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Sachin Gupta & Akshay Phatak

ADC Guide, Part 13: Input Impedance

Sachin Gupta
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UdyRegan
UdyRegan
9/26/2018 8:11:11 AM
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Newbie
Re: Z-in for ADC
This is what we really need on how to clarify things out on a brief scale without omitting the important details. We need to bear in mind that the circuit we are about to get our hands on is compact but holds a complicated underlying setup. A clear explanation like this entails just what we need on the surface before drilling into the ground. A customization may look simple but the intricate details are all that matters.

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Brad Albing
Brad Albing
3/27/2013 6:56:44 PM
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Z-in for ADC
Gentlemen - thanks for an easy to understand explanation of the input structure and of the problems one may encounter if one does not pay careful attention. Good useful details on the sample and hold circuit.

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Brad Albing
Brad Albing
3/27/2013 4:46:44 PM
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Re: switched capacitance adc's
Well sure, but look at the price difference.

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Brad Albing
Brad Albing
3/27/2013 4:44:28 PM
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Re: switched capacitance adc's
Entirely reasonable. Altho', a bit more pricey.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
3/21/2013 12:52:30 PM
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Master
Re: switched capacitance adc's
LM324 is a nice opamp. However, it is not rail to rail opamp and this is a distinct disadvantage.

MCP604 is even better and swings from rail to rail.

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pmoyle111
pmoyle111
3/20/2013 4:39:30 PM
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Teacher
Re: switched capacitance adc's
Yes, I experieced those problems wiht it as well on another design. I had an automatic gain and offset control, digitally controlled by an 8051, some adc's and dac's and I used the LM324 to shift the offset up or down. My signal requirements were not that much, a 117 triangle wave on one channel and a 1019 hz one on the other. But I experiencenced a nasty little "nick" on each side. After digging thru data sheets, trying a bunch of techniques to make sure power pins had nothing bad going in or out) and talking with National, I just substituted a BB OPA 4131 and the problem vanished.

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Brad Albing
Brad Albing
3/20/2013 4:15:31 PM
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Re: switched capacitance adc's
The '324 was a decent op-amp, but it certainly had its limitations. as you noted. Its crossover distortion was problematic in audio applications, tho' I've seen it used in a bunch of designs 20 years ago.

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Brad Albing
Brad Albing
3/20/2013 4:02:33 PM
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Re: Sensors Interfacing
VP - glad you like the material these guys have prepared. There's more coming (soon) so stick around.

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pmoyle111
pmoyle111
3/20/2013 1:39:02 PM
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Teacher
switched capacitance adc's
Back in the early '90's I did a design using a switched capacitor adc (unbuffered) that on one channel was a temperature reference voltage buffered by a unity gain LM324. All other channels which had instument amps were buffered by BUF634's (which can drive something like 200ma - 20 yr's ago, so I don't really remeber - but a lot). The entire circuit worked fine....until the actual equipment was installed in a Chineese auto plant which had no heat and got down to about 10 deg C. The LM324 no longer could produce enough current to satisfy the inrush of the ADC which resulted in a sort of negative half sine wave sync'd to every conversion start.


Afterwards, looking at the spec for the ADC it was observed that the ADC required a source that could drive 100 ohms. The LM324 could only drive a 600 ohm load. Of course since the signal was a steady state signal, it wasn't as brutal as borne out by the numbers but it was apparently close to the edge and the temperture change made either the LM324 source less or the adc drink more.

That had been my first experience with a switched capacitor type, os I wasn not used to an adc requireing that much current, but I have paid attention to boht sides ever since.

 

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SunitaT0
SunitaT0
3/19/2013 3:03:12 AM
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Master
Re : ADC Guide, Part 13: Input Impedance
In buffered ADC, buffer provides isolation between sample capacitors and input network resulting in reduced charge injection transients. While in unbuffered ADC, charge injection from sample capacitors reflects back onto the input network.

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Page 1 / 2   >   >>
More Blogs from Sachin Gupta & Akshay Phatak
In this installment, weíll see how to design an interfacing circuit for a given signal source to match the output impedance of the source to the input impedance of the ADC. This process is referred as impedance matching.
In previous part of this series we discussed the noise specifications of an ADC. This installment of the series talks about the ADC input configuration.
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