What is tPA and tJitter in ADC?

I'm trying figure out the sample and hold time of an ADC. In other
words, not the sample rate, but how fast the sample and hold works.
I'm comparing two chips.

Chip #1
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,P11972,D8597
tAP (Sample-and-hold acquisition delay time) is 0 ns.
tJitter (Sample-and-hold acquisition delay time jitter) is 0.2 psRMS.

Chip #2
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,P1644,D2894
Advertises a 250MHz Internal Sample-and-hold
tAP (Sample-and-hold acquisition delay time) is 3 ns.
tJitter (Sample-and-hold acquisition delay time jitter) is 5 psRMS.

Could someone please explain what this means? I didn't see any sample
and hold frequency for Chip #1. I can understand why ADC need a sample
and hold because the signal is most likely changing. So the ADC wants
to work on a non-changing voltage. Chip #2 explanation of the sample
and hold seems easy to understand-- 250MHz. If we associated a
frequency to chip #1 then what would that be?

Thanks,
Paul

 

[Andrew Holme]

pmlonline@gmail.com wrote:

I'm trying figure out the sample and hold time of an ADC. In other
words, not the sample rate, but how fast the sample and hold works.
I'm comparing two chips.

Chip #1

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,P11972,D8597
tAP (Sample-and-hold acquisition delay time) is 0 ns.
tJitter (Sample-and-hold acquisition delay time jitter) is 0.2 psRMS.

Chip #2

http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,P1644,D2894
Advertises a 250MHz Internal Sample-and-hold
tAP (Sample-and-hold acquisition delay time) is 3 ns.
tJitter (Sample-and-hold acquisition delay time jitter) is 5 psRMS.

Could someone please explain what this means? I didn't see any sample
and hold frequency for Chip #1. I can understand why ADC need a
sample and hold because the signal is most likely changing. So the
ADC wants to work on a non-changing voltage. Chip #2 explanation of
the sample and hold seems easy to understand-- 250MHz. If we
associated a frequency to chip #1 then what would that be?

Thanks,
Paul

Datasheet #1 quotes the "full power bandwidth" of the input circuit as 640
MHz.

 

[Tim Wescott]

pmlonline@gmail.com wrote:

I'm trying figure out the sample and hold time of an ADC. In other
words, not the sample rate, but how fast the sample and hold works.
I'm comparing two chips.

Chip #1
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,P11972,D8597
tAP (Sample-and-hold acquisition delay time) is 0 ns.
tJitter (Sample-and-hold acquisition delay time jitter) is 0.2 psRMS.

Chip #2
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1155,C1001,P1644,D2894
Advertises a 250MHz Internal Sample-and-hold
tAP (Sample-and-hold acquisition delay time) is 3 ns.
tJitter (Sample-and-hold acquisition delay time jitter) is 5 psRMS.

Could someone please explain what this means? I didn't see any sample
and hold frequency for Chip #1. I can understand why ADC need a sample
and hold because the signal is most likely changing. So the ADC wants
to work on a non-changing voltage. Chip #2 explanation of the sample
and hold seems easy to understand-- 250MHz. If we associated a
frequency to chip #1 then what would that be?

Thanks,
Paul

tAP is shown in the "Timing Diagram" on page 11 of your first data

sheet. It has nothing to do with the highest sample _rate_ of the
thing, just the apparent amount of delay from the clock. It doesn't
necessarily have anything to do with the sampling _rate_ that you can
get from the chip as a whole -- since that rate is determined by the
converter architecture the performance of the actual sampler is moot.

tJitter is just what it says -- the amount of timing jitter that you can
expect between your clock input to the chip and the chip's response
_assuming that you drive the chip as specified_. Any timing jitter here
will multiply by the slope of the incoming signal and appear as noise in
the reading.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com