bluerabbit97 said:
Hi Chris,
I'm in university doing Final Year Project.
This ADC is just a side track that is needed for the whole project. I
wish to complete in a month's time.
Currently I'm not given the make model of the gyro, just an insight
that it will provide a max of +/-5v. I was told to get the ADC tested
using a sine-wave generator.
Thanks
Blue
Hi, Blue. Here's something that might get you started, if you have to
use the single supply ADC.
Let's assume gyro zero is significant, but you have to get pretty much
a full scale range from -5V to 5V, with inputs pretty much limited to
-2.5V to +2.5V. You can start with the idea above of using a voltage
divider to level shift, and work from there.
Without further information, we'll have to assume your gyro output
center is actuually at 0.000V, rather than (Vcc + Vee) / 2. Find this
out -- it's critically important, and you'll have to change your plan
if it isn't.
I'm going to also make the assumption that you've got other analog
voltages available to power things, like, say, a +/-12V for the gyro.
Doing the problem this way, you'll have to start out with a good quad
op amp with low voltage offset and drift (U1). Choice of op amp is
dependent on your power supplies, and what you have available now.
You'll need a reliable +5V reference, and there are a number to choose
from. But you've already got a 2.5V reference on chip, and you can use
the op amp to make that 5V, like this (view in fixed font or M$
Notepad):
|
| 2.5V |\|
| o------|+\ 5.00V
| | >--o---o
| .--|-/ |
| | |/| .-.
| | | | 10K 0.1%
| |1/4 U1 | |
| | '-'
| | |
| '--------o
| |
| .-.
| | |10K 0.1%
| | |
| '-'
| |
| ===
| GND
|
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
In this setup, the important thing is stability of the voltage
reference, not precision.
Now, look for analog switch IC (U2) that will switch bipolar voltages
with a logic level control signal from your uC. High OFF resistance is
more important than low ON resistance. Choose the one that works for
you based on availability. Remember that pinouts for these ICs are
somewhat standardized, so you can always drop something in and order an
upgrade.
You'll need time for the rest of the show -- try to get something up
and running from a hardware standpoint as soon as possible, so you can
start learning what else you need.
By obtaining these components, you can set up a unipolar signal input
of 0 to +2.5V with center or zero at 1.25V, and typical signal riding
from 0.625V to 1.875V and a uC-controlled autozero calibration like
this:
|
| .-----.
| | | 20K 0.1% X 2
| Gyro |1/4U2| ___ ___ 5.Vref
| o------------o o---o---|___|-o-|___|----o
| -5V/5V | | | |
| | | | |
| '--o--' | |
| | | | 1/4U1
| |\ | | |
| .-----| >O--' | | |\| 20K 0.1% X 2
| | |/ | '---|+\ ___
| | | | >---o-|___|-o----o
| | | .---|-/ | | To ADC
| | .-----. | | |/| | .-.
| | | | | | | | |
| | |1/4U2| | | | | |
| | .----o o---' '----------o '-'
| | | | | |
| | | | | ===
| | === '--o--' GND
| | GND | U1 = Quad low offset low drift op amp
| CAL | | U2 = Quad analog switch
| o---o-----------'
|
|
(created by AACircuit v1.28.6 beta 04/19/05
www.tech-chat.de)
This should give you a good start -- one of these for each of your 3
axes. Since your ADC is faster than your data requirements, you can
occasionally switch CAL on, and do an autozero cycle to be able to
cancel out offset and drift. One quad op amp IC, two quad analog
switch ICs, a handful of precision resistors in easily obtained values,
and you're done.
The devil, as always, is in the details on this. To get a realistic
precision of 1 or 2mV, you'll have to be particularly careful with
ground paths. Ask your adviser for other suggestions on this. Take
the time to read Analog Devices AN-345, AN-404 and the venerable
AN-202, "An IC Amplifier User's Guide to Decoupling, Grounding,
and Making Things Go Right for a Change" By Paul Brokaw.
Good luck, and feel free to post again if you run into problems or if
the assumptions about the gyro are incorrect. Hurry early.
Good luck
Chris
