Hi there. I'm an analog electronics design engineer at EADS Astrium in the UK.
I work alongside digital hardware designers and software developers covering aerospace applications. Max (our illustrious editor in chief here at All Programmable Planet) told me that many FPGA designers could do with a little help understanding how to interface the analog world to their digital domain. Eventually, I could no longer find the strength to struggle against Max's pitiful pleadings and I agreed to explain the wibbly-wobbly analog world in ways that even he could understand (his words, not mine). So here we go...
Many transducers, such as electrochemical gas sensors, for example, produce very low-level signals. Depending upon the transducer in question, the signal can be a voltage, or a current, or even a varying electrical resistance. When I say "low-level," I'm talking about values such as 41μV per degree Celsius from a thermocouple or 10nA per "unit" from an electrochemical sensor. The point is that it is not viable or desirable to apply such signals to an analog-to-digital converter (ADC) without first using an intermediate analog signal amplification stage.
While some applications use digital methods to filter out noise from an analog signal after analog-to-digital (A/D) data conversion has been performed, there is another problem with analog signal amplification -- DC voltage offset errors. Not only do such errors add to, or subtract from, the analog signal of interest, but they may also drift with respect to temperature, time, and power supply voltage variations. Unlike noise signals, these are DC drift errors. They pose a significant problem when attempting to amplify the analog signal, because the amplifier must allow the desired DC signal through, which means it cannot block DC drift from influencing the signal of interest.
Let's consider a very high-level view of the signal path as it makes its way from some form of transducer/sensor, through an amplifier block, into an ADC, and eventually -- in digital form -- into an FPGA:
Now, the first thing Max asked me when he saw this diagram was "Why do we have to have multiple amplification stages in the DC and AC amplifier block -- can’t we simply use a single amplifier?" Well, I will be covering all of this in more detail in future blogs, but -- for the purposes of this column -- I can summarize things in a (simplified) nutshell as follows: If you have an amplifier with a voltage gain of 1 and a bandwidth of 100MHz, then using the same amplifier with a voltage gain of 10 reduces the bandwidth to 10MHz. That is, multiplying the voltage gain by some value "X" means the available bandwidth is divided by the same value "X." Thus, if we were to use the same amplifier with the voltage gain set to 100, this would reduce the bandwidth to only 1MHz. However, if we were to use two of these amplifiers in series, both with their voltage gains set to 10, then we would retain a bandwidth of 10MHz while achieving a total voltage gain of 10 x 10 = 100 (ta-dah!).
Titanium Andy 3/23/2013 9:00:24 AM User Rank Blogger
Re: CDS to compensate offset
Sunita
Thank you. What you have described is a very good method of detecting, and then removing the error caused by the offset voltage error within an operational amplifier. In fact I shall be detailing two methods, one which uses the offset null terminals of some operational amplifiers, usually single devices, but when they are not available, as per dual operational amplifiers, another method along the lines you describe becomes applicable. Then add the extension to using operational amplifiers to form a differential/instrumentation amplifier, or using a dedicated instrumentation amplifiers, and another method is then required.
What I am hoping to demonstrate is how, similar to the method that you have suggested, is to easily detect the error and its magnitude within the analogue domain, and then use digital methods to reduce the error to an insignificant level sufficient for subsequent signal processing.
"For this reason, there is a desire to have a method of DC voltage amplifier error detection and correction; that is, an automatic DC error nulling process that operates without user intervention or signal degradation."
CDS(Corelated Double Sampling) is the one process used to nullify the DC error offset.In CDS, first reference volatge(usually tied to ground) is measured from which we will get the offset value.Next input signal is measured which will include input signal and the offset. Difference of these values will remove DC offset.
@Andy - "I shall be revealing weak elements of analogue circuit function and performance and show how, by using digital signals, compensate for these inadequacies within the analogue domain, thereby minimising further signal correction."
Hoping to see this very soon, sounds really interesting.
Analog Steve 3/6/2013 4:34:59 PM User Rank Beginner
Re: Great FPGA front end
Hi Andrew,
I'll just echo what everyone else is saying about being glad to have an analog guy posting on APP. Between your writing for this site and Adam Taylor's past (and hopefully future) signal chain work on Xcell Journal, I'd say this community will be well served learning about the intersection between analog and digital.
@Gina: Nice to meet you, Gina. Although I acknowledge your presence kinda late, I'm happy to meet tomii's friend. Hope to see you at DW wearing the Prop CapNet Hat. I think I will leave my only Panama hat at the hotel so Max doesn't take it to precociously install a propeller in its roof.(grin) But don't worry, I will introduce myself, hopefully also wearing one of those CapNet hats. :0)
@max: He doesn't have to drink in his office -- we can do that for him :-)
This blog is going very well, Adam. Max and I are even getting into the minute detailed plans for our trip together to Brazil. Tonight I need to rush and call Sidney to become a member and be an active participant of this discussion.
Later on Sidnei can address his favorite filtering techniques.
Titanium Andy 3/5/2013 12:05:15 PM User Rank Blogger
Re: Great FPGA front end
Steve - Thank you. All being well, I shall be revealing weak elements of analogue circuit function and performance and show how, by using digital signals, compensate for these inadequacies within the analogue domain, thereby minimising further signal correction.
hash.era 3/5/2013 10:38:22 AM User Rank Clever Clogs
Re: Great FPGA front end
"Looks like your series will be really helpful to people using analog features, on or off of the FPGA."
Sleibson: Even I'm interested in it. I have some findings too which involves analog but would prefer to have a look on this 1st and then I'll dish out mine.
Digital engineers are often confused among operational amplifiers, differential amplifiers, and instrumentation amplifiers; this is exacerbated by the fact that their circuit symbols can be similar.
Digital engineers are often confused between operational amplifiers, differential amplifiers, and instrumentation amplifiers; this is exacerbated by the fact that their circuit symbols can be similar.
For an operational amplifier to perform its role, there has to be some method to feed a voltage back from the output to one of the two inputs and achieve voltage stability.
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