ADS8866 - Daisy-Chain Operation

Hiroshi Katsunaga

Mastermind 6480 points

Other Parts Discussed in Thread: ADS8866, OPA333

Hello,

My customer have some questions about ADS8866.

[Q1]

Is it possible to use as the simultaneous sampling ADC with 3 devices in daisy-chain operation ?

[Q2]

If they use 3 devices, is it possible to share a reference drive circuit ?

(They are going to refer to Figure 55 in the datasheet with respect to the reference circuit.)

Best Regards,

Hiroshi Katsunaga

over 10 years ago

0 Sandeep Kasargod over 10 years ago

TI__Expert 4405 points

Hiroshi-san,

As mentioned in the datasheet, the ADS8866 does support daisy chaining.

However it is not recommended to share the reference drive circuit across the three ADCs. The reference voltage is critical to the performance of a SAR ADC and must remain stable during conversion to avoid errors. In a shared configuration, high-current transients at the reference pin as an ADC as it makes its bit decisions can impact the results on the other ADCs.

This phenomenon and the care to be taken when designing references for SAR ADCs is very nicely described in the following blog by Luis Chioye: e2e.ti.com/.../sar-adc-pcb-layout-the-reference-path

Regards,
Sandeep

0 Hiroshi Katsunaga over 10 years ago in reply to Sandeep Kasargod

Mastermind 6480 points

Hi Sandeep,

Thank you for your fast response.
I understood the reference voltage circuit for a CDAC SAR ADC is very sensitive.

Then, is it possible to share the reference source (REF30xx) and to separate the buffers ?
REF30xx --+-- OPA333 --- ADS8866
|
+-- OPA333 --- ADS8866
|
+-- OPA333 --- ADS8866

Also can you answer the previous Q1 ?
[Q1(again)]
Is it possible to use as the simultaneous sampling ADC with 3 devices in daisy-chain operation ?
Does the daisy-chain operation mean the simultaneous sampling ?

Best Regards,
Hiroshi Katsunaga

0 Sandeep Kasargod over 10 years ago in reply to Hiroshi Katsunaga

TI__Expert 4405 points

Hiroshi-san,

Yes, it is fine to share the reference source and individually drive the ADCs with separate buffers as you have described.

The CONVST signal defines the instant at which the analog input signal is sampled by the ADS8866. If you look at Figure 49 in the datasheet, which shows the connection diagram for the daisy-chain configuration, the CONVST pins of all ADCs are tied together and therefore they will sample simultaneously.

Regards,

Sandeep

0 Hiroshi Katsunaga over 10 years ago in reply to Sandeep Kasargod

Mastermind 6480 points

Hi Sandeep,

Thank you for your fast response.
I understood your comment.
I became clear.

Thank you for your support!

Best Regards,
Hiroshi Katsunaga

0 henning larsen over 7 years ago in reply to Sandeep Kasargod

Intellectual 335 points

the mentioned link to the blog by Luis Chioye: e2e.ti.com/.../sar-adc-pcb-layout-the-reference-path appears broken - also when searching specifically on ti.com for the document.

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ADS8866 - Daisy-Chain Operation