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Anonymous Poster

### Sample and Hold Circuit

11/05/2009 4:41 PM

I have been trying to figure out the calculations for a sample and hold circuit (basic) with respect to the capacitor size for parameters.

For an input of 1v sinewave (10khz) being sampled at 100khz. I will be using a mosfet for switching and this will be controlled by a square wave.

Another problem I am having:

I got the circuit done with a voltage buffer input/output and a mosfet/capacitor etc. However by output has very noisey harmonics in the output. Can anyone suggest anything for that?

Thank you for your time.

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Guru

Join Date: May 2006
Location: Reading, Berkshire, UK. Going under cover.
Posts: 9100
#1

### Re: Sample and Hold Circuit

11/05/2009 6:55 PM

Is this a college project, a bit of DIY, or what?

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Guru

Join Date: Jan 2008
Location: NY
Posts: 1218
#2

### Re: Sample and Hold Circuit

11/05/2009 7:24 PM

Given:

1 [V] (RMS or peak-to-peak?) sine wave at 10 [kHz]
10 [microseconds] between samples

Please supply the following parameters:

MOSFET Rdson value [ohms]
S&H capacitor C value [microfarads]
Buffer amplifier input impedance and/or bias current [ohms] [nanoamps]

Guru

Join Date: Dec 2008
Location: Long Island NY
Posts: 11186
#3

### Re: Sample and Hold Circuit

11/06/2009 10:13 AM

I think your ill defined noisy harmonics comes from the charge injection that your solid state switches generate. Depending on the topology and component selection, with sufficient time settling after acquisition this error will either be proportional to the voltage to be measured or a nominal constant value. In either scenario though this should be a predictable offset that can be compensated for, once understood.

Obviously though this relies on the idea that your noisy harmonics are harmonics of your sampling frequency. If you mean something else then please clarify what these harmonic frequencies are multiples of what anticipated frequency. Also don't forget to identify where you measure these signals; after your input buffer, on your storage capacitor, after the storage capacitor buffer. Lastly identify what you used to measure these harmonics; analog oscilloscope, digital oscilloscope with FFT capabilities, dynamic signal analyzer, analysis of data set from A/D converter.

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