Re: balancing sensors


redbelly wrote:
redbelly wrote:
. . . you could try putting a
voltage divider on the sensor output that gives the larger signal.

The resistance of the divider should be:
1. Large enough so that it does not alter the sensor output
appreciably, and
2. Small enough so that the microchip does not alter it's output
appreciably.

lerameur wrote:

Yes, how about putting the voltage divider at the input supply. ?

It really needs to go on the output voltage.

Here is the chip I am using:
http://www.taosinc.com/product_detail.asp?cateid=20&proid=2

Nice. That 8 Meg resistor makes it pretty sensitive.

Also I saw about 3%-4% of ripple (noise). Maybe just adding a small
capacitor with a resistance in parallel (from Vout to ground)? or would
a feedback op-amp low pass filter be a better idea?

k

A simple way to both have a low-pass filter and make adjustments to the
DC signal would be to put this on the output of your sensor:

R1
. o--,/\/\/'---+----+---o
. | |
. \ |
. / |
. R2 \ --- C
. / ---
. \ |
. / |
. | |
. o------------+----+---o
. |
. -----
. ---
. -

The filter's time constant is Rp*C, where Rp is the parallel
combination of R1 & R2.
R1 & R2 form a simple voltage divider for the DC level of the signal.
R2 could be a trimpot; adjust it on one sensor to match the response of
the other.

A low-pass filtering op-amp is better, if you can hook one up. Then
you could use a trimpot to adjust the DC gains of the two sensors to
match each other, in addition to having filter caps.

Question: is your ripple:
... at line frequency?
... twice the line frequency?
... some other frequency?
... just randomly fluctuating, not at any particular frequency?


I added a capacitor and a resistor in parallel at the output like you
said and it solved the problem. I already had a a resistor their. The
max current these chip can take is about 20ma for input anyway. I also
has some back ac lighting,weak but still affected the circuit, but when
in total darkness and just the white led, it works good.
thanks

ken

.

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