Timeline for RFI/AC Filter Rejection for Instrumenation Amplifiers
Current License: CC BY-SA 3.0
13 events
| when toggle format | what | by | license | comment | |
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| Apr 7, 2013 at 14:25 | vote | accept | Saad | ||
| Sep 26, 2012 at 21:16 | comment | added | Saad | Thanks. And since the wire is being connected directly to ground, should I be worried that I may introduce significant noise to my ground through this wire? | |
| Sep 26, 2012 at 21:02 | comment | added | Olin Lathrop | If you're going to ground one side, you don't need a inamp, just a ordinary opamp that can work with input down to ground, or with a small negative supply. With a single ended signal, I'd put two poles of R-C right before the amp, one to get rid of high frequencies, the next to limit the frequency to the minimum you need. Then you can use the output of the opamp directly, no buffering required. | |
| Sep 26, 2012 at 20:51 | comment | added | Saad | @OlinLathrop Assuming that I ground the other end of the wire and do a single-ended measurement using an in. amp as you recommended, do I just use a single RC filter (1.6 kHz cut off) at the +ve input of the In. amp and then have another more dominant filter after that one? And you recommended a buffer before the in. amp, is this because of the relatively high output impedance of this more dominant filter? | |
| Sep 26, 2012 at 16:07 | comment | added | Olin Lathrop | Just a few volts negative is enough, which can easily be done with a ordinary charge pump running from 5V. After the transistor and diode drops, you might end up with -2.2V. That's plenty for most opamps. For "rail to rail" input opamps just a few 100 mV would help. Some truly go down to ground on the input. The simplest answer would be to find one of those. | |
| Sep 26, 2012 at 13:59 | comment | added | Saad | @OlinLathrop That's a VERY neat solution! Does the -ve voltage need to be -5? I don't think so, I think even -1V would do. Though I reckon I will need to reference the output to ground as thats what the ADC would expect. I think I will read up on that and ask questions here if I need. | |
| Sep 26, 2012 at 13:57 | comment | added | Olin Lathrop | Yes, many amps don't really sense all the way down to ground, often despite what the datasheet says. However, I'd fix that by making a small negative supply, not by making the signal differential and all the complications that go with that. A small charge pump can make a few volts negative at a few mA, good enough for many opamps. | |
| Sep 26, 2012 at 13:35 | answer | added | Olin Lathrop | timeline score: 1 | |
| Sep 26, 2012 at 13:01 | comment | added | Saad | @OlinLathrop I haven't yet picked a In.Amp but it seems most of them don't have inputs that can reach rails completely (even those who are marketed as rail to rail). They need to be above gnd by a few mV at least. | |
| Sep 26, 2012 at 12:57 | comment | added | Olin Lathrop | What's the purpose of D1? Why can't one end of the cable be tied to ground and this then become a single ended measurement? | |
| Sep 26, 2012 at 9:55 | history | edited | stevenvh |
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| Sep 26, 2012 at 9:39 | answer | added | stevenvh | timeline score: 2 | |
| Sep 26, 2012 at 9:27 | history | asked | Saad | CC BY-SA 3.0 |