increasing an A/D's range
increasing an A/D's range
(OP)
We have a channel with a 16-bit A/D and a 2.5 Vpp range (38uV/count). There is an unused channel available on the same board so a colleague has suggested increasing the signal and using one A/D for the top half and the other A/D for the bottom half. The software then has to stitch everything back together. Is this standard practice, or are there some gotchas?
Here are the catches that I've thought of so far:
- With one A/D offset for a range of 0V to +2.5V and the other A/D offset for a range of -2.5V to 0V, A signal swinging +/-2.5V will swing outside the absolute maximim ratings of the input. I can get around this by clamping the inputs for each A/D and careful setting the clamping current.
- Once I clamp the inputs, I'll need time to come out of saturation. When I have a fast change (high dV/dt) this will effect my transition from one A/D to the other. As long as the recovery time is fast enough, I can get around it by overlapping the A/D ranges. So maybe I offset one A/D with a range of -0.5 to 2.0V, and the other A/D with a range of -2.0V to +0.5V. As long as I'm out of saturation before the signal changes by 1.0V I'll still have quality data.
- The offset voltage will need to be very clean. Otherwise it will degrade the signal to noise ratio.
- The A/D must be have enough gain before it that the additional gain won't degrade the overall Noise Figure.
So now I have 38uV resolution and a range of 4V. That is the equivalent of 104858 counts, 60% more range than I started with at 65536 counts over 2.5V. It isn't much in terms of bits (16.678) but I do have 60% more range. While adding an entire second channel doesn't seem very efficient, if it running unused, why not?
Am I missing anything?!?
Here are the catches that I've thought of so far:
- With one A/D offset for a range of 0V to +2.5V and the other A/D offset for a range of -2.5V to 0V, A signal swinging +/-2.5V will swing outside the absolute maximim ratings of the input. I can get around this by clamping the inputs for each A/D and careful setting the clamping current.
- Once I clamp the inputs, I'll need time to come out of saturation. When I have a fast change (high dV/dt) this will effect my transition from one A/D to the other. As long as the recovery time is fast enough, I can get around it by overlapping the A/D ranges. So maybe I offset one A/D with a range of -0.5 to 2.0V, and the other A/D with a range of -2.0V to +0.5V. As long as I'm out of saturation before the signal changes by 1.0V I'll still have quality data.
- The offset voltage will need to be very clean. Otherwise it will degrade the signal to noise ratio.
- The A/D must be have enough gain before it that the additional gain won't degrade the overall Noise Figure.
So now I have 38uV resolution and a range of 4V. That is the equivalent of 104858 counts, 60% more range than I started with at 65536 counts over 2.5V. It isn't much in terms of bits (16.678) but I do have 60% more range. While adding an entire second channel doesn't seem very efficient, if it running unused, why not?
Am I missing anything?!?





RE: increasing an A/D's range
Range can be increased much more efficient with a voltage divider - you know that - and if you need more resolution, a simple SW filter can take care of that. It does reduce your BW, though. Is that acceptable?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: increasing an A/D's range
RE: increasing an A/D's range
TTFN
Eng-Tips Policies FAQ731-376
RE: increasing an A/D's range
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: increasing an A/D's range
RE: increasing an A/D's range
RE: increasing an A/D's range
Z
RE: increasing an A/D's range
Felixc is suggesting dithering and oversampling. This is a valid technique, but loses bandwidth. By averaging the two data streams you do not lose bandwidth and get a sqrt(2) improvement in the noise.
RE: increasing an A/D's range
RE: increasing an A/D's range
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...