connected.

The sum of these Input Capacitances is 50pF,
Exceeds the ADS8557 Load Capacitance rating of 30pF max.

		Input Capacitance(pF)	Rated Load Capacitance(pF)
DSP	OMAP-L138	3
FPGA	10M04DCF256I7G	8
ETHERNET	W5300	5
DAC	DAC7728SPAG	14
ADC1	ADS8557IPM	5	30
ADC2	ADS8557IPM	5	30
ADC3	ADS8557IPM	5	30
ADC4	ADS8557IPM	5	30
	Total	50

In this case, for the circuit configuration
1) If it is judged that there is no problem, present the reason.
2) Proposal of countermeasures when judged not to be recommended
please.

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I appreciate your great help in advance.

Best regards,

Shinichi

over 2 years ago

0 Tom Hendrick over 2 years ago

TI__Guru**** 176431 points

Hi Shinichi,

We cannot guarantee the performance of the ADS8557 if you exceed the MAX load capacitance as slew rates may impact the ability of the FPGA or DSP to properly read/write data. You could put the four ADS8557 devices behind a bus transceiver to isolate them from the rest of the bus.

0 Shinichi Inoue over 2 years ago in reply to Tom Hendrick

Guru 20926 points

Hi Tom,

Thank you for your reply.

I understand ADS8557 can not accept 50pF load capacitance even in this case.

It should be add bus transceiver between ADS8557 and bus.

I'll share this information with the customer.

I appreciate your great help and cooperation.

Best regards,

Shinichi

0 Shinichi Inoue over 2 years ago in reply to Shinichi Inoue

Guru 20926 points

Hi Tom,

The customer has an additional question.

Could you please let us know what type of the bus transceiver should be used and device part number.

I appreciate your great help and cooperation.

Best regards,

Shinichi

0 Tom Hendrick over 2 years ago in reply to Shinichi Inoue

TI__Guru**** 176431 points

Hi Shinichi,

You have a pretty wide selection which mainly depends on how you are powering the BVDD of the ADS8557 and if you plan on using byte or word mode of operation. Can you provide that detail please?

0 Shinichi Inoue over 2 years ago in reply to Tom Hendrick

Guru 20926 points

Hi Tom,

Thank you for your reply.

I'll confirm with the customer and then feedback to you.

Could you please wait a couple of time.

I appreciate your great help and cooperation.

Best regards,

Shinichi

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ADS8557: Load Capacitance when bus connected