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Impedance Calculation

R

Rob

Hello,
I have a quick question.
I have an RC low pass filter and I need to calculate the impedance so
I can match it to a SAR ADC.

What would the steps be to calcuate the impedance?

Thanks,
Rob
 
B

Bob Masta

Hello,
I have a quick question.
I have an RC low pass filter and I need to calculate the impedance so
I can match it to a SAR ADC.

What would the steps be to calcuate the impedance?

Thanks,
Rob

You probably shouldn't be doing any such thing as
"impedane matching". The SAR should have a
high input impedance. That will not only prevent it
loading down the LPF and changing its response,
but also prevent the LPF from messing up the SAR.

Best regards,


Bob Masta

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
Science with your sound card!
 
N

Noway2

Rob said:
Hello,
I have a quick question.
I have an RC low pass filter and I need to calculate the impedance so
I can match it to a SAR ADC.

What would the steps be to calcuate the impedance?

Thanks,
Rob
You need to be careful about the impedance you drive the ADC with. The
sample and hold needs to be able to quickly charge / discharge to the
next sample value. In this sense quickly is relative to the sample rate
and the operational parameters of your converter. Some converters
have a setting for the track time before they will isolate the sample
cap from the driving circuit. For best results, you really need to
understand the characteristics of your converter.

In the absence of other advice, I would suggest driving the circuit with
a low impedance buffer or amplifier circuit rather than attempting to
match its impedance. You will want at an impedance at least 10X greater
at the input to the converter than the driving circuit to account for
loading.
 
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