# Sample and Hold Circuts

Discussion in 'Electronic Design' started by Sunil, Feb 16, 2008.

1. ### SunilGuest

hi, i was reading about sample and hold circuits and i dint quite
understand a few sentences......it said that s and h ckts are not
required for low speed signals while they are of the utmost necessity
for high speed circuits. can someone explain y?
thanx
sunil

2. ### D from BCGuest

A slow moving turtle is easy to capture. It doesn't move much and one
can just run up to it and grab it.
But if you try that with rabbits..You'll miss and miss and miss..
So you set out a trap (sample).
Once trapped, the rabbit is 'holding' and that gives you time to do
whatever.

D from BC
British Columbia

3. ### Tim WescottGuest

Because whatever follows the sample and hold needs time to do it's job on
the measurement. Most commonly, the thing following will be an ADC, and
you want to (a) sample at a well-controlled point in time and (b) give
the ADC time to do a conversion. A successive-approximation converter
goes through several steps that need to happen when the input voltage is
constant, if the input signal moves significantly during this conversion
period then the measurement will be confused.

Note that life is more complicated than this: Few ADCs require sample-
and-hold circuits these days. Unless you're doing something special with
a special ADC chances are that the ADC has a built-in sample & hold
circuit, or is a switched-capacitance type that incorporates the sample &
hold function into the fabric of the A to D conversion.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

4. ### donaldGuest

Tims very good response cut.

I am going to save questions like these for the next project I get, that
says they want to take their project off-shore.

If guys like this can not figure out these functions in the classroom,
what good will they be on a real project/product.

My \$.02

donald