ADC, Do I need a sample and hold chip?

[rosexu@fsmail.net]

Dear all,

I have a question about the ADC.

If I use a ADC chip and a MUX etc., and the ADC has the internal MUX
and internal sample and hold, do I still need a sample and hold after
the MUX before the ADC?

Many thanks,

Regards

 

[Eeyore]

Dear all,

I have a question about the ADC.

If I use a ADC chip and a MUX etc., and the ADC has the internal MUX
and internal sample and hold, do I still need a sample and hold after
the MUX before the ADC?

Depends on various things. Even if it's a succesive approximation converter you
can calculate if it needs a s/h by examining the maximum input slew rate. I've
managed without one.

Graham

 

[rosexu@fsmail.net]

Depends on various things. Even if it's a succesive approximation converter you
can calculate if it needs a s/h by examining the maximum input slew rate. I've
managed without one.

Graham

Thanks, Graham.
Can I say if I put a S/H, it will be safer?
Could you tell me how do you calculate it?

Cheers,

 

[John Larkin]

Dear all,

I have a question about the ADC.

If I use a ADC chip and a MUX etc., and the ADC has the internal MUX
and internal sample and hold, do I still need a sample and hold after
the MUX before the ADC?

No; it already has a s/h, right? But anyhow, if the chip has an
integrated mux and s/h, how could you get between them to add another
s/h?

The two common integrated adc architectures, these days, are
delta-sigma and charge-balance successive approximation, and neither
neeeds an external s/h.

A little anti-aliasing filtering is always prodent.

John

 

[Rich Grise]

I have a question about the ADC.

If I use a ADC chip and a MUX etc., and the ADC has the internal MUX and
internal sample and hold, do I still need a sample and hold after the MUX
before the ADC?

If "the ADC has the internal MUX...", then why do you need a separate
MUX at all?

Thanks,
Rich

 

[rosexu@fsmail.net]

If "the ADC has the internal MUX...", then why do you need a separate
MUX at all?

Thanks,
Rich

Thanks for your response.
The ADC is not able to give enough channels and I need to condition
the signal before the ADC. The maximum ADC channel I can have for one
ADC is 32 single end channels. But there are 24 differential channels
to sample.
To save the cost and the number of componennts, I have to use several
multiplexers in the front.

Cheers,

 

[Eeyore]

Thanks, Graham.
Can I say if I put a S/H, it will be safer?

If it doesn't need a s/h in the first place it doesn't need to be 'safer'.

Could you tell me how do you calculate it?

What's your application ? What are you measuring. What chip are you using ?

Graham

 

[rosexu@fsmail.net]

If it doesn't need a s/h in the first place it doesn't need to be 'safer'.


What's your application ? What are you measuring. What chip are you using ?

Graham- Hide quoted text -

- Show quoted text -

The application is to measure DC voltages. I am going to use the
ADG509F (Analog Device) - MUX, and ADS8326 (TI) - ADC.
There will be a low pass filter before the ADS8326.

Cheers,

 

[Eeyore]

The application is to measure DC voltages. I am going to use the
ADG509F (Analog Device) - MUX, and ADS8326 (TI) - ADC.
There will be a low pass filter before the ADS8326.

What frequency LPF ? That virtually assures me that you won't need a s/h from first
principles since the filter will limit the rate at which the input voltage can change.

Graham

 

[rosexu@fsmail.net]

What frequency LPF ? That virtually assures me that you won't need a s/h from first
principles since the filter will limit the rate at which the input voltage can change.

Graham- Hide quoted text -

- Show quoted text -

Not sure yet. For DC voltage measurement, what freq. do you think is
right? Probably 30Hz, which will filter out the mains 50 or 60Hz.

 

[Peter Bennett]

Not sure yet. For DC voltage measurement, what freq. do you think is
right? Probably 30Hz, which will filter out the mains 50 or 60Hz.

The low pass filters should go _before_ the mux. The signal path
between mux and ADC has to pass the mux switching frequency - if you
put a LP filter there, you have to scan slow enough that the filter
will settle between readings, which would mean, with a 30 Hz LP, the
switching frequency of the mux will have to be significantly below 30
Hz to get any sensible readings.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
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[Rich Grise]

The ADC is not able to give enough channels and I need to condition the
signal before the ADC. The maximum ADC channel I can have for one ADC is
32 single end channels. But there are 24 differential channels to sample.
To save the cost and the number of componennts, I have to use several
multiplexers in the front.

As long as your MUX can get the signal to the ADC in time (i.e., before
the sample window) and is stable enough for the sample interval that it
doesn't disturb the reading, then I'd say, you probably don't need another
S/H.

You'd switch your MUX to the channel in question, tell the ADC to make
the reading, and _its_ S/H should operate normally.

So, no, you shouldn't need an additional sample & hold.

Good Luck!
Rich

 

[Eeyore]

Not sure yet. For DC voltage measurement, what freq. do you think is
right? Probably 30Hz, which will filter out the mains 50 or 60Hz.

If you want to filter out 50 or 60 Hz you'll need a very high order filter @ 30Hz. It all
depends on the characteristics of what you're measuring of course.

It seems to me that you're going at this blind. You ought to have a better idea of your
actual requirements.

Graham

 

[Eeyore]

The low pass filters should go _before_ the mux. The signal path
between mux and ADC has to pass the mux switching frequency - if you
put a LP filter there, you have to scan slow enough that the filter
will settle between readings, which would mean, with a 30 Hz LP, the
switching frequency of the mux will have to be significantly below 30
Hz to get any sensible readings.

That's very true.

Condition the signals and then mux them.

Graham

 

[Hal Murray]

That's very true.

Condition the signals and then mux them.

Putting the filter after the mux avoids duplicating the filters.

It works find as long as your sample rate is slow enough.

 

[rosexu@fsmail.net]

Putting the filter after the mux avoids duplicating the filters.

It works find as long as your sample rate is slow enough.

Thanks for all your answers.
I will probably put the 30Hz filter before the MUX, since the RC
filters aren't going to add too much on the budget. Apart from the
cost, the other reason is the MUX has to switch quickly enough to get
all 24 channels samples at least less than 0.05 sec.

Cheers,

 

[YD]

Late at night, by candle light, rosexu@fsmail.net penned this immortal
opus:

Thanks for your response.
The ADC is not able to give enough channels and I need to condition
the signal before the ADC. The maximum ADC channel I can have for one
ADC is 32 single end channels. But there are 24 differential channels
to sample.
To save the cost and the number of componennts, I have to use several
multiplexers in the front.

Cheers,

Looks like you might want to use diff to single amps on the inputs.
Then you get to use them as filters/conditioners as well. Depends on
what kind of overhead you can tolerate, 24 of them do take up a bit of
real estate and spare change. It's always nice to have an extra layer
between the DAC and the cruel, cold outside world.

- YD.

 

[Peter Bennett]

On Sat, 22 Sep 2007 19:31:51 -0500,

Putting the filter after the mux avoids duplicating the filters.

It works find as long as your sample rate is slow enough.

True, but for a 30 Hz low pass filter, the scan rate would have to be
Bloody Slow - a half second per channel might work. And the OP now
says he wants to scan all 24 channels in 50 mS.


--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
new newsgroup users info : http://vancouver-webpages.com/nnq
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Vancouver Power Squadron: http://vancouver.powersquadron.ca