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HI,
I am checking on example project and found the sample and hold example. The sample and hold component can use internal and exteranl as Vref, but I could not find any reference on why and when you should use the internal or external vref. Any idea where can I find the information?
Tks
Solved! Go to Solution.
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I know two reasons for using an external reference:
- the internal one is not precise enough, ot to unstable (temp coefficent)
- the internal one has the wrong voltage (e.g. you need 2.048V but the internal one only goes to 1.2V)
On the -050 kits there is even a spare footprint for an external reference with higher precision (and I think on the -030 kits too). The internal reference is accurate for about 0.9%, with a temp coefficent of 50ppm. The proposed LM4140 gives you 0.1% and 3ppm.
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I know two reasons for using an external reference:
- the internal one is not precise enough, ot to unstable (temp coefficent)
- the internal one has the wrong voltage (e.g. you need 2.048V but the internal one only goes to 1.2V)
On the -050 kits there is even a spare footprint for an external reference with higher precision (and I think on the -030 kits too). The internal reference is accurate for about 0.9%, with a temp coefficent of 50ppm. The proposed LM4140 gives you 0.1% and 3ppm.
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1) You have to do an end to end error analysis to see what reference tolerance you need.
2) The internal reference can be scaled with PGA, its gain and drift would contribute to ref error analysis.
3) The S/H probably does not warrant need for high precision external reference, as its own errors
probably dominate end to end error analysis. But my conjecture can only be established by doing
the error analysis.
4) LM4140 A grade an expensive part, and only a 0 - 70 C part. Digikey shows ~ $4 - $5
part. The 10 ppm part more like $ 1.50 - $ 3.00.
The onchip reference is speced over I temp range, and its specs look like -
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But how is the reference affecting the S&H?
May be I missed something here. Where is the reference come into play with the S&H process?
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This should help -
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Also, is the effect of Vref post-samplig or pre-sampling?
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Went thru the TRM, it seems that using the internal one would be a bad option. I think it should state this in the datasheet and give more details of what would happen as well as the requirement of having external reference to more than 500mV.
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I think Cypress should update the data sheet.
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I filed a case with Cypress as there are two different schematics showing two different
T/H, S/H capacitor sizes.
I also address this step size issue. I think this reference is to mixer operation, CMR
allowable. But I have seen ap notes discussing an allowable CMR operation that
does not comply to datasheet.
Will keep forum informed when I get answer.
Regards, Dana.
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I filed a CASE on this to clarify -
1) Two different schematics in TRM show two different cap sizes for S/H, T/H
2) I think the Vref comment about step size in mixer is a CMR issue of OpAmp,
not sure that it applies to S/H, T/H, just mixer. But I have been wrong before on guesses,
so have asked Cypress to clarify. However the Vref issue in module datasheet
totally incomplete as to what it is doing when applied to S/H, T/H. I think it is a CMR
issue again with internal OpAmp.
Will keep forum informed.
Regards, Dana.
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I thought the post had not occured, and lost the text, so retyped,
Sorry for confusion.
Dana.
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Thanks, dana.
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