ADC fast sampling pot - Page 1
Forum section: electronics / projects

author: sirhaggis
author_rank: Regular Contributor
timestamp: April 14, 2016, 05:09:38 am
content: Hi all,I am working on a project in which I am building an analog bus, on which I want to measure multiple voltage values with an ADC by rapidly switching between them. I have a single 12bit ADC into which is connected up to 32 bus switches, only one of which can be active at any moment to provide a direct connect between the analog source and ADC. The circuitry switches rapidly between the various bus switches, connecting each of the analog sources directly to the ADC for a short period, on which I then trigger a conversion to read its value. The concept seems to be sound in principle, however I am having issues with the speed at which I can switch the analog sources. I would like to be able to switch to a source and sample it, ideally within a ~500ns or so period of time. The bus switches I am using are certainly more than capable of switching at this speed (<20ns switch speed), however the concern is overcoming the inherent capacitance of the circuit and charging the ADC capacitor within the appropriate amount of time to get an accurate reading.For example, I am currently have a 10K potentiometer wired as a voltage divider as one of the analog sources. When I switch to this there is a ramp up of voltage that takes ~600ns, an excessive amount of time for my purposes. Would the simple solution be to just use a lower impedance pot, or is there a better way to achieve the results I am looking for? Clearly low impedance solutions are the ones I need to go for - is there a better way to get a low impedance voltage from an analog input like a pot? I am somewhat new to many elements of electronics so there are possibly obvious things that I am missing here.Any suggestions you might have would be greatly appreciated.Thanks

author: sirhaggis
author_rank: Regular Contributor
timestamp: April 14, 2016, 06:38:37 am
content: Ok I'm partially following you, although perhaps I should explain my circuit more clearly:I have a 10K pot configured as a voltage divider with 3.3V source. The pot output is connected to the A input on a 74CBTLV1G125 (http://www.mouser.com/ds/2/302/74CBTLV1G125-839142.pdf) into which I provide an OE signal when it has bus time. Each of the analog sources is configure the same way with their own independent switch, and a central decoder controls which currently has the bus (by pulling OE low). The outputs of each of these switches is combined to a single bus line attached to the ADC. Although this works fine, the output signal B on the switch for less than max pot rotation gives an unacceptable ramp up time, which to me indicates that there is too high impedence on the switch input which is being passed through the switch?? The ramp up time increases from an acceptable 20-30ns when the pot is at full rotation, up to ~600ns as the pot is rotated towards ground.From what I can derive from your comment, could I replace the 74CBTLV1G125 with some other kind of buffer to reduce the output ramp up time without changing the input impedance attached to it? I'm not familiar with buffers in general, do you have any component suggestions that could be used in a similar fashion (i.e. OE signal to activate from high impedance state, rapid settling of output voltage to that of input even if input is highish impedance)?Thanks for your time, much appreciated.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 14, 2016, 06:46:06 am
content: If the signals are slowly changing (i.e., pots turned by human beings, temperature sensors, etc.), then just parallel them with ceramic caps at least 1000x bigger than the ADC sampling cap - typically 10 nF to 22 nF is enough. This drops the output impedance of your analog sources to a few ohms or less. This cap should be as close to the ADC (close to mux pins, in this case) as possible to avoid inductive impedance.

author: dannyf
author_rank: Super Contributor
timestamp: April 14, 2016, 10:51:50 am
content: "Could the simple solution be to just use a lower impedance pot"Use a follower to transform the impedance.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 14, 2016, 11:29:09 am
content: Quote from: dannyf on April 14, 2016, 10:51:50 amA simple capacitor does exactly same much easier and cheaper (and without the worry about capacitive loading making it unstable). The limitation is poor bandwidth, but this is not a problem with a pot.

author: sirhaggis
author_rank: Regular Contributor
timestamp: April 14, 2016, 11:40:34 am
content: I stuck a capacitor in and it gives a nice tight edge, but now the voltage range of the pot output has been significantly reduced from a useable 1V-3.2V down to an unusable 2.9V-3.2V when I activate the bus switch. I'm a bit of a newbie so I'm possibly missing something obvious here...?

author: sirhaggis
author_rank: Regular Contributor
timestamp: April 14, 2016, 11:49:22 am
content: Never mind, I had a loose wire so I wasn't grounded properly. It seems to work fine with a 15nf ceramic. Thanks for the help