On 08/01/2011 07:21 PM, Jonathan Cameron wrote:
Hi Michael / All,
We have a quite a few capacitance drivers. They are all simple and so
would be easy to clean up, except for the fact that we don't have an abi
for them.
So lets make one up. How about the following? Main choice is the units...
Doing it with Farads leaves us with a lot of needed decimal places, but then
we need a lot anyway for these so what the heck. We are going to need those
types with holes in them..
Hi Jonathan,
These devices typically feature an single digit pF input range(2..8 pF).
Going with a scale in Farads is probably not going to work.
What do you mean by types with holes in them?
We need something scanf() and friends can eat...
In addition we have enough scale bits before the decimal point as well.
I would make the scale targeting pF of uF.
What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY_raw
What: /sys/bus/iio/devices/iio:deviceX/out_capacitanceY_raw
What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY-capacitanceZ_raw
KernelVersion: 3.1
Contact: linux-iio@xxxxxxxxxxxxxxx
Description:
Raw (unscaled no bias removal etc) capacitance value from/to
channel Y. After application of _offset and _scale, units will
be Farads.
Additional entries for:
What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY_offset
What: /sys/bus/iio/devices/iio:deviceX/in_capacitanceY_scale
With the ad7745 the capdac does seem to be available off chip.
Why do you think the CAPDAC is off-chip on the AD7745?
The CAPDAC can be seen as a negative capacitance connected internally
to the Cin pin.
If I understand
their use correctly it could just be treated as a _calibbias parameters?
Yes - it can be seen as a bias - however why do you have
out_capacitanceY_raw then?
(the complexity being that there are two..)
Yes - one for each Cin(+|-) pin.
More tricky on the AD7746, since there are only two CAPDACs for two
Cin(+|-) pin pairs.
Naturally there is also a
calibration register so this gets somewhat tricky...
The calibration register typically holds the zero-scale calibration
coefficient.
It's automatically updated following the capacitance offset calibration.
Tricky here is that there is only one on the AD7746, so the values must
be saved
and restored when switching between the input pairs.
Is there an optimum
combination for a given desire measurement range?
Yes there is - large offsets > 1 pF should be eliminated by the CAPDACs.
from the datasheet:
CAPACITIVE SYSTEM OFFSET CALIBRATION
The capacitive offset is dominated by the parasitic offset in the
application, such as the initial capacitance of the sensor, any
parasitic capacitance of tracks on the board, and the capacitance of any
other connections between the sensor and the CDC. Therefore, the
AD7745/AD7746 are not factory calibrated for capacitive offset. It is
the user’s responsibility to calibrate the system capacitance offset in
the application.
Any offset in the capacitance input larger than ±1 pF should first be
removed using the on-chip CAPDACs. The small offset within ±1 pF can
then be removed by using the capacitance offset calibration register.
One method of adjusting the offset is to connect a zero-scale
capacitance to the input and execute the capacitance offset calibration
mode. The calibration sets the midpoint of the ±4.096 pF range (that is,
Output Code 0x800000) to that zero-scale input.
Another method would be to calculate and write the offset cali-bration
register value, the LSB is value 31.25 aF (4.096 pF/217).
The offset calibration register is reloaded by the default value at
power-on or after reset. Therefore, if the offset calibration is not
repeated after each system power-up, the calibration coefficient value
should be stored by the host controller and reloaded as part of the
AD7745/AD7746 setup.
On the AD7746, the register is shared by the two capacitive channels. If
the capacitive channels need to be offset calibrated individually, the
host controller software should read the AD7746 capacitive offset
calibration register values after performing the offset calibration on
individual channels and then reload the values back to the AD7746 before
executing a conversion on a different channel.
All comments welcom.
Jonathan
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Greetings,
Michael
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